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Old 06-07-2011, 07:55 PM   #311 (permalink)
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The sender has a resistance of between about 1.5K stone cold & about 200 ohms at 100C so initially I used a 1K variable resistor across the sender wire & ground. Had the scope & the DMM connected across the sender & ground.
Has a similar pulse to the fuel gauge but the amplitude is much smaller. Also noticed that if I took the resistance too low, the amplitude fell to almost zero & the gauge suddenly dropped from max to minimum.
I have not tried a divider yet. Could do one with a 3K & a 33R. That would get me down to 15mV.

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Old 06-07-2011, 08:04 PM   #312 (permalink)
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I don't understand what you did here. I thought the sender was just a variable resistor, how come it changes so much with temp?
Also, your DMM readings are irrelevant with such a short pulse width. 15 mV dc is not going to do anything.
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Old 06-07-2011, 08:16 PM   #313 (permalink)
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The sender is a variable resistor & that is what it measures.
I had it in water in a saucepan & started heating it up.
Remember though, the gauge only shows 50C to 120C.
The gauge is much more sensitive to resistance changes at the top end of the scale.
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Old 06-07-2011, 08:23 PM   #314 (permalink)
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Ah, I get you now. You are talking about the temperature gauge! I thought you were still on the fuel gauge, I've been away for a week and haven't read all the posts here.
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Old 06-07-2011, 09:36 PM   #315 (permalink)
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Yeh, the fuel gauge should be sorted. Just have to get a couple of bits.

What I am currently attempting to do is use the existing temp gauge as a battery amp meter.
I just did a couple of careful measurements with the scope & the dmm.
Minimum gauge gives 556 ohms across the variable resistor. The scope showed the pulse amplitude of 2.54V.
Max gauge gives 33.5 ohms across the var resistor & an amplitude of 0.54V
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Old 06-07-2011, 09:48 PM   #316 (permalink)
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Interesting, sounds like the gauge is in series with the sender. What sort of pulses do you get when the sender is disconnected? (ie open circuit).
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Old 06-07-2011, 10:04 PM   #317 (permalink)
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The pulse width straight across the gauge wire to ground is the same, only the amplitude is a little bit higher at 3.4V.
It is similar to the fuel gauge except the voltage is lower & the gauge does not appear to be linear.
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Old 06-07-2011, 10:18 PM   #318 (permalink)
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Well, it certainly looks like the current through the gauge is what controls it. ie low sender resistance = more current during the pulse = high gauge reading. But it is very non-linear. The 2.5V pulse across 556 R is only 4.5 mA while .54 across 33.5 is 16 mA.
I'd be inclined to connect my variable power supply direct to the sender wire (sender disconnected) and see what gauge reading corresponds to what voltage, plot a graph and then you can maybe work out what would drive an op-amp connected instead of the sender.
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Old 06-07-2011, 10:28 PM   #319 (permalink)
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My problem is that my variable power supply does not go lower than 1.34V.
I tried a 3K-33R divider with a 68R ohm on the output (cause thats what worked with the fuel gauge). The gauge will go immediately to about half & stay there. If I bypass the 68R the gauge goes to min & stays there.
Not having much luck with a direct voltage drive yet.
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Old 06-08-2011, 09:26 AM   #320 (permalink)
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Ian,
My guess would be that if you connected a supply capable of going down to 0V thru a ~33ohm series resistor to sender wire, you'd find 0V=needle @120C and slightly less than 2V=needle @50C. (The 1.5V battery in series with the supply as mentioned earlier does work. I've used it before).
If you wanted a larger usable voltage swing to control this, then connect the sender to a 33ohm directly to ground (this becomes the ballast resistor). Then connect the power supply to the sender wire as well, but through a larger value series resistor. You could play around using a 1K pot to determine what this value should be. Key first is power supply that can get to 0V

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