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Old 03-03-2012, 07:34 PM   #76
qman8
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rethink...

Quote:
Originally Posted by JoelWisman View Post
20 mA, as in 0.020 amps, is the MAXIMUM short circuit current you would want to send through the fuel strip sensing circuit.

Voltage is immaterial to melting or burning things up as long as your power source is limited to no more then 20 mA

You are not trying to heat the circuit trace with current, you are trying to create an arc across where the circuit trace is broken between the laminated layers of plastic.

The strip sensing circuit, when working, is around 2,700 ohms. A power supply that is "current limited" to a maximum of 20 mA will fall to 54 volts once the broken part of the circuit is bridged. (ohms law for voltage I*R=E (I=intensity of current, or amps. R=resistance, or ohms. E=electro motive force, or voltage))

So we now know from ohms law that a voltage source that is limited to a MAXIMUM of 20mA will in fact supply 54 volts once the higher voltage arcs across the break and creates a carbon trace or whatever it does that fixes them.

To find Power, which it the case of this resistive circuit will also be heat, ohms law says (I*E=P (P is for power, otherwise known as watts)) 54*0.02=1.08 watts.

1 watt is not going to melt the strip or the sensing circuit in my opinion.


Now, if the strip were delaminated, or there was so much capacitive current that it melted through the strip, you bet, thats a good ignition source for anything flammable! So is the fuel pump which has carbon brushes that arc the whole time the bike is running, as are static discharges that occur between the body of the tank and metal parts such as the pump all the time.

Why don't gasoline tanks blow up all the time??? Simple, fuel vapors inside a tank are normally not in a flammable state because they lack oxygen. In other words that might seem more understandable to mechanics, the fuel mixture in the tank is normally to rich to ignite.

The lower flammability limit of gasoline is 1.4%. Uncompressed, anything less then a 1.4% gasoline to air mixture won't ignite.

There is also an upper limit. Anything more then 7.6% fuel to air and it also won't ignite.

Atmospheric pressure and many other variables have an effect, but only small ones in regard to conditions a fuel tank will be in.

On a cool spring day, gasoline mixtures in a fuel tank will be upwards of 50%

Now, if the tank is nearly empty and you have been blowing air in the open cap for a while, you might get a boom.

If it is negative 50 degrees F out and the cap is open, you probably will get ignition because this is so cold that the fuel is not evaporating, but at any sane temperature, gasoline rapidly evaporates and turns any sealed container into a vessel that is WAY to fuel rich to ignite let alone explode.


This principal is why as a child you could make molotov cocktails with gasoline, light them and let them burn all day without a boom till you tossed them and broke the glass. This is why when you poured too much alcohol in your spud gun, it wouldn't light. This is also why when you drilled a hole near the bottom of a 55 gallon drum, dumped a bunch of trash, wood and gasoline in, and lit it from the hole, you lost all the hair on your arm and burning logs and trash rained down in a 75 foot diameter. The top of the vessel was open so you DID get a flammable, or in my case, explosive mixture.

A note on flight 800 who's center tank exploded in NY as I recall. Jumbo jets run on kerosine. Kerosine does not start evaporating in mass till around 130 F, so jet plane tanks are normally too lean to ignite but when nearly empty and heated by all the things that heated that tank climb into the flammable range. This would have never happened if planes ran on gasoline.

Lastly, a note on all the cars that blow up on TV. That is TV, it is not real and not one single person from Hollywood would know reality even if it crawled up their ass and died. Those cars explode because special effects guys hook remote detonated explosives to their fuel tanks. The boom is from the bomb, the fire afterward is from the fuel that was blown around after the bomb flattened the fuel tank, which is the same thing 60 minutes did when they and Ralph Nader were trying as usual to destroy American car manufactures on behalf of the Asian manufactures they are paid lobbyist for, but I'm getting far afield now

At normal temperatures in a sealed gas tank (cap closed), gasoline will not ever burn or explode.

I have been wrong before, so if you think this may be one of those times and you don't wish to become a screaming alpha, remove the strip from the tank before working on it, font create any sparks are have any static discharges as you do cause once the cap and fuel pump access is removed, theres plenty of air to support combustion in the tank :)
Wow...keystroke quantity impressive...but keep in mind that current is entirely dependent on voltage. Thats where your logic fails. Point is that HIS circuit is NOT the same as the megohm meter and i want to keep the poorbastard from at the very least blowing the strip and worse injuring himself or the bike....for anyones sake please DO NOT USE the circuit shown.
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Old 03-03-2012, 08:18 PM   #77
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Originally Posted by qman8 View Post
Wow...keystroke quantity impressive...but keep in mind that current is entirely dependent on voltage. Thats where your logic fails. Point is that HIS circuit is NOT the same as the megohm meter and i want to keep the poorbastard from at the very least blowing the strip and worse injuring himself or the bike....for anyones sake please DO NOT USE the circuit shown.
Voltage does not determine current. In a DC circuit V/R, or voltage divided by resistance determines current. This is not my theory or logic, it is an accepted law of physics but was first postulated by George Ohm way back in the 1830's.

Now, where I think we are failing to communicate is on the subject of resistance. There are two prime points of resistance in this instance. 1 is the fuel strip, which if working properly would have resistance value near 2,700 ohm.

If an unlimited current supply at 1000 volts was hooked to the fuel strip sensing circuit, 0.37 amps would flow. This is WAY to high and would equal 370 watts! ( 1000/2700=0.37 ) ( 0.37 X 1000= 370)

But there is a second resistance and that is the resistance of the power supply. When I say "current limited to 20mA max" or "short circuit current of 20mA max" I am using common terms to say "use a power supply that won't put out any more then 20 mA under any circumstance" Or for those not into metrics, 0.020 amps maximum.

If the power supply can't put out more then 20 mA, then the voltage will be 54 volts because that is the voltage that will flow 20 mA through a 2,700 ohm load.

The circuit linked in the post way above is a regulated 1000 volt supply, and without testing it or spending days working up the math, it's hard to say what it's short circuit current will be but likely higher then desired. It won't be insane because it is powered by an 800 mA 12 volt transformer. If efficiency was perfect, and it never is, this circuit is designed to output 9.6 watts (0.8 X 12 = 9.6)

9.6 watts, less losses is it's operating power. Thats too much power and is going to have WAY more then 20 mA of short circuit current, so there we agree, though the circuit is most defiantly NOT 20 watt. Power out can not be greater then power in or we wouldn't need energy anymore.


So to you qman8, or anyone else contemplating this project, don't proceed further until you understand what the terms "current limited" and "short circuit current" mean. I am suggesting a power source with a minimum of 1 mA of short circuit current and a MAXIMUM of 20 mA and an open circuit voltage around 1000 volts.

Also make sure the circuit does not have greater then 40pF of capacitance. Think of a capacitor as a battery that can instantly discharge all the energy it is storing. Regardless of the current available from your power source, it will eventually charge the capacitor up to the source voltage. A bigger capacitance will blow shit up.

Is everyone with me on the whole "20mA current limited" or specified another way that means the same thing "20mA short circuit current". It is important and what I think qman8 is not understanding.
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Old 03-04-2012, 06:04 AM   #78
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Quote:
Originally Posted by JoelWisman View Post
Voltage does not determine current. In a DC circuit V/R, or voltage divided by resistance determines current. This is not my theory or logic, it is an accepted law of physics but was first postulated by George Ohm way back in the 1830's.

Now, where I think we are failing to communicate is on the subject of resistance. There are two prime points of resistance in this instance. 1 is the fuel strip, which if working properly would have resistance value near 2,700 ohm.

If an unlimited current supply at 1000 volts was hooked to the fuel strip sensing circuit, 0.37 amps would flow. This is WAY to high and would equal 370 watts! ( 1000/2700=0.37 ) ( 0.37 X 1000= 370)

But there is a second resistance and that is the resistance of the power supply. When I say "current limited to 20mA max" or "short circuit current of 20mA max" I am using common terms to say "use a power supply that won't put out any more then 20 mA under any circumstance" Or for those not into metrics, 0.020 amps maximum.

If the power supply can't put out more then 20 mA, then the voltage will be 54 volts because that is the voltage that will flow 20 mA through a 2,700 ohm load.

The circuit linked in the post way above is a regulated 1000 volt supply, and without testing it or spending days working up the math, it's hard to say what it's short circuit current will be but likely higher then desired. It won't be insane because it is powered by an 800 mA 12 volt transformer. If efficiency was perfect, and it never is, this circuit is designed to output 9.6 watts (0.8 X 12 = 9.6)

9.6 watts, less losses is it's operating power. Thats too much power and is going to have WAY more then 20 mA of short circuit current, so there we agree, though the circuit is most defiantly NOT 20 watt. Power out can not be greater then power in or we wouldn't need energy anymore.


So to you qman8, or anyone else contemplating this project, don't proceed further until you understand what the terms "current limited" and "short circuit current" mean. I am suggesting a power source with a minimum of 1 mA of short circuit current and a MAXIMUM of 20 mA and an open circuit voltage around 1000 volts.

Also make sure the circuit does not have greater then 40pF of capacitance. Think of a capacitor as a battery that can instantly discharge all the energy it is storing. Regardless of the current available from your power source, it will eventually charge the capacitor up to the source voltage. A bigger capacitance will blow shit up.

Is everyone with me on the whole "20mA current limited" or specified another way that means the same thing "20mA short circuit current". It is important and what I think qman8 is not understanding.
"Voltage does not determine current"...Dude! it DOES..thats fundamental electronics 101....Your stated formulas are correct, but resistance is normally constant. Stated another way, you can have voltage without current flow, but you cannot have current flow without a voltage differential, so again, current is completely dependent.
First, the 2700 ohm figure is meaningless, because the fuel strip is broken, so resistance = infinity (theoretically). Now you are going to introduce enough voltage to achieve an ionizing arc, then your resistance drops dramatically . Thats when the internal impedance really comes into the picture. So, when that arc forms you want just enough current to do the job,but not enough to damage the device. With the megohm meter, the voltage will plummet when the arc starts, the cap will try to keep a constant voltage and discharge to do so, and then the system drops to a steady state current. Here is where the damage can occur. SO, I'm saying keep enough resistance in series to keep that damage from happening...what that minimum resistance can be is determined by the max current rating of the strip. If you know that, then you can estimate a substitution reisistance.

qman8 screwed with this post 03-04-2012 at 07:36 AM
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Old 03-04-2012, 08:23 AM   #79
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Wow,
Seems there should be a cheap-safe way to zap this thing.

I would probably take the sensor out of the tank anyway. The Fluke is way to $$$.
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Old 03-04-2012, 11:47 AM   #80
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Quote:
Originally Posted by qman8 View Post
"Voltage does not determine current"...Dude! it DOES..thats fundamental electronics 101....Your stated formulas are correct, but resistance is normally constant. Stated another way, you can have voltage without current flow, but you cannot have current flow without a voltage differential, so again, current is completely dependent.
First, the 2700 ohm figure is meaningless, because the fuel strip is broken, so resistance = infinity (theoretically). Now you are going to introduce enough voltage to achieve an ionizing arc, then your resistance drops dramatically . Thats when the internal impedance really comes into the picture. So, when that arc forms you want just enough current to do the job,but not enough to damage the device. With the megohm meter, the voltage will plummet when the arc starts, the cap will try to keep a constant voltage and discharge to do so, and then the system drops to a steady state current. Here is where the damage can occur. SO, I'm saying keep enough resistance in series to keep that damage from happening...what that minimum resistance can be is determined by the max current rating of the strip. If you know that, then you can estimate a substitution reisistance.
Oh jeeze, Im arguing with a physicist, engineer, or someone who likes to read a lot. I'm familiar with all 3 and have argued with all 3 while teaching "electronics 101", to bring everyone up to the same playing field before diving into instructing industrial automation and other classes using real power in real classrooms. The electricians that were taking the classes for their certification or continuing education requirements were easy. The physicists, engineers, and electronics majors were a pain in the butt.

So lets stew this down qman8

Are you suggesting that a "current limited power supply" that has a short circuit current no greater then "20 mA" is going to fall to a steady state of dissipating more then 1.08 watts of power through this fuel level sensing strip?

Are you suggesting that a current limited power supply with a short circuit current of no greater then 20 mA will fall to a steady state of greater then 54 volts once the arc ends after creating a carbon track and powering into a 2,700 ohm circuit?

Are you suggesting that 20 mA at 54 volts will dissipate more then 1.08 watts into this circuit?

Are you suggesting that 1.08 watts is more then the sensor circuit of this fuel strip can safely dissipate forever?

Have you ever seen a resistance fuel strip?


As to the strips normal operating envelope, I have never bothered to measure it. It is likely the same as the other 99% of sensors in the automotive world, 5 volts from a 10k ohm power source.

What it's normal operating voltage, current, or power dissipation are is not material because we are trying to do something that is not normal and will do something that 5 volts from a 10k ohm source will not, fix the strip!

P.S. After the whole part about an arc creating an ionized field with the impedance plummeting to essentially zero (absolutely correct though it is still in series with the 2.7k ohm of the rest of the circuit), and went on to talking about keeping enough resistance in series to prevent damage, was that you agreeing with me while studiously refusing to agree with me?

The power supply which is "current limited" and has no greater then a "20 mA short circuit current" IS THE RESISTANCE IN SERIES TO LIMIT CURRENT.

If you are suggesting bad things will happen if a current is steadily passed through the strip sensor circuit greater then it it normally is from the bikes computer..... Well thats probably 5 volt through 12,700 ohm,or 0.39 mA.....

I would suggest the following. Grab a broken strip, dealerships are littered with them so shouldn't be hard, and find out.

I suppose an EE would want to see a white paper. I have never bothered to try to find out who makes the strip BMW uses but it's probably Bosch, Siemens, or Marelli.

A physicist is going to want to explode it. I'm down with that, take video :)

A person that reads a lot, not sure, but look at an actual strip if you haven't already which I'm thinking you haven't. It,s a fat trace with a lot of surface area.

So lets have it, what that I have specifically stated do you disagree with? And if I get off my duff, grab a strip, pass 20 mA through it for an hour and it doesn't melt, do I get a prize?

What about if I run a loop of 22AWG wire into my fuel tank wrap some silicone tape around the individual conductors as they pass through the gas cap so they don't short there, shut the gas cap on the wires and pass 20 amps through the wire so that it heats white hot and burns in half. Do I get a prize if it doesn't blow up or ignite the tank?

I passed more then 20 mA continuously through the volvo strips which look about the same and we tried everything under the sun to ignite fuel vapors inside a sealed gas tank, so i'm cheating but would still like a prize and will put it all up on youtube for a good enough one :)
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Old 03-04-2012, 01:11 PM   #81
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Voltage DOES determine current - simple ohms law 1st year electrical.

On a given fixed resistance the higher the voltage the higher the current. Simple stuff, eventually you have fire.
and if you don't believe that, try the experiment on the bench for yourself.
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Old 03-04-2012, 01:21 PM   #82
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Quote:
Originally Posted by GS Addict View Post
Voltage DOES determine current - simple ohms law 1st year electrical.

On a given fixed resistance the higher the voltage the higher the current. Simple stuff, eventually you have fire.
and if you don't believe that, try the experiment on the bench for yourself.
Wait, I'm being greatly educated here and learning new fundamental principals of the universe! Wake George Ohm, Kirchoff and Einstein from the dead, they were all wrong!!!!!!!!!!!!!!!!!!!!!!!!!

Since "VOLTAGE DETERMINES CURRENT", as you say, WHY IN THE TIDDY HELL do you go on to state

Quote:
Originally Posted by GS Addict View Post
On a given fixed resistance the higher the voltage the higher the current
???????????????????

I mean, resistance does not matter oh wise ones. Thats what you are saying because "VOLTAGE DETERMINES CURRENT"

SO DO ENLIGHTEN ME. Why are you saying anything about resistance?

Look, I'm bored. Do what you like, or better yet, don't do anything but type.

I can not get this through some of your heads with a truck load of 2X4s.

I have stated REPEATEDLY to use a current LIMITED power source with a short circuit current no GREATER then 20 mA.

If you can't understand that or what it means. Your loss, bye bye.
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Old 03-04-2012, 02:07 PM   #83
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Quote:
Originally Posted by JoelWisman View Post
Wait, I'm being greatly educated here and learning new fundamental principals of the universe! Wake George Ohm, Kirchoff and Einstein from the dead, they were all wrong!!!!!!!!!!!!!!!!!!!!!!!!!

Since "VOLTAGE DETERMINES CURRENT", as you say, WHY IN THE TIDDY HELL do you go on to state



???????????????????

I mean, resistance does not matter oh wise ones. Thats what you are saying because "VOLTAGE DETERMINES CURRENT"

SO DO ENLIGHTEN ME. Why are you saying anything about resistance?

Look, I'm bored. Do what you like, or better yet, don't do anything but type.

I can not get this through some of your heads with a truck load of 2X4s.

I have stated REPEATEDLY to use a current LIMITED power source with a short circuit current no GREATER then 20 mA.

If you can't understand that or what it means. Your loss, bye bye.
Don't get your knickers in a knot!
Sorry I missed the key word 'limited'
I thinks others did too
Obviously in a current limited circuit once the setpoint has been reached, additional voltage will have no effect until component voltage/insulation ratings are exceeded.
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Old 03-04-2012, 02:22 PM   #84
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For other then those who understand ohms law wrongly.

http://www.allaboutcircuits.com/vol_1/chpt_2/1.html

Untitled

Notice, "voltage over resistance determines current"



http://en.wikipedia.org/wiki/Ohm's_law

Untitled

Notice, "voltage over resistance determines current"


http://youtu.be/-mHLvtGjum4



http://hyperphysics.phy-astr.gsu.edu...ic/ohmlaw.html

Untitled

Notice, "voltage over resistance determines current"


http://www.ndt-ed.org/EducationResou...ty/ohmslaw.htm

Untitled


Voltage does NOT determine current. Voltage over resistance determines current.

Some of you are as right as you can get, this IS as fundamental as it gets in the world of electricity, so the fact that some you don't understand it is concerning since you are making statements of false fact on a thread dealing with electricity :)

In my suggestion of what may fix a fuel strip sensor circuit, I have made the following suggestion worded a bunch of different ways to hopefully get the message across, apparently without luck.

Apply around 1000 volts from a CURRENT LIMITED power source with no greater then 20 mA OF SHORT CIRCUIT CURRENT and no greater then 40 pF of CAPACITANCE.

This means we are dealing with two resistance values in series. One resistance is the fuel strip, around 2,700 ohm when working properly. The other resistance that is in series is the CURRENT LIMITED POWER SUPPLY!

I have stated repeatedly that this power supply should be limited to NO GREATER THEN 20 mA of SHORT CIRCUIT CURRENT.

This means, regardless of the load impedance and regardless of the voltage NO GREATER THEN 20 mA of current will flow because by whatever means, though usually fixed resistance, the power supply will NOT supply more then 20 mA at ANY voltage.

It does not matter what the voltage is at the power supply because current will NOT be following voltage because the power supply is LIMITED and as it happens, current is VOLTAGE DIVIDED BY RESISTANCE.


I am not going to get to which of the 4 pins on the fuel strip connector are the heater and which are the sensor, where the connector is, how you pull the strip out of the tank if you desire to, How you force the bike to recalibrate without a MOSS, how tight the fuel pump collar goes to prevent fuel leakage, why capacitance matters, or anything else because I have people vigorously asserting facts about current and safety that are basic, simple, and wrong.

Thats too bad, but this isn't my job, I'm not getting paid, my knickers are already in a knot :P and i have to get back to things more fundamentally important to me, changing the driven pulley oil seal on my scooter or sticking a battery in my caponord since its removed for Lead/acid versus LiFePo4 battery trials, so I can ride across town and eat.

Now, somebody play with a darn fuel strip instead of a keyboard and perhaps I will pop back in some day, but I need to get busy on other things before my knickers permanently knot LOL
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Old 03-04-2012, 02:50 PM   #85
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Quote:
Originally Posted by JoelWisman View Post
For other then those who understand ohms law wrongly.

http://www.allaboutcircuits.com/vol_1/chpt_2/1.html



Notice, "voltage over resistance determines current"



http://en.wikipedia.org/wiki/Ohm's_law



Notice, "voltage over resistance determines current"


http://youtu.be/-mHLvtGjum4



http://hyperphysics.phy-astr.gsu.edu...ic/ohmlaw.html



Notice, "voltage over resistance determines current"


http://www.ndt-ed.org/EducationResou...ty/ohmslaw.htm




Voltage does NOT determine current. Voltage over resistance determines current.

Some of you are as right as you can get, this IS as fundamental as it gets in the world of electricity, so the fact that some you don't understand it is concerning since you are making statements of false fact on a thread dealing with electricity :)

In my suggestion of what may fix a fuel strip sensor circuit, I have made the following suggestion worded a bunch of different ways to hopefully get the message across, apparently without luck.

Apply around 1000 volts from a CURRENT LIMITED power source with no greater then 20 mA OF SHORT CIRCUIT CURRENT and no greater then 40 pF of CAPACITANCE.

This means we are dealing with two resistance values in series. One resistance is the fuel strip, around 2,700 ohm when working properly. The other resistance that is in series is the CURRENT LIMITED POWER SUPPLY!

I have stated repeatedly that this power supply should be limited to NO GREATER THEN 20 mA of SHORT CIRCUIT CURRENT.

This means, regardless of the load impedance and regardless of the voltage NO GREATER THEN 20 mA of current will flow because by whatever means, though usually fixed resistance, the power supply will NOT supply more then 20 mA at ANY voltage.

It does not matter what the voltage is at the power supply because current will NOT be following voltage because the power supply is LIMITED and as it happens, current is VOLTAGE DIVIDED BY RESISTANCE.


I am not going to get to which of the 4 pins on the fuel strip connector are the heater and which are the sensor, where the connector is, how you pull the strip out of the tank if you desire to, How you force the bike to recalibrate without a MOSS, how tight the fuel pump collar goes to prevent fuel leakage, why capacitance matters, or anything else because I have people vigorously asserting facts about current and safety that are basic, simple, and wrong.

Thats too bad, but this isn't my job, I'm not getting paid, my knickers are already in a knot :P and i have to get back to things more fundamentally important to me, changing the driven pulley oil seal on my scooter or sticking a battery in my caponord since its removed for Lead/acid versus LiFePo4 battery trials, so I can ride across town and eat.

Now, somebody play with a darn fuel strip instead of a keyboard and perhaps I will pop back in some day, but I need to get busy on other things before my knickers permanently knot LOL

I think what we all agree on is that theres a lot of wasted energy here!
But again I must disagree when you say "current will NOT be following voltage" ...It must..current is always the dependent factor. The Power supply will limit the current ONLY by lowering output voltage!!!! And your statement "the power supply will NOT supply more then 20 mA at ANY voltage" Is wrong simply by using the ohms law formulas you reproduced. Put your volt meter on your limited supply at 20mA and slowly reduce the resistance....and tell me the output voltage does not drop to limit current to 20mA!

All that aside, my only reason for commenting is to protect the the guy who posted the circuit that could potentially be unsafe. And when you questioned earlier about 1.08 watts, I will say yes, this is my main concern...If you know for sure that the strip will take that power for extended length of time, then there is no issue if the current is limited to the 20mA....UNLESS that 2700 ohms changes by a significant factor due to damage either by the arcing, or otherwise.

I'm prepared to end this discussion....
Thank you.
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Old 03-04-2012, 06:00 PM   #86
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Quote:
Originally Posted by JoelWisman View Post
This principal is why as a child you could make molotov cocktails with gasoline, light them and let them burn all day without a boom till you tossed them and broke the glass. This is why when you poured too much alcohol in your spud gun, it wouldn't light. This is also why when you drilled a hole near the bottom of a 55 gallon drum, dumped a bunch of trash, wood and gasoline in, and lit it from the hole, you lost all the hair on your arm and burning logs and trash rained down in a 75 foot diameter. The top of the vessel was open so you DID get a flammable, or in my case, explosive mixture.

Sounds like you had a lively and dangerous childhood (not that I didn't play with gas myself from time to time)!

Great posts by the way. Thanks.
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Old 03-05-2012, 03:16 AM   #87
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Originally Posted by GS Addict View Post
Don't get your knickers in a knot!
Sorry I missed the key word 'limited'
I thinks others did too
Obviously in a current limited circuit once the setpoint has been reached, additional voltage will have no effect until component voltage/insulation ratings are exceeded.
Sorry but i dont want you to mislead people... You cannot raise voltage without increasing current unless resistance is infinite and current is zero.
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Old 03-05-2012, 07:13 PM   #88
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Apparently you are not prepared to end this discussion lol

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Originally Posted by qman8 View Post
Sorry but i dont want you to mislead people... You cannot raise voltage without increasing current unless resistance is infinite and current is zero.
Really? How about something like a tetrode? I can think of 10 instances where resistance is NOT infinite yet increasing voltage does NOT raise current but in a region actually lowers it, and thats off the top of my head without resorting to google or text books.

Ohms law, correctly stated as " I = E over R " has plenty of exceptions. Stating that " current follows voltage " is not even right half the time because it implies a linear relationship that rarely exists.

A simple incandescent lightbulb is a good example. apply a small amount of voltage and X current will flow. Double the voltage and the current will practically double. Raise the voltage more and you will not get a linear raise in current.

This is because as the tungsten element heats its resistance raises.

At any given instance in time, ohms law is still correct with a incandescent lightbulb, as correctly stated E over R is still in effect, it's just that R is changing.

Place emphasis on E being the driver, as in " current follows voltage " and you are lead astray because resistance is raising with current.

" current follows voltage " is something taught to people as a law when they aren't going to need to know much about electricity. It is a way of simplifying a complex subject. It is also WRONG, but right enough it needn't be corrected if people are just going to be scratching the surface of electricity, much the same as the current flow model versus the electron flow law.

Trust me, I know this is frustrating. While taking automotive classes I was taught that current flowed from the positive battery terminal to the negative battery terminal. Ok, simple, that makes sense, next topic... Later when I entered electrical, engineering and chemistry classes I found out that the current model is wrong. Electrons ARE the charge carrier and since they are negatively charged, they flow from the negative battery terminal to the positive battery terminal.

This isn't just semantics, you need to know the direction of electron flow for many many things to make sense, including why the chemical processes that take place in a battery take place.

As a technician, I didn't need to know these things. Put a battery tester on a battery, if it fails, the battery is bad...

As a field service engineer or electrician responsible for half a million dollars of UPS batteries, I needed to know the truth, because I had to understand the chemistry side of batteries to be able to remove the plates of the battery and determine what had damaged them. The chemistry of batteries does not make sense with the current model, it does with the electron flow law.

So, with all of that, electrons flow from negative to positive, ohms law correctly stated as " I = E over R " has plenty of exceptions, and the statement " current follows voltage " has more holes then swiss cheese and is a way of thinking that does not impress the correct way of understanding the dynamic that " at a given instance in time, " current flow will USUALLY equal voltage divided by resistance ". There is NO prime driver to simplify matters, voltage, resistance, and current are all interrelated.

Are you quoting "ELI the ICE man" when you state "current follows voltage". Thats capacitance my friend.

Even that statement is a simplification. More often correct is that " current will USUALLY equal voltage divided by IMPEDANCE " Because nearly every circuit in the world contains at least SOME inductive and capacitive reactance.

Explicit in all of this is that increasing voltage will not ALWAYS increase current.

My suggestion to use a " 1000 VOLT CURRENT LIMITED" power source with a short circuit current " NO GREATER THEN 20 mA " is one such instance.

Here is a typical diagram of an active current limiting circuit.

Active current limiting

Raising the voltage at the source will NOT result in increasing the current to the load beyond the threshold for which the current limiting circuit is set for regardless of load resistance unless of course you raise the source voltage beyond the level at which the circuit is designed to function.

You could also use a passive current limit where the source has enough resistance that with a source of 1000 volts, even were the resistance of the load zero ohms, not more then 20 mA would flow.

I am not suggesting anyone build their own current limiting circuit or 1000 volt source, though it's fine if you want to, but supporting such an endeavor is beyond the scope of what I had intended.

As a practical matter, it is easy to test current limiting. Hook the output across a multimeters current sensing probes. If less then 21 mA flows, I am suggesting we are good. If more then 20 mA flows, I am suggesting we are not.

Lastly, qman8. It is frustrating to be wrong, especially when you are repeating what you have been taught by others you believed to be right. I suspect this is what is happening, and have been in the same place myself. We could take this subject all the way to the quantum physics level, but it would not further the goal of this thread and likely bore a lot of people. Google is your friend as are text books :) Try googling the string "current does not follow voltage" and you will get plenty of hits and some good information.


The subject at hand for this thread is: "using a 1000 volt power supply, current limited to a short circuit current of no greater then 20 mA, not more then 1.08 watts will be dissipated by a 2,700 ohm fuel strip sensing circuit and if the fuel strip should happen to have lower resistance or a passive current limiting source is used, even less energy will be dissipated.

I am not insisting a 20 mA current limited 1000 volts power supply will be ideal for fixing the BMW fuel strip sensor circuit. I used exactly those figures on a fuel strip Volvo was testing out and nothing melted. With BMW I used a power source that had a measured 1 mA short circuit current and a measured 40 pF of capacitance.

IT IS JUST MY BEST GUESS that up to 20 mA of short circuit current from a 1000 volt power supply with no greater then 40 pF will be alright to hit BMW fuel strips with.. Thats just my best guess, but at least I have a good understanding of electricity

If you disagree that 1.08 watts can safely be dissipated by the fuel strip, this can be empirically proven using a fuel strip :)

If you disagree that a current limited 1000 volt power source with a short circuit current no greater then 20 mA will dissipate no greater then 1.08 watts into the fuel strip sensing circuit with a resistance of 2,700 ohms, at the least, show me some math so I can debate something other then your vigorous assertions :)
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JoelWisman screwed with this post 03-05-2012 at 07:20 PM
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Old 03-05-2012, 08:18 PM   #89
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[QUOTE=JoelWisman;18146413]Apparently you are not prepared to end this discussion lol

Thanks for jumping in so quick.
I have many other things to do like running my business and cannot check in and respond every 5 minutes.
This site does not pay my bills.
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Old 03-05-2012, 09:36 PM   #90
JoelWisman
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[QUOTE=GS Addict;18146985]
Quote:
Originally Posted by JoelWisman View Post
Apparently you are not prepared to end this discussion lol

Thanks for jumping in so quick.
I have many other things to do like running my business and cannot check in and respond every 5 minutes.
This site does not pay my bills.
Yeah yeah rub it in lol, You have to work and all I have to do is sit at home and wait for my next job to materialize :P

This sight isn't paying my bills either, Are you looking for employees? I was a licensed industrial electrician in Oregon a few centuries ago :)
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