Variable resister/fuel level sensor - fuel gauge - change resistance range

Alec_t

Joined Sep 17, 2013
13,224
Hi garanito. Welcome to AAC!
The circuit in my posts above is for the situation where the actual sender resistance range is lower than the required emulated range. Your problem is the reverse, requiring a different circuit. The one below should do what you want and has been tailored to suit your sender :-
SenderAdapter.jpg
D1 protects agains reverse polarity. D2-4 and the capacitors protect against voltage transients.
I'm also attaching the LTspice simulation asc file.
 

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garanito

Joined Dec 2, 2021
2
Hi garanito. Welcome to AAC!
The circuit in my posts above is for the situation where the actual sender resistance range is lower than the required emulated range. Your problem is the reverse, requiring a different circuit. The one below should do what you want and has been tailored to suit your sender :-
View attachment 254136
D1 protects agains reverse polarity. D2-4 and the capacitors protect against voltage transients.
I'm also attaching the LTspice simulation asc file.
Thanks Alec_t! I will get the components and give you the feedback! Thank you!
 

Thread Starter

nicktruman

Joined Feb 4, 2019
73
It could be made on a small piece of strip-board (e.g. Veroboard). The pinout details of the opamp are in its datasheet. Through-hole versions of the IC and the other components would be preferable if you have little circuit assembly experience.
Hi Alec_t
I have had to buy a bigger fuel tank that has come with a different sender. Can you tell me what I need to change for the new sender please?
My fuel gauge (built into my dash cluster) expects the fuel level sender to have a resistance range from 50 to 350 ohms; however my new fuel level sensor has a resistance range from 9 empty to 179.5 ohm full
Is there a calculator that I can work these values out my self?
Regards
Nick
 

Alec_t

Joined Sep 17, 2013
13,224
Can you tell me what I need to change for the new sender please?
In my post#7 circuit, make R3=39Ω and R2=140kΩ. The fact that the new sender minimum resistance is 9Ω rather than zero introduces non-linearity in the conversion factor of the circuit. The suggested resistance value changes result in the gauge reading erring slightly on the low side for low sensor resistances.
Bear in mind the caveat in that post regarding the gauge type.
Because of that non-zero minimum I can't offer a simple formula for choosing resistor values.
 
Maybe I'm missing the point here, but it sounds like your fuel gauge is more sensitive than the one intended for a 0 to 100 ohm sender which is good news. Presumably the gauge is displaying a measure of the current through it so if you put a resistor in parallel with the gauge some of the current through the sensor will not go through the gauge. Another resistor in series with the sender should complete the adjustment for both full and empty.

Just spotted that you have a new sender with range 9 to 179.5 Ohms. Try a 19.5 ohm resistor in series with the sender which will give a range 28.5 to 199 which is the same ratio as 50 to 350. An 18 or 20 ohm resistor should be close enough. From the resistance of the gauge you can work out (ohms law) the resistance you need to put in parallel with it so less current flows through the gauge.
 
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Thread Starter

nicktruman

Joined Feb 4, 2019
73
In my post#7 circuit, make R3=39Ω and R2=140kΩ. The fact that the new sender minimum resistance is 9Ω rather than zero introduces non-linearity in the conversion factor of the circuit. The suggested resistance value changes result in the gauge reading erring slightly on the low side for low sensor resistances.
Bear in mind the caveat in that post regarding the gauge type.
Because of that non-zero minimum I can't offer a simple formula for choosing resistor values.
Thank you Alec, I’ve ordered the resistors, hopefully be here soon. The adapter works perfectly with my cluster, so i guess the Audi gauge is nothing special
I measured the resistance with the tank empty and full, those were the readings i got. I guess if the sender is on a bench it would zero at empty
I’ll let you know
 
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