Re: Fual Gauge Resistance

Jim Warren wrote:
Alan Smith wrote:
Jim Warren wrote:
Alan Smith wrote:
Jim Warren wrote:
Provided you use a 10V voltage stabiliser, I reckon 50 ohms will just get the gauge to Full, and 1000 ohms will be just about empty. If you bought a handful of 10 ohm ones, you should be able to test this, but assuming your 80 ohms reading 3/4 full is when using a 12V battery, it is a working assumption that the full mark is reached when 200 ma are running through the meter. So that is what I based my calculations on.

You might want to make the 1000 ohm one a bit bigger so that when the gauge says empty there will be a little drop of petrol left in reserve. You could try 1600 or 1800 ohms.

Don't forget that the 200ma current running through the 50 ohm resistor will make it warm. To be on the safe side, get a 2W resistor for that one.

Jim
Once again thanks Jim, got a few busy days so might not get back onto it till after the weekend, but will call round at either RS or Maplins to pick some resistors up. If you recommend 2W for the 50 ohm, what do you recommend for the others.

Alan...
You didn't say how many steps you were planning to include from full to empty, so I have done some calculations based on steps of 1/8 of a tankful.

The other thing I would advise is that you get your 10V stabiliser and do a couple of tests with the resistors you have got to see whether the assumptions I started with (eg 200ma gives Full) are actually right before you buy higher wattage resistors. You can get away with lower wattage ones for testing but don't forget that a fuel gauge is powered all the time the ignition is on so there is a gradual heat build up: don't under-scale the ones you are going to use permanently. You can over-size the wattage with no ill effects but undersized ones are going to fail eventually.

So wire up the voltage stabiliser and the gauge and check that 50 ohms is FULL and 80 ohms is about 5/8 full and 100 ohms is about half full (the calculations are on a logarithmic scale not a linear one). If you get somewhere near those results (they wont be exact - most resistors are nominal value plus or minus 20%), then you can trust the rest of my calculations. If you don't, then tell me what you do get and I will rework my figures.

If you get somewhere near my rough results, then based on the resistor values that are actually sold http://www.radio-electronics.com/info/data/resistor/resistor_standard_values.php

You are aiming for these values (the wattage shown is for the last one in the chain):
FULL 50 ohms 2W
7/8 50+6.8 ohms 2W
3/4 50+6.8+10 ohms 2W
5/8 50+6.8+10+12 ohms 2W
1/2 50+6.8+10+12+22 ohms 1W
3/8 50+6.8+10+12+22+33 ohms 1W
1/4 50+6.8+10+12+22+33+68 ohms 1/2W
1/8 50+6.8+10+12+22+33+68+200 ohms 1/2W
Empty 50+6.8+10+12+22+33+68+200+1000 ohms 1/2W


Jim

Thanks again Jim, just waiting for the stabiliser to arrive, might struggle to get so many resistors in the sender, it's only a 15mm tube, already has the reeds in there, was hoping for just 2 per set. As you appear to be knowledgeable in this area if I wanted a light to come on at say 1/8 full is that possible?

Alan...

You only need the reeds in the tube. You can run wires out from the reeds to a little box conveniently placed which holds the resistors, and you will probably have to do that because that many 2W resistors might not fit in a 15mm tube.[1]

As for the light coming on, it is possible, but you will need more components. I will have a think about it, but which components will depend on your reeds. Are yours going to be normally open and closed by the magnet, or normally closed and opened by the magnet? Or have you gone for the posh double pole type?

Jim

[1] After what Dave Plowman said about Maplins further down this thread, I remembered I had a Maplins catalogue at home and I looked up their resistor range. The bad news is that their 2W resistor range doesn't go below 100 ohms. So from Maplins you are going to be getting 3W resistors which are physically a bit bigger (but go all the way down to 0.1 ohms). The 3W range are 33p each (inc VAT), so not dreadfully expensive in the grand scheme of things. I haven't got a Radio Spares catalogue so I can't do any comparisons.

Thanks again Jim, of course you're right about not having the resistors in the tube, it was just me not doing the lateral thinking, I've put them in for testing. Currently & this is open to change if a better idea comes along, the reeds are normally open & are closed by the magnet, they again are currently are in pairs which are offset to each other as I found that a single reed goes closed/open/closed as a magnet passes over them. However this does change if you orientate the magnet the other way, you then got just a single close but for a very short duration.

Alan...
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Relevant Pages

  • Re: Fual Gauge Resistance
    ... I'm making my own fuel sender for my kitcar, the system works in principle, using a float, reed switches, magnets & resistors. ... My only problem is the calibration, I'm using a Smiths fuel gauge out of a Triumph but I do not know what the range of the gauge is, how do I find out? ... Once you have set up your stabiliser, find out which way your dial works - it might read Full with an open circuit or it might read Empty. ... your tank unit showing 80 Ohms when empty doesn't tell me whether when it is full it goes to 0 Ohms or to a number higher than 80. ...
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  • Re: Fual Gauge Resistance
    ... I'm making my own fuel sender for my kitcar, the system works in principle, using a float, reed switches, magnets & resistors. ... My only problem is the calibration, I'm using a Smiths fuel gauge out of a Triumph but I do not know what the range of the gauge is, how do I find out? ... Once you have set up your stabiliser, find out which way your dial works - it might read Full with an open circuit or it might read Empty. ... your tank unit showing 80 Ohms when empty doesn't tell me whether when it is full it goes to 0 Ohms or to a number higher than 80. ...
    (uk.rec.cars.classic)
  • Re: Fual Gauge Resistance
    ... I'm making my own fuel sender for my kitcar, the system works in principle, using a float, reed switches, magnets & resistors. ... My only problem is the calibration, I'm using a Smiths fuel gauge out of a Triumph but I do not know what the range of the gauge is, how do I find out? ... Once you have set up your stabiliser, find out which way your dial works - it might read Full with an open circuit or it might read Empty. ... your tank unit showing 80 Ohms when empty doesn't tell me whether when it is full it goes to 0 Ohms or to a number higher than 80. ...
    (uk.rec.cars.classic)
  • Re: Fual Gauge Resistance
    ... If you bought a handful of 10 ohm ones, you should be able to test this, but assuming your 80 ohms reading 3/4 full is when using a 12V battery, it is a working assumption that the full mark is reached when 200 ma are running through the meter. ... You might want to make the 1000 ohm one a bit bigger so that when the gauge says empty there will be a little drop of petrol left in reserve. ... Once again thanks Jim, got a few busy days so might not get back onto it till after the weekend, but will call round at either RS or Maplins to pick some resistors up. ...
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