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"Fixing" an electric heater for lower wattage

 
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gliggo



Joined: 06 Jan 2009
Posts: 1

PostPosted: Tue Jan 06, 2009 10:15 pm    Post subject: "Fixing" an electric heater for lower wattage Reply with quote

I have an old house and the kid's room was designed with inadequate heat vents. Last year I bought a nice baseboard heater with a digital thermometer, but it's 120V/1500W and with the old wiring it blows a breaker once in a while, which is annoying. Also, the room is small that 1500W is more than needed.

What I want to do is limit the power to about half that, and I recently picked up a big industrial rectifier, 40A and 200V, so it should be more than adequate. It will let half the wave through and I think it should work, it's a resistive only heater, no fan. I was thinking of soldering in the rectifier inside and sealing the unit back up.

I was wondering, since during half the cycle it's full power, would it still blow a breaker? Also since half the wave is used and half not, would it create any interference or other issues with devices on the same circuit?

Thanks.
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Wed Jan 07, 2009 7:16 am    Post subject: Reply with quote

Sounds like you may have some knowledge about the theory, but a bit confused.... heater is a passive resistive load and using a rectifier(regardless of the size) is not going to cut the total power in half. The whole concept of conduction is valid when controling semiconductors like a SCR or Triac.
Effectively, you are changing the AC to a halfwave DC. A resistive element on the receiving end, still exhibits the same resistance and the total current draw stays the same--it will simply attempt to dissipate the energy thru heating element and the rectifier.
Depending on the design of the built-in thermostat(some will simply monitor the time, some take advantage of a multi-section heating element or a bimetallic switch), the correct way is to either use it's thermostat or better yet, upgrade your wiring/circuit breaker.
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PostPosted: Wed Jan 07, 2009 10:29 am    Post subject: Reply with quote

vtech wrote:

Effectively, you are changing the AC to a halfwave DC. A resistive element on the receiving end, still exhibits the same resistance and the total current draw stays the same--it will simply attempt to dissipate the energy thru heating element and the rectifier.

That is not how I understand rectifiers at all. When the AC is on the negative half-wave, the rectifier is reverse biased, and this means it is not conducting or sinking any current, it acts like an open circuit. therefore the total power consumption will be about half.

Am I missing something? If so, what?
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Wed Jan 07, 2009 3:55 pm    Post subject: Reply with quote

By connecting a half wave rectifier in series with a resistive load, you will get the half effective power in the load. But the RMS value of the current will not drop to half its rated value, just to 0,707 times its rated value. And then it depends, whether that current is enough to blow the circuit breaker. There is also a far possibility that the DC current component that will be created by the half wave rectification can cause an earth fault breaker or a circuit breaker with magnetic momentaneous tripping to behave strangely.

I don't know how it is in the US, but in the EU, it is nowadays prohibited to manufacture equipment that draws a DC current from an AC mains. 15-20 years ago, it was common that electric heaters had a half/full power switch that simply connected a diode in series with the heater in the half power position, but more recent such units must be manufactured with two separate heating resistors, one of whom is switched of in the half power position. There are two reasons for that: first, the risk of malfunctioning earth fault breakers and circuit breakers, and second, the risk that DC currents drawn by several equipments add upp and cause saturation of the core in some transformer in the distribution network, this would mean high losses and possibly overheating of the transformer.
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PostPosted: Wed Feb 04, 2009 5:37 am    Post subject: Reply with quote

Something to think about.

What is your life worth?

Rewiring an electric heater to save energy is dangerous. An AC switch will not carry DC current. That switch will burn out and cause a fire.

In 2008 Australia, there were 11,000 electrical house fires caused by people trying to carry out electrical repairs/renovations/new wiring in their homes.

Go out and purchase a new heater; Do you really want to die in a house fire?
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