John, In your first solution, I think you have your circuit backwards. If I read the issue correctly, the new sender has a resistance of 0 - 30 ohms and what the gauge expects is 0 - 90 ohms. I will be honest though, I don't have a clue of how a gas gauge and sender work.
-joe On Sun, Nov 17, 2013 at 9:19 PM, John Rehwinkel <[email protected]> wrote: > > 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. > -- 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/CAE%2BVk6NdExiFWBYMkgu6jsHWJGPS%2B5oSp%3DAc3B9NQf7o9%2BYS9A%40mail.gmail.com. For more options, visit https://groups.google.com/groups/opt_out.
