> Electronic design is not my strong suit so I thought I ask if someone here > could offer some advice.. I'm not even sure if this is possible. > > I need to make a fuel tank sender and it's gauge compatible. The gauge is > expecting to see a sender (basically a variable resistor) that has a value 0 > to 90 ohms. The current sender in the tank has a value of 0 to 30 ohms. The > original gauge, changed long ago is NLA and while an appropriate sender is > available, changing it is going to be an issue. > > I was hoping for a simple circuit that would accomplish this. It would have > to be slightly adjustable and not affected by the supply voltage that vary by > several volts depending on the battery load and charge condition.
There are a few ways to approach this. I'll address them in order of increasing complexity. The easiest approach is to make the existing gauge look like the gauge the sender expects. To do this, parallel it with a resistor. In effect, you want a resistor that's half the resistance of the existing gauge, so it in parallel with the existing gauge, will be 1/3 the resistance. Then the (approximately 3x) current from the sender will be divided unevenly, with about 1/3 the current (equivalent to a sender with 3x the resistance) going through the gauge. Making part of this resistance adjustable will give you the adjustment you're after. Note that this is the approach I would use if it were mine. Note that there might be some odd effects on linearity, but fuel gauges aren't famous for linearity anyway (I had a VW where full to half barely moved the pointer, but it had lots of resolution near empty, where it mattered more). The next most complicated approach is a lashup known as a "current mirror". It uses transistors to make the current through one transistor control the current through another (or several more). By playing with transistor betas and paralleling transistors, you can make it assymetrical, so the transistor driving the gauge tracks the one driven by the sender proportionally. After that, you get to relay type circuits, where the sender operates something like an optocoupler (ideally one of the linearized kind), which then drives an amplifier that in turn drives the gauge. This can also be done mechanically, with the sender driving a galvanometer hooked to a potentiometer that acts as a surrogate sender, with the desired resistance range. After that, you get to microcontroller-based arrangements, where the sender drives an analogue-to-digital converter, and then the microprocessor uses a lookup table to drive a digital-to-analogue converter which then drives a transistor controlling the gauge (PWM works just fine for automotive gauges, as they don't respond quickly). Once you have a microcontroller and a lookup table, you could also drive a nixie display or a bargraph indicator if it'll fit (just to offer something on-topic). A side note - some cars (like my old Volvo) actually have a voltage regulator in the dash so the gauges can run on a more stable voltage supply (I think it was 8 volts). - John -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/D0DBC2B4-F25D-414E-A652-9D2B52D1C688%40mac.com. For more options, visit https://groups.google.com/groups/opt_out.
