Electrical question please help :)

[Chrishuff1]

I'm putting a new gauge in my van and its 0 Ohms emtpy and 90 Ohms full, but I need it to drop down to 75 Ohms when full, is there a way to do this?

 

[ArmenK]

put it in parallel with a 450 Ohm resistor

 

[juancferrer]

can't do it...your van will explode!!!

 

[dighn]

putting a resistor in parallel will work but it will change the way the meter moves a bit . but since it's just a fuel gage i doubt you need high accuracy anyway

 

[Chrishuff1]

Explanations would be nice

 

[ArmenK]

Originally posted by: Chrishuff1
Explanations would be nice

1/x + 1/90 = 1/75

solve for x

that is the equation for two resistors (90 ohms and x ohms) in parallel

 

[vshah]

what armenk said. Req will equal 75ohms

 

[TuxDave]

It's all about using the water pipe example. Putting an additional water pipe in parallel to an existing water pipe allows more water to flow, hence less resistance.

 

[olds]

Get the right gauge and or sending unit.

 

[PowerEngineer]

Originally posted by: ArmenK

Originally posted by: Chrishuff1
Explanations would be nice

1/x + 1/90 = 1/75

solve for x

that is the equation for two resistors (90 ohms and x ohms) in parallel

You are right about what it would take to come up with an equivalent resistance of 75 ohms (when the gauge is at 90 ohms), but I don't think that will really fix the gauge reading which I assume is actually related to the current flowing through it (and therefore unchanged by any resistance in parallel with it).

If you needed to increase the resistance, then adding a resistor in series might work. But since it's the other way around, you may want to invest in a new gauge like oldsmoboat suggests.

 

[element]

Originally posted by: PowerEngineer

Originally posted by: ArmenK

Originally posted by: Chrishuff1
Explanations would be nice

1/x + 1/90 = 1/75

solve for x

that is the equation for two resistors (90 ohms and x ohms) in parallel

You are right about what it would take to come up with an equivalent resistance of 75 ohms (when the gauge is at 90 ohms), but I don't think that will really fix the gauge reading which I assume is actually related to the current flowing through it (and therefore unchanged by any resistance in parallel with it).

If you needed to increase the resistance, then adding a resistor in series might work. But since it's the other way around, you may want to invest in a new gauge like oldsmoboat suggests.

Putting a resistor in parallel passes some current through the resistor and the rest through the meter. If the resistance is low enough (like a short) the meter won't even budge.

This works if the sending unit is sending a voltage to the meter to move it. The OP really ought to find out what voltage is sent to the meter for full scale and empty.

Power Engineer, how you can say that the meter is unaffected by what resistance is in parallel to it is beyond belief. What if you short it? Is it still going to be unaffected?

 

[Chrishuff1]

I guess that I could just use it as is, the only thing is, when it says full, it will be full but it will be empty at 1/4 instead of the E so its not that big of a deal, but it'd be nice if I could switch some things up.

 

[element]

Just put the resistor in parallel and let us know how it goes.

 

[PowerEngineer]

Originally posted by: element

Power Engineer, how you can say that the meter is unaffected by what resistance is in parallel to it is beyond belief. What if you short it? Is it still going to be unaffected?

Beyond belief? It shouldn't be...

We're all making some assumptions here, and maybe it'd help if I stated mine. I'm assuming that we can treat the van's 12v power system as an "ideal" voltage source (i.e. no internal resistance with no limit on current) and that the wires have negligible resistance. These are common assumptions for simple circuit problems.

If you accept these assumptions, then there will be 12 volts across both the parallel resistor and the gauge -- no matter what the value of the parallel resistor is. And this would mean that the current through the gauge would not be changed by the existance of the parallel resistor.

Of course these assumptions only approximate the van's electrical system, and would require the battery to supply infinite current if a short circuit is applied. The battery's internal resistance and the resistance of the wires become important in that case, and can't be assumed away. But for less extreme impedances, I'll stand by my assumptions.


Which brings up a point... There must be some sort of resistor in series with the gauge to limit current to some maximum value (maybe listed on the gauge) when the gauge itself is at zero ohms. You might be able to make the 90 ohm gauge work by putting a resistor in parallel with this other series resistor. I'm thinking that the value of the parallel resistor should be 5 times that of whatever the series resistor is.

Still the best idea is to get the right gauge.



P.S. -- My guess is that the gauge will read just a tad less than full when full...but it wouldn't be beyond belief if I were wrong!

 

[olds]

Originally posted by: PowerEngineer
Still the best idea is to get the right gauge.

I concur

 

[dighn]

isn't the gage a fuel level sensor that i assume outputs the fuel level as a resistance? or is it something else?

<-- dont know much about cars

 

[element]

Originally posted by: dighn
isn't the gage a fuel level sensor that i assume outputs the fuel level as a resistance? or is it something else?

<-- dont know much about cars

oh by gauge I thought he meant the gauge in the dashboard in the form of a meter. the sensor in the tank is just that, a sensor, not a gauge.

 

[Krakerjak]

I don't see how a resistor will help you if the guage reads 1/4 when the tank is empty.
You say the guage registers 0 ohms on the E but when your tank is empty the guage shows
1/4 full, something does not seem consistent.

 

[dighn]

Originally posted by: Krakerjak
I don't see how a resistor will help you if the guage reads 1/4 when the tank is empty.
You say the guage registers 0 ohms on the E but when your tank is empty the guage shows
1/4 full, something does not seem consistent.

when the gaue is at 0 ohm the equivalent parralel resistance is still 0

it's just that the resistance vs level curve gets less linear as the fuel level gets higher