SCR dimmer control for portable electric heater??

[clare]

The diode across the thermostat connection would prevent the heater from ever shutting off. It would switch from high to low - assuming a single pole thermostat.

[clare]

[Jim Yanik]

[e-mail] wrote in news: [e-mail]:

the triac may shut off current flow at a part of the sine wave less than peak voltage,so it -can- reduce the voltage to the load. It would have to be at less than 50% setting,though.

[clare]

Which covers both SCRs and Triacs

[George]

It -does- reduce the voltage. Connect any volt meter to the load and observe the RMS voltage (what meters display) decrease as you decrease the dimmer setting. You are thinking of p-p voltage which is always constant and isn't what a typical meter displays.

I don't understand the significance of 50%. The moment the triac changes the waveform is the moment the RMS voltage is no longer line voltage.

[me]

Yes.... that's the problem.....finding one for 1500 watt load.....and one that is "off the shelf" and cheap enough

[Ralph Mowery]

When a triac is used to control the ammount of heat in a resistance load, a meter can show almost anything up to the peak of the voltage. The common meter with a coil is only calibrated to show the RMS of a sine wave. If any other waveform is measured, the calibration is no longer valid. Some digital meters are suspose to be calibrated for a true RMS, but I would think they have some limitations.

The 50 % part comes in because any conduction angle less than 50% will not have the full line voltage as the peak value. That is the point the meter can not reach the full input voltage to the triac on the output side. I really should say scope instead of meter, but a peak or peak to peak meter with a peak hold function could be used.

All this is for simple circuits. There are some triacs that only cut off and on at the zero crossing and there is a timming circuit that will let it conduct for so many full cycles out of a period of time depending on the ammount of heat wanted. If a voltmeter is put across the load it will try to reach the full line voltage and will almost do it lots of times. It will only stay there for a fraction of a second. Almost looks like a windshield wiper on a car. The same with an amp meter. It will almost reach the full current. The meters can be dampened to show a more 'averaged' value.

[Joe]

You can buy a 20 A Powerstat or Variac autotransformer on eBay for

40-60 bucks or so. Use it to change the voltage to the heating elements and leave the thermostat function alone. The units are good sized, but from a safety and simplicity standpoint you won't hurt yourself with such a system.

Joe

[George]

I understand p-p vs RMS. And you are correct depending on how well the meter integrates the waveform it may not be accurate. But it still shows an approximation of the RMS voltage value which does continuously change with the dimmer setting.

RMS is used because it allows us to treat the reading as DC for basic calculations.

[me]

wont a variac just dissipate a TON of heat and act like a secondary mini heater?

[The Daring Dufas]

I would suggest taking a cheap dimmer and combining it with a triac from an old microwave oven. If you're handy with electronics and working with sheet aluminum, you can easily make your own high current control. A flat piece of aluminum with strips of aluminum angle and a bit of heat sink grease can make a great heat sink for the triac. It could be built out of parts from a junk pile with a minimum of new parts for insulating things.

TDD

[AZ Nomad]

why bother gutting a microwave oven? Just take a cheap dimer and replace the triac with a higher current version bought new. Did a look up @ digikey: For example, a 800V 16A (good for 1900 Watts @ 120V) is $1.27. 25A is $1.33. It is far cheaper than the labor involved with gutting a uwave oven.

Heaters are a resistive load; they're easy to control with a dimmer.

[The Daring Dufas]

example, a 800V 16A

But where's the fun in that? I like to tinker with things and turn them into other things. When I was a kid, I was very good at turning electrical devices into smoke generators.

TDD

[bud--]

You may be thinking of a rheostat - which is a resistor.

A variac is a type of transformer and has a high efficiency like a transformer.

[AZ Nomad]

example, a 800V 16A

Electrical devices run on smoke. If you let the smoke out, they quit working.

[The Daring Dufas]

example, a 800V 16A

"Magic smoke", you big silly.

TDD

[clare]

Not the way they work. The triac turns off when the voltage reverses. The turn -ON is all that is controlled, so the actual voltage would always be the same, but average voltage (and therefore average power) would be reduced.

[clare]

A variac is an autotransformer, not a rheostat, and as such is pretty darn efficient

[clare]

example, a 800V 16A

For maximum efficiency you want the device to be rated just over the maximum operating voltage. Higher voltage rated devices generally have a higher voltage DROP as well - meaning higher dissipation - or is that only in MosFets?

[Don Klipstein]

If the triac is turned on later in the half-cycle than the voltage peak, the actual voltage is reduced.

One more thing: Power is determined not by average voltage, but RMS voltage. RMS is root-mean-square, or square root of average square of voltage. And AC voltmeters that do not boast about being "true RMS" do not read RMS voltage, but 1.111 times the average voltage, which is the same for an unchopped sine wave but not for most other waveforms. A usual AC voltmeter will read lower than the actual RMS voltage applied to a load that is dimmed by a dimmer. If the meter reads half the normal voltage, the RMS voltage will be closer to 70.7% of normal voltage. (The relationship is only a square root one in sinewave AC at the point of turn-on halfway through a half cycle.)

- Don Klipstein ( [e-mail])