rectifier diode in series with AC heating element to save energy? - diyAudio
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Old 10th June 2007, 03:10 AM   #1
star882 is offline star882  United States
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Default rectifier diode in series with AC heating element to save energy?

I remember reading that installing a rectifier diode in series with a resistive heating element saves energy. I understand that the heating element will receive half the power. Although this, if done correctly, will make the heating element last longer (lower temperature) and possibly safer (again, lower temperature), wouldn't the element have to be on for twice the time to produce the same amount of heat?

I don't think it's true because if it is, every electric heater will have a rectifier. (Some heating equipment do contain a large rectifier but that's for control.) I think I read it while googling for "true-rms" but I couldn't find it anymore.
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Old 10th June 2007, 03:54 AM   #2
sandyK is offline sandyK  Australia
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Default rectifier diode in series with AC heatin elementg

Probably not a good idea. It would take ages to heat up, and the element may even buzz. It may however, be worthwhile switching in the diode when the required temperature is reached.
A thermosat would be far better , though.
I am not sure how nearby A/V gear would like this method.

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Old 10th June 2007, 04:25 AM   #3
Duo is offline Duo  Canada
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If a heater designer wants the heater element to last longer, he/she may make the element larger (but rated for the same amount of power as a smaller element) to distribute the heat, and thus thermal stress, over a larger area. Upon that, one might cause airflow around the element to prevent it from heating up as much.

A diode in the path to the element would induce very bad harmonic switching noises, and possibly EMI, into the environment immediate to the heater.

Diodes to handle the amount of current for most typical heater elements would probably be somewhat costly as well...
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Old 10th June 2007, 12:47 PM   #4
star882 is offline star882  United States
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I recall there was a device that effectively reduced the voltage to a water heater during off-peak times and restores full voltage during peak times. I forgot how it did that, but since it advertises something about half power, a rectifier diode can definitely be a way it did it.

I'll make a model using a small rectifier and a 5w power resistor. Then I'll experiment to find out exactly what the rectifier does.
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Old 10th June 2007, 01:16 PM   #5
AndrewT is offline AndrewT  Scotland
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can we assume this heater runs direct off the mains voltage?
The diode half wave rectifies the mains.
Output power is halved.

The reverse voltage of the diode will need to exceed the peak voltage on the supply. 200V is a bit too close for 120Vac.
In the UK we would need 600V for 240Vac.

This power reduction is what was fitted to the trigger of two speed power drills. Massive reduction in torque made them run slow. Big drills then made them run even slower. Slow turning fan and very high half wave currents = burnt out motor.
regards Andrew T.
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Old 10th June 2007, 01:47 PM   #6
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Isn't this idea commonly used in hairdryers?
I've never dismantled one to find out but assumed it to be true because I've read so many people complaining about hairdryers affecting the sound quality from HiFi.
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Old 10th June 2007, 02:52 PM   #7
Eva is offline Eva  Spain
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See this post:

With the series diode in place only half the power will be applied to the heater, and DC will be drawn from mains line. This is not a recommended practice.
I use to feel like the small child in The Emperor's New Clothes tale
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Old 10th June 2007, 03:47 PM   #8
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I have a diode in the power cord for my bench soldering iron. When I need more power, I engage a bypass switch. I guess it is not too bad on the line power at that lower power level. The set-up is simple, small, and lightweight.
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Old 10th June 2007, 03:50 PM   #9
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It has gotten me thinking if I can easily redo it though.
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Old 10th June 2007, 03:55 PM   #10
star882 is offline star882  United States
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I have seen it used in portable heaters, heat guns, and even coffee makers.

The experiment had some strange results. With a 1:1 isolation transformer powering a 3k9 resistor with and without a 1N4007 rectifier, adding the rectifier halved the voltage, current, and delta-T. A true-RMS meter was used to measure the voltages and currents. Based on voltage and current, the resistor was drawing only 1/4 its original power, but based on delta-T, it was producing only 1/2 the heat. This makes no sense so I think I made a mistake somewhere.
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