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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 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 :)
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.
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
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.
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.
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
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.
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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