Circuit to match fuel sender to fuel guage

[3dOptics]

12v
Sender: 0 ohms Empty, 70 ohms Full
Gauge: 0 ohms Empty, 280 ohms Full

 

[MikeMl]

Use the forum search feature to research this topic; it has come up many times before....

 

[3dOptics]

I built the circuit in the following thread, but MultiSim Analog Devices edition is showing 1.17 GigaOhm to 1.189 GigaOhm.

www.electro-tech-online.com/threads/raising-a-resistnace-4x-through-a-range.33294/

 

[MikeMl]

You need to determine how the original gauge is wired. Is it a simple DC ammeter circuit, or is it one of the heated-wire gauges?

 

[3dOptics]

You need to determine how the original gauge is wired. Is it a simple DC ammeter circuit, or is it one of the heated-wire gauges?

The gauge has three terminals: +12v, Sender, and GND.

 

[MikeMl]

The gauge has three terminals: +12v, Sender, and GND.

Then this is hard to do without knowing more about the gauge.

To do this, with the gauge powered, I would first measure the open-circuit voltage between the sender terminal on the gauge and ground using a DMM.

Second, I would put my DMM in DCmA mode, and measure the current that flows from the sender terminal on the gauge to ground.

Third, I would connect a 70Ω resistance (68Ω+2.2Ω, or a pot set to read 70Ω) from the sender terminal on the gauge to ground, and measure the voltage between the sender terminal on the gauge and ground.

With these numbers in hand, we stand a chance of developing a converter circuit to make the 70Ω sender act like a 280Ω sender. Without these measurements, you don't have a chance...

 

[dr pepper]

Heated wire guages tend to be 2 wire, 3 wire could be an air core, there are 2 diffrent windings, the guage has a moving magnet that positions to the magnetic vector of the energising coil and the measurement coil in series with the sender.

 

[3dOptics]

To do this, with the gauge powered, I would first measure the open-circuit voltage between the sender terminal on the gauge and ground using a DMM.

5.58v

Second, I would put my DMM in DCmA mode, and measure the current that flows from the sender terminal on the gauge to ground.

2.02 amps -> 0.01 amp

Third, I would connect a 70Ω resistance (68Ω+2.2Ω, or a pot set to read 70Ω) from the sender terminal on the gauge to ground, and measure the voltage between the sender terminal on the gauge and ground.

3.0v

 

[MikeMl]

5.58v

OK

2.02 amps -> 0.01 amp

I don't understand this. I expected a steady reading. Is it pulsing? Is the meter jumping between two stable readings?

3.0v

OK

 

[3dOptics]

I don't understand this. I expected a steady reading. Is it pulsing? Is the meter jumping between two stable readings?

At first the amps are 2.02 and then after 20 seconds the reading is a constant 0.01 amp.

 

[MikeMl]

At first the amps are 2.02 and then after 20 seconds the reading is a constant 0.01 amp.

Ok, by using a more sensitive setting on the DCmA scale of the DMM, read the current to more significant figures after the initial transient settles out...

e.g., change the scale so that you read 10.3mA, or 9.8mA..., or whatever...
'
ps: if you are worried that the 2A surge will damage your meter when it is set to a more sensitive scale to read the steady-state current, initially short the sender terminal on the gauge to ground using a clip lead. Then place the meter probes, wait a bit, and then remove one end of the clip lead. That will protect the meter, but allow you to measure the steady-state current more accurately.