Electronic Design

RAP's Multirange Wattmeter

Although the basic wattmeter covers 1.501 kW, some people might be interested in other ranges. So, we will show here a multirange version, with full calibration (see the detailed schematic). The basic explanation is also applicable to this version with multiple ranges.

We'll start with the 1.0-m‡ copper shunt shown in the April 29 column for the 1.501-kW range (www.elecdesign.com/Articles/Index.cfm?ArticleID=17028&Extension=pdf). Then, I made a smaller shunt, two strands paralleled of about 12 in. of 22-gauge wire, to make up the 9.0-m‡ shunt. This gives 150.1-W full scale, with 0.1 W of resolution.

If you want a 15.01-W range with 0.01 W of resolution, you can add a 90-m‡ shunt made of 5 ft of #24 wire. For a 1.501-W range, I used a hank of 50 ft of #24 wire. Extra switch positions may be needed. These shunts are all added to the basic Wattmeter circuit. Each resistor should be trimmed within about 3% of nominal to make final calibration easier. Measure twice, then cut carefully!

The calibration for different ranges is easily made by adding suitable switches for each range. If you have a four-position, two-deck rotary switch, you could use that to connect up the shunts and the multiple gain-adjust pots, as shown. Myself, I prefer to use four DPST switches, one to turn on each range. I just have to remember to turn off each unwanted range.

Note that most rotary selector switches aren't rated for 15 A. But it's easy to find 15-A-rated DPST switches. If you are going to use a rotary selector switch, don't change ranges when heavy load current is (or will be) flowing! Remember, a load of just six 100-W bulbs would draw 25 A of turn-on transient! That would soon ruin most rotary selector switches.

If you calibrate this at about 2 A so VMETER = 1.01 × 2 × I × (VIN - 1.2 V), the calibration will be right on at 6 A, about 1% high at 1 or 2 or 3 A, about 1% low for 12 A dc, about 2% low for 12-A sine waves, and about 5% low for 15-A sinusoidal ac loads, or as much as 10% or 20% where the rectifiers are pulling significant peak currents at the sine peaks. In all cases, this meter computes the true RMS multiplication of the V × I, but the scale factor just tails off a couple percent, for currents above half-scale.

I computed that if you use this meter with a load that draws relatively large pulses of current, such as 0.75 A, on a 10% duty cycle, just at the peaks of the sine waves it will actually draw 120 W average power. If you read this with the meter in the 1500-W scale, it will read about 119 watts. But in the 150-W scale, it will read about 74.4 W.

All multipliers have limitations on duty cycle and pulse widths. So I just showed you where this one has its limitations. If you have some equipment that draws a lot of watts during the line voltage's peaks (because the rectifiers are charging up capacitors right at the peaks), and it seems to be up above 50% of range, try a higher range. You will get less resolution, but better accuracy.

The temperature compensation is not perfect. But at all rated loads, the errors should be well inside 2% between 0°C and 50°C.

If it is well calibrated at 110 V ac, it will tend to be off at 80 V ac by about -0.2%, or at 140 V by about +0.2%. Not too bad. This wattmeter is not suitable for reading the watts in loudspeakers, because it loses a lot of accuracy if VIN is less than 60 or 80 V. (If you wanted to add the Howland Current Pump using A1 to compensate down to 2 V, you could try that. It might work pretty well up to 1 kHz, but I haven't built this.)

You can calibrate the respective ranges with 2.0 A, then 0.20, 0.020, 0.0020, and so on. Of course, you will have to readjust the zero offset or allow for some offset error in the lowest range, as there may be a small but noticeable offset in the lowest range. Can't be helped.

If you want intermediate ranges, such as 5 kW, 500 W, or 50 W, be my guest. Make all the fancy switches and shunts you need. You could even compute 15 kW or more, using a 0.1-m‡ shunt. But just remember, the shunt will dissipate over 2 W at full scale!

P.S. The basic wattmeter design is based loosely on an old design by Carl Nelson in NSC Application Note AN222. I added the ranges, the dc calibration, and several other features.

P.P.S. I haven't had time to measure the inductance, but the shunt as laid out in my April 29 column is not going to be very suitable for audio work, not even at 5 or 1 KHz. If you want to try to use this for an audio wattmeter, send me an e-mail, and tell me your snail-mail address. I'll reply by July 1. Maybe twisted pairs will make a good shunt out to 15 KHz!

All for now. / Comments invited!

RAP / Robert A. Pease / Engineer
[email protected]—or:
Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

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