Thread: run 9v appliance from 12v, what resistor?

you could put in a resistor, but I'd go for a voltage regulator. if you have an old lighter socket mobile charger, this can be a good start as it will already have the required circuit, you just need to alter the components values to give a 9v output instead of whatever the designed voltage is.

alternatively, put a rechargeable 9v battery in the multimeter and add a 12v feed to charge the battery for the specified time before removal (should be close enough to work well).

this is a good example: Car NiCad Charger

the more I think about it though, you can probably buy a cheap, multi voltage output lighter socket charger from tandy or whatever for less than the parts to make one...

I looked at the multi-voltage lighter output thing in Tandy $28ea.
The multimeter was $10

I was just hoping for the quick cheap n nasty method

This may or may not be feasible.

Most meters are not designed to be driven by the power supply they are testing.

Without the circuit diagram in front of me, I would be reluctant. If the meter probes are connected the wrong way round (normally that wouldn't be a problem) it could zap the meter.

Originally Posted by p38arover

This may or may not be feasible.

Most meters are not designed to be driven by the power supply they are testing.

I can power them from different sources, (not the one on test) but they would still be 12v.
The probes would be fixed position, (correct polarity).

If this is a tough technical issue I'll just shove it in the too hard basket
I thought I could just solder in a XXX resistor and that would be that, oh well

Originally Posted by HangOver

I thought I could just solder in a XXX resistor and that would be that, oh well

No, the resistor value is dependent on ther current drawn. You'd need to measure that then work out the resistance.

What you trying to do would help? use it as a permanent volt/amp gauge?

Simply, you cant. The meter will draw varying current, meaning a resistor will change in voltage. you NEED a voltage regulator. Albeit a single chip, you will need to get the data sheet for the chip you buy and follow their circuit diagrams.

cheers!

Originally Posted by beforethevision

Simply, you cant. The meter will draw varying current, meaning a resistor will change in voltage. you NEED a voltage regulator. Albeit a single chip, you will need to get the data sheet for the chip you buy and follow their circuit diagrams.

cheers!

Yes... and no.

Will a digital meter vary much in current draw? If it does, then yes, a regulator will be required. If it varies very little, one could get away with a resistor.