dimmers?

[jfk]

I see they make standard forward phase dimmers (aka triac, aka leading), AND also there are magnetic low voltage dimmers.

What's the issue with using a triac dimmer with a transformer?

 

[Martaine2005]

Hi jfk,
I suppose nothing if it's AC from the transformer.
It will obviously depend on the current needed for your device.
Most household dimmers are given in wattage. A typical electric drill speed controller is very similar. As are the chinese christmas flashing lights with hundreds of LEDs.
What are you trying to control? there is probably a far more efficient way..
If DC is available, pwm is a much better choice.

Martin

 

[jfk]

These are wall dimmers such as sold at Home Depot. Rather than a lamp hooked to it, I want to use a transformer.
They suggest MLV type for transformers. I have triac.

My question is whats the issue using triac?

 

[AnalogKid]

A TRIAC has a minimum voltage drop across it that can be 2 V or more even when the dimmer is set to 100%. If the triac is in series with a low voltage secondary, that voltage drop might be a problem for whatever is downstream. If the TRIAC is in the line voltage primary, then a possible problem is the voltage wave distortion created by the TRIAC turning off and on every half-cycle of power line. Remember, a TRIAC does not "turn down" the input sinewave, it truncates it. This reduces the input wave's average value, and the thermal mass of an incandescent light bulb acts as a lowpass filter so you do not see anything other than a dimmer bulb. But the AC voltage waveform is severely distorted, and has the same peak value as an un-dimmed waveform even when at 50%.

At its heart, a TRIAC dimmer is designed and built to do one thing - dim an incandescent light bulb. Everything else has potential problems. Not unsolvable, but something to keep in mind.

ak

 

[duke37]

A triac passes current in both directions on an AC suppy if everything is optimum.
If the system is not quite symmetrical, then current can be passed in one direction more than the other. This can cause the transformer core to saturate and current will not be limited by the transformer inductance. BOOM.

I made a transformer based supply to heat a hot wire cutter. I put an incandescent light bulb in series with the transformer primary to limit the current, this worked well and the brightness of the bulb was a help in setting the voltage. This was much cheaper and lighter than a Variac.

 

[jfk]

A triac passes current in both directions on an AC suppy if everything is optimum.
If the system is not quite symmetrical, then current can be passed in one direction more than the other. This can cause the transformer core to saturate and current will not be limited by the transformer inductance. BOOM.

I made a transformer based supply to heat a hot wire cutter. I put an incandescent light bulb in series with the transformer primary to limit the current, this worked well and the brightness of the bulb was a help in setting the voltage. This was much cheaper and lighter than a Variac.

I'm curious what you mean by "not quite symmetrical".
This is a small magnetic transformer that powers a low voltage light.

 

[jfk]

A TRIAC has a minimum voltage drop across it that can be 2 V or more even when the dimmer is set to 100%. If the triac is in series with a low voltage secondary, that voltage drop might be a problem for whatever is downstream. If the TRIAC is in the line voltage primary, then a possible problem is the voltage wave distortion created by the TRIAC turning off and on every half-cycle of power line. Remember, a TRIAC does not "turn down" the input sinewave, it truncates it. This reduces the input wave's average value, and the thermal mass of an incandescent light bulb acts as a lowpass filter so you do not see anything other than a dimmer bulb. But the AC voltage waveform is severely distorted, and has the same peak value as an un-dimmed waveform even when at 50%.

At its heart, a TRIAC dimmer is designed and built to do one thing - dim an incandescent light bulb. Everything else has potential problems. Not unsolvable, but something to keep in mind.

ak

I appreciate your answer.

I hooked up the transformer, and watched its secondary output vary about as I expected, and saw no problems.

When you say "not unsolvable", what type of solution might be needed?

Thanks!

 

[duke37]

A transformer run on AC sine wave has the magnetic flux varying from one direction to the other. The average flux is zero.
The triac will be turned on by a diac and if this is perfectly symmetrical, the positive and negative parts of the waveform will be identical and the average flux will be zero. If however the positive and negative pulses are not equal, then there will be a net flux in one direction with the possibility of core saturation.

 

[Bluejets]

Used a dimmer for years on a 36v 5 amp transformer for wire cutting,,,no problem.

 

[jfk]

A transformer run on AC sine wave has the magnetic flux varying from one direction to the other. The average flux is zero.
The triac will be turned on by a diac and if this is perfectly symmetrical, the positive and negative parts of the waveform will be identical and the average flux will be zero. If however the positive and negative pulses are not equal, then there will be a net flux in one direction with the possibility of core saturation.

In practical terms, would this mean extra heat, or short transformer life, or another issue? Is this a common failure in the real world?

 

[duke37]

The effect of saturation will depend on the core. An E/I core will likey cope better than a torroid where the core is more tightly controlled.
The higher primary current will give more heating and the transformer may well cater for this. A close rated fuze will help.
I do not think that this is a common failure since triacs driving transformers are not common. My practical knowledge is very limited.

Some dimmers I have seen say not to use transformers and some say that they will drive inductive loads. I postulate that this a due to the quality of the positive/negative match of the diac.

 

[AnalogKid]

Is this a common failure in the real world?

Not that I've seen. As long as the input AC powerline is reasonably symmetrical, the energy conducted by the TRIAD curing the positive and negative half-cycles will be equal and the net transformer flux will be zero. An asymmetrical failure in the trigger circuit is very rare.

ak

 

[Fredx-3gtvi]

In practical terms, would this mean extra heat, or short transformer life, or another issue? Is this a common failure in the real world?

Yes, extra heat in transformer, and a shorter transformer life. Another problem is that as the output from the dimmer is not symmetrical (and no triac dimmer is ) then at some point one will be at zero, while the other half will still be giving voltage, ie pulsed DC ant this will take out a transformer in a short time. This zero to V may be as high as 30Volts. But even two volts will cause a problem.

Yet another problem could be very high voltages generated by the switching on voltage times. This wont be much of a problem when you have a lamp for a load, but if the lamp blows, there could be very high voltages across the transformer output. So don't touch thinking there is only 12 volts there, there could be lots more !

 

[Bluejets]

Iron core transformers in the real world tend to be fairly hardy devices.
I have seen supply authorities for example, regularly run at 150% for years on end.

 

[Minder]

If you do a search for hot wire foam cutters there are many out there that are using the Triac dimmer in the primary, having said that, I made one for mine and use it in the secondary, there is not that much to them and easily constructed.
M.