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I need a simple power meter for a phase-controlled heater.

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LAN_403

radiotech

Tue Feb 21 2017, 10:34PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The elevator lights transformer may mess with the chopped AC.

You also could hook two clear 120 volt Christmas lights in series to make a 240 volt lamp.

klugesmith

Wed Feb 22 2017, 02:42AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

>> Place the two lamps in a shroud so you can judge if both are lighted equally.
>> If both are glowing equally, then the power applied to both lamps is equal.

That's my favorite approach, for reasonable accuracy at low cost. Lamps respond to extreme crest factors a lot like heaters do.

For objective visual comparison of brightness, you can arrange the lamps to cast partly-overlapping shadows or spots of light on a white surface.
One alternative, which I read about decades ago, puts the lamps inside an opaque box at opposite ends. At the midplane is a thin sheet of white paper prepared with a small spot of oil or melted wax in the middle. Observing the paper through a viewport from either side, the spot appears brighter or darker than the surrounding field, according to the relative illuminance on far and near sides.

The DC voltage on lamp #2 can be servo'd to automatically keep the brightnesses matched.
Visual or electronic matching accuracy can be improved by using the same sensor for both lamps, with a mechanical optical chopper. Or switching a single lamp between unknown AC and reference DC voltages -- the human eye could be a pretty good null meter if the blink frequency is well chosen. The brightness of a single lamp could be measured with an analog lightmeter and non-linear scale markings, calibrated with DC.

- - -

If low wattage 240 volt lamps are hard to come by, you could use a low voltage lamp(s) with power resistor instead of transformer. Then no worries about transformer frequency response. Voltage division ratio would no longer be constant (not even close!), but that's easy to handle with DC calibration or identical DC reference lamp circuit.

- - -

As for analog op-amp methods, it's not easy to do better than the RMS-to-DC converters in mainstream RMS-reading multimeters.
Unknown AC voltage (typically AC-coupled!) is precision-rectified, then squared and low-pass filtered. A DC voltage source maintains a matching output from an identical analog voltage squarer. Readout settling time depends on the low-pass filter.

Of course this is about measuring RMS voltage or current, not measuring true power to non-resistive loads.

-Rich

Dr. Slack

Wed Feb 22 2017, 05:44AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Posted: Tue Feb 21 2017, 10:20AM
Thanks for all of the feedback,
I've decided to go with a simple linear scale and 'callibrate' it later when I have access to a true rms meter

Not sure what you mean by 'simple linear scale'. The thing we've been discussing is either how to square effectively, or to measure heating power in a some sort of resistor, the point about a phase-controlled switching waveform is the mapping between mean current and power is non-linear with phase angle. But at least it is consistently non-linear.

Sulaiman

Wed Feb 22 2017, 09:39AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I meant;
I will use a ready made linear dial (e.g. 0 to 10) for the potentiometer
and have a look-up table/graph for actual power vs. dial setting.

This way I immediately get a scale that allows me to use a previously tested power setting.
(this is just to control a heating mantle for a flask to do fractional distillations at a constant boil-up rate,
as I am learning the techniques of distillation for my chemistry hobby)

Physikfan

Sun Feb 26 2017, 05:22PM

Registered Member #60240 Joined: Mon May 16 2016, 07:01PM
Location:
Posts: 304

Hi Sulaiman

Your main goal is:
"I will be measuring the effective heating power by the rate of boiling and condensing of water,
and I want to compare it to the actual electrical power input."

I would replace your triac controlled AC source by a dc source.
In my opinion it would be the easy way to solve your problem, measurement of the rate of boiling and condensing of water
as a function of the actual electrical power input.

Sulaiman

Sun Feb 26 2017, 06:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

dc would be easier to measure, but the Â£1.24 controller works well and I do not want huge electrolytic capacitors etc.
(cost, volume, reliability etc.)

I only wondered if anyone had a simple circuit that could measure the electrical power input using phase-control,
because I could not remember or conceive of a simple circuit,
just so that I can determine heating efficiency, which in fact, is not that important.

I will just use a callibration curve/graph to convert between dial setting and boil-up rate, which is the important parameter.

I can use and easily measure mains voltage direct for full power, and half-wave rectified for 1/2 power
then use those two callibration points only ... good enough for this application.

I THINK THAT THIS THREAD CAN NOW BE CONSIDERED CLOSED.

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