Current source?

[rexxar]

I bought a multimeter kit, thinking it'd be fun to build and anything would be better than what I already had. It didn't occur to me that I'd have to calibrate the thing, and now I'm stuck.

I figured I'd buy a DMM Check to get the resistance and volts ranges "close enough". While they have versions with current sources, they're more expensive, and are only in the milliamps range. I need to calibrate the 20A range on this meter, and I don't have anything like a precision current source.

I was thinking that once I got resistance and volts in the general area of calibrated, I could get one of those huge power resistors, stick it across my power supply, and apply Ohm's law to figure out how much current should be passing through.

Is that sensible, or are there ICs or something that will do a better job at low cost?

[Simon]

No, you can't do that. Your meter will have it's own resistance causing burden voltage and so many things to account for. Not exactly precision. Have a look at Daves videos on his "dummy load" you need to make something that works on the same principle with precision resistors.

[ve7xen]

You can probably get in the ballpark if you measure the current shunt, measure your power resistor, add the two and then use Ohm's law. Power resistors don't tend to have very good temperature coefficients though, so do the measurement quickly before it has a chance to heat up.

If you want more precision you need a reference and a feedback loop of some sort.

[sub]

If you have a working ohms range, perhaps you could derive the high-current calibration from the resistance of the shunt+connectors.

[Zero999]

You could use another volte meter to measure the voltage across the resistor and calculate the current.

[Marco]

Is that sensible, or are there ICs or something that will do a better job at low cost?

A high current version of Dave's constant current load is relatively cheap component wise. A 20 Ampere shunt is a couple of bucks, a low offset voltage opamp is a couple of bucks (Dave's LM324 is a bit dodgy, maybe an AD8638 or something) a 100 Watt MOSFET is a couple of bucks, a MOSFET driver less than a buck (zetec totem pole) ... so less than 10 bucks of components.