Control/Dimming 120v SCR dimmers + low voltage transformers

[Keith Chapman]

Hi Folks,

I was just curious if anyone would be willing to explain or point me in the right direction for more information about how low voltage transformers work with standard 120v SCR dimming. I've used them for silly props with spliced x-mas lights without a problem, but am interested in knowing a bit more about them.

I found this from another thread:

Lightbox Fan and a dimmer

I'm curious as to why the voltage spikes, and if I'm running any potential for fires. I might have gotten away with the props due to a short periods of use.

Any advice would be appreciated,

Keith

 

[JD]

The short version: Dimmers work by chopping the waveform. Transformers do strange things when fed a chopped waveform. Result: unexpected voltage climb at certain settings.
A heavy "resistance" load, such as incandescent lamps, minimizes the effect, but other devices may actually be damaged by the over-voltage.
The waveform produced by a dimmer is called a sawtooth waveform. To be accurate, it is "like" a sawtooth waveform. The sharp rise on the tooth of the waveform causes a very high overshoot on the output winding of the transformer, which may actually ring or resonate. If the transformer is sufficiently damped with a heavy resistive load, the overshoot will not be as bad. The worst thing you want connected is something that converts the output to DC, like the switch mode power supply found in almost everything these days. In those cases, the DC voltage may end up at double the voltage you would expect it to be!

 

[Keith Chapman]

Thanks John,

I had understood how SCR's chop the waveform, but did not realize that transformers created the harmonics/resonate/ringing problem. This helped me understand it greatly.

So I have two follow-up questions:

1: Is there a cheap and safe way to convert SCR dimmers from 120 volts AC to 12v DC for my props applications that would still be ok with dimming the intensity for fade ins and outs?

2: For curiosity's sake... would transformers be fine with IGBT reverse phase control dimming, as this would remove the spikes?

Thanks,

Keith

The short version: Dimmers work by chopping the waveform. Transformers do strange things when fed a chopped waveform. Result: unexpected voltage climb at certain settings.
A heavy "resistance" load, such as incandescent lamps, minimizes the effect, but other devices may actually be damaged by the over-voltage.
The waveform produced by a dimmer is called a sawtooth waveform. To be accurate, it is "like" a sawtooth waveform. The sharp rise on the tooth of the waveform causes a very high overshoot on the output winding of the transformer, which may actually ring or resonate. If the transformer is sufficiently damped with a heavy resistive load, the overshoot will not be as bad. The worst thing you want connected is something that converts the output to DC, like the switch mode power supply found in almost everything these days. In those cases, the DC voltage may end up at double the voltage you would expect it to be!

 

[JD]

Thanks John,

I had understood how SCR's chop the waveform, but did not realize that transformers created the harmonics/resonate/ringing problem. This helped me understand it greatly.

So I have two follow-up questions:

1: Is there a cheap and safe way to convert SCR dimmers from 120 volts AC to 12v DC for my props applications that would still be ok with dimming the intensity for fade ins and outs?

2: For curiosity's sake... would transformers be fine with IGBT reverse phase control dimming, as this would remove the spikes?

Thanks,

Keith

2) Not reverse phase, but IGBT pure sine wave dimmers, or old autotransformers.

1) A snubber circuit can be designed but it takes some playing around. The other way around this is to use parts in a design that can handle the overvoltage without damage.
So, if we look at the problem, first we need to make sure the dimmer is loaded sufficiently. If the lamp in the box is the core load, that should not be a problem. Dimmers get flaky below 50 watts load.

I assume the transformer has a 120 volt primary and a 12 volt secondary. We need to put some resistance loading on the secondary. Something along the lines of a 20 ohm 15 watt resistor should do the trick.

Next we add a diode and a capacitor to make it DC. If your fan motor is designed to draw 200ma (for example), then we can use a 60 ohm resistor in place of the fan for a test run. Measure the voltage across the cap as you bring the dimmer up.
It will probably crest around 20 volts before dropping back to 12 volts when the dimmer is at 100%.

At this point, we can do one of two things:

A) use a fan that is rated for 24 volts.
B) use a one piece regulator voltage regulator like a LM7812 to limit the voltage to 12 volts. LM7812ACT Fairchild Semiconductor | Mouser

 

[Keith Chapman]

Thanks John,

I'll have to play around with this circuit.

Keith