JD
Well-Known Member
A lot of us have been going down memory lane with recent posts about some of the grand autotransformer dimmers that are still in use. [See the thread http://www.controlbooth.com/forums/lighting-electrics/31887-shubert-theater-house-lighting.html . --DL] What a wonderful and quiet way to dim house lights!
Still, there is a quirk to these grand monsters that may have gotten lost to time and is worth bringing up just in case someone wants to implement one of these wonderful beasts.
We live in an error of phase chopping dimmers that work like a high speed switch in series with the load. This circuit is the norm, and because it is a series circuit, we expect that the input current to the dimmer and the output current to the load will always be about the same. Modern dimmers are protected by a circuit breaker that cuts the hot feed to the module should an overload occur.
THIS WILL NOT WORK ON AN AUTOTRANSFORMER DIMMER!!!
Autotransformers are at their core, a transformer. A transformer can change the ratio of voltage to current. Effectively, the primary is the whole core, and the secondary is the section of windings below the position of the brush. One would think they are protected if they feed a 2.4kw autotransformer with a 20 amp breaker, but this is not true. Under short-circuit conditions, the output of the autotransformer could be above 70 amps, even though the primary is only drawing about 5 to 10 amps !! The result could be a fire or burned windings on the transformer, or both. Because of this, autotransformers REQUIRE protection on the OUTPUT as well as the input. This is something we are not used to on solid state dimmers.
Typically, the old transformer boards included breakers on the outputs of the transformers. However, if you purchase a raw transformer, they do not include this protection. If you intending to use one, remember, you must use secondary (output) protection.
Still, there is a quirk to these grand monsters that may have gotten lost to time and is worth bringing up just in case someone wants to implement one of these wonderful beasts.
We live in an error of phase chopping dimmers that work like a high speed switch in series with the load. This circuit is the norm, and because it is a series circuit, we expect that the input current to the dimmer and the output current to the load will always be about the same. Modern dimmers are protected by a circuit breaker that cuts the hot feed to the module should an overload occur.
THIS WILL NOT WORK ON AN AUTOTRANSFORMER DIMMER!!!
Autotransformers are at their core, a transformer. A transformer can change the ratio of voltage to current. Effectively, the primary is the whole core, and the secondary is the section of windings below the position of the brush. One would think they are protected if they feed a 2.4kw autotransformer with a 20 amp breaker, but this is not true. Under short-circuit conditions, the output of the autotransformer could be above 70 amps, even though the primary is only drawing about 5 to 10 amps !! The result could be a fire or burned windings on the transformer, or both. Because of this, autotransformers REQUIRE protection on the OUTPUT as well as the input. This is something we are not used to on solid state dimmers.
Typically, the old transformer boards included breakers on the outputs of the transformers. However, if you purchase a raw transformer, they do not include this protection. If you intending to use one, remember, you must use secondary (output) protection.
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