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4hv.org :: Forums :: General Science and Electronics
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I need a simple power meter for a phase-controlled heater.

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Sulaiman
Sat Feb 18 2017, 10:25AM Print
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I am using a simple phase-controlled-triac 'dimmer' like this Link2
to control a 240 V ac 300 W resistive heater.

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 want to measure the electrical heating power but I do not have access to a true rms meter at the moment,
is there a simple measurement technique that requires only simple callibration,
for say 10% to 90% power, with 5% accuracy or better ?
(preferably passive circuitry with a 100uA or 1mA d.c. f.s.d. moving coil meter :)

an indication of (rms current).squared would suffice
as the heater will not change resistance significantly in the intended working temperature range.

At a minimum, once min. and max. are set, a 'callibration' formula to mark the knob positions/scale would do,
(I really don't want to wake up the calculus bit of my brain if avoidable - it hurts :)
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Dr. Slack
Sat Feb 18 2017, 03:50PM
Dr. Slack Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
so many options, so few meet your requirements.

Any current meter must also be constant resistance, as any tempco that's different to the heater will mean a gain error with respect to the heater.

The simplest passive type would be a 'hot wire' ammeter, which naturally reads true rms. A man of your skills ought to be able rig one up and calibrate it fairly easily. Use a 'resistance wire' rather than steel or copper for low tempco.

Hot filament power substitution method is used for RF meters. A small incandescent lamp is fed with DC through an RF choke, and heated to a temperature where its resistance is 50 ohms, to match impedance. RF is fed in through a capacitor, and the DC feed is servoed to maintain the temperature by reducing its power input. The difference in DC power is the amount of RF arriving. For a low frequency measurand, you could switch to a high frequency substitution supply.

You could make a 'square and average' front end for your meter. Apart from the standard AD835 multiplier circuit, I've always had a hankering to build a sigma delta multiplier, but never got round to it. The general idea is that the output of a sigma delta converting an input is used to control a multiplexer that switches between the input and its inverse, so the mean of that output is proportional to the input squared. The first sigma delta converter I built when I invented the concept independently (a year after Phillips, Sony and Toshiba, so no patents on that) ran at 10kHz with a TL072 and HC74, so it's very easy (for appropriate meanings of the word 'very').

Then again, borrowing a true RMS meter, or even buying the one you've always wanted, might be easier.
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Sulaiman
Sun Feb 19 2017, 01:16AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I was hoping for some magic trick ... no surprises.
I gained a Fluke dmm from work as it was not eonomical to repair, worth a try
last year I remembered to remove the expired batteries ......

No problem, I'll just callibrate the dial in effective heating power




DONE
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Hazmatt_(The Underdog)
Sun Feb 19 2017, 01:36AM
Hazmatt_(The Underdog) Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I've got several RF ammeters that are thermocouple type and are true reading from DC to 10 MHz.

Surplus Sales of Nebraska has a 20A thermocouple for this purpose, and I bought one but I never had enough current that I could source to see any action out of it.

I checked and it looks like they don't have any RF shunts anymore. They are getting a bit scarce, but they do have a 40A RF ammeter.
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Electra
Mon Feb 20 2017, 12:12AM
Electra Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
I think The next most accurate method if you can't measure the rms by any means.

If you can measure the phase angle using a scope,and use that to draw a calibrated dial for the control

Calculate the rms voltage or power from the phase angle.

Found this formula on line, as I couldn't recall it


Po = ( V Squared / pi * R ) * [pi - α + (sin2α / 2) ]


the firing angle α = ωt, where ω = 2πf

V = rms volts.

I have not checked it's correct mind you.
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Uspring
Mon Feb 20 2017, 10:28AM
Uspring Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711
It's correct. If you don't have a scope but a meter, which can measure average current, you can determine the phase angle from there and plug it into the equation for the power.

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johnf
Mon Feb 20 2017, 06:40PM
johnf Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
The biggest problem with using a multimeter is that phase fired waveforms have too high a crest factor for the rms circuitry. In good multimeters ie Fluke there is a crest factor spec in the AC ranges.

If you simultaneously multiply your instantaneous voltage with instantaneous current you get iinstantaneous power you can use a linear multiplier then a voltage to freq convertor and count the pulses for a very accurate power reading.
I did this at work and got to better than 1 part in 36million for power applied to an electrolysis cell but this did require a very special current sense resistor from germany to get to this accuracy
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Hazmatt_(The Underdog)
Tue Feb 21 2017, 01:39AM
Hazmatt_(The Underdog) Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I always wanted to try the analog route and see how that worked out.

The approach being taking the scaled voltage, and current measurement (CT) and applying them to a log amp, sum them, and take the anti-log to the meter. The major drawback to this is the typical BB log amp chip is in the $10 range.

The other approach would be cram it into the arduino, code whatever filter you want, and a look-up table with scaled correction factors would probably be cheaper then the log amp.
Plus, you could code peak and average, and moving average history all on a 16x2 LCD.

All depends on what you want.
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radiotech
Tue Feb 21 2017, 09:38AM
radiotech Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Connect a small wattage light bulb in parallel with your 240 volt 300 watt phase controlled heater.

Now connect a similar light bulb up to a DC power supply that you can easily measure current and
voltage to easily determine power level.

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.

The effective voltage in both lamps will be the same. This voltage squared times the resistance
of the 300 watt heater will be the true power.
Many


This topic was discussed here in 2010 in this posting , and I (radiotechnician) mentioned the power lamp.

Link2
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Sulaiman
Tue Feb 21 2017, 10:20AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
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.

radiotech ... I had considered a front panel filament lamp in parallel with the heater as a visual power indicator.
my pathetic excuse for not doing it is
a) I find low wattage 240 V lamps have a short life, a false 'nopower' indication is worse than not having an indicator
b) I don't have one in stock
Somewhere I've got one of those small circular transformers intended for running elv filament panel lamps on 240 Vac ....
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