# chinese car voltmeter calibration

Discussion in 'General Electronics Discussion' started by dorf, Aug 28, 2015.

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Apr 7, 2015
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The calibration is probably determined by the accuracy of the TL431.

Since it is set for the minimum voltage the TL43 can be set to, if your meter reads too high, all you can probably do is find another TL431 with a slightly lower voltage.

If it reads too low, you can use a pair of resistors (or a resistor and a trimmer) to change the voltage.

I might have this exactly the wrong way around -- I've not been up for long.

3. ### Colin Mitchell

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Aug 31, 2014
"I might have this exactly the wrong way around -- I've not been up for long."

Yes, you have it around the wrong way.

The adjustment is the 1% resistors.

Put 10k to 100k across either resistor and see what happens.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
yeah, Colin is right. I missed that it is reporting the power supply voltage.

What I said is still correct, but changing the voltage divider is way easier.

5. ### Colin Mitchell

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Aug 31, 2014
"What I said is still correct, "

I don't think you are correct because if the circuit is as shown, the regulator will drop out of regulation when the supply is 5v.
I think the chip has an internal reference.

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
No, the chip doesn't appear to have an internal voltage reference.

Without doing more calculations I can't confirm that the minimum voltage is 5V, but it seems reasonable. It equates to about 10mA which is comparable to the typical sink current of a single output.