120V accurate current source - how?

I'm seeking opinions on the best way to generate an accurate current at 120V AC. I need to calibrate a power meter design, and it calls for the ability to accurately generate specific currents between 1A and 5A. I've considered the use of power resistors, but 5A would require a 600W resistor, which, while not impossible to find, is a bit drastic. I also considered six 100W bulbs, but since it's a power meter I need a degree of accuracy greater than the tolerance of a light bulb.

Any suggestions on other possibilities I might explore, or old equipment that I might be able to pick up from eBay?

Thanks all!

Power meters generally have 2 inputs, one to measure voltage and one to measure current. Calibrating them is usually done with a variable 120V voltage into the high impedance voltage input and a variable current input at a low AC voltage into the low impedance - close to zero - current input. It is not necessary to have the current input come from a 120 volt source. Commericial current sources suitable for calibration are available and are expensive. Ideally the phase angle between the voltage and current sources is also adjustable to test the effect of inductive or capacitive loads.

Dan

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1:What's your definition of "accurate"???? Just how accurate?

2:What's your definition of "power"??? If you expect to measure RMS power but calibrate it using some kind of (non-true RMS) voltage measurement and a resistor...see #1.

3: If you expect to get a pure sine wave out of the wall plug...see #1.

4: Do you expect your measurement bo be accurate with reactive loads...over what PF range? see #1.

5: How about non-sinusoidal currents? Range of peak to average ratio? See #1.

The theory is trivial. Acutally (accurately) measuring power over a range of non-ideal conditions is not so easy.

mike

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I needed to do something similar without spending any money. Used the workshop Variac (every workshop should have a Variac) and 2kW electric fanheater (every workshop should ... ). Monitored the current with a DVM. regards john

Incorporate a 10:1 shunt in your power meter design. Calibrate the circuit at at 12 volts and 1/2 watts. For 120 volt measurements, use the shunt.

Ed

Ecellent idea. If the OP doesn't have a DVM for the rated current, he can always use a known small resistance shunt and measure the drop to determine current.

Also, note that even without the variac there are devious ways to handle high power in lower-power resistors. First off, you can apply the power just for the second it takes to get a reading, then give them a long cool-down. Better yet, immerse the resistors in a paint can full of oil to dissipate the heat. This trick was used back in the '60s by hams needing dummy loads for transmitters. There are of course limits to just how far you can go with this... don't use a 1/4 watt resistor and expect it to work. But if you can get something in the 10s of watts, I think you will be fine.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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