A Lutron 3 wire dimmer

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gar

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Ann Arbor, Michigan
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EE
141227-2414 EST

A Lutron three wire dimmer, that is one that requires a neutral, has an approximate phase shift adjustment range for turn on of 45 degree (2 mS) to 150 degree (6.8 mS). This requires no load to operate. Based on a sample of one.

A highly inductive load will cause problems, and requires sufficently low shunt resistance across the load to correctly function.

You can run your own experiments with a scope for verification.

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141228-0823 EST

This is a 120 V dimmer, and 5 years or so ago when I purchased it the cost was around $ 50. Way too expensive for what is in it.

This dimmer was designed for dimming fluorescent fixtures and has a switched full 120 output as well.

A major advantage ot this dimmer vs a 2 wire dimmer is that at any setting, in particular at minimum, the dimmer will restart at its original low setting on power loss and restoration in addition to not requiring a minimum load. Most 2 wire dimmers won't restart at a minimum setting after a power loss.

When I originally called Lutron to find out what models were 3 wire dimmers I had a hard time getting the person to understand what I needed.

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The electronic low voltage dimmers- 3 ways are over $80. I had forgotten about the fluorescent dimmers- they too have a neutral. There should be a model number on the unit.
 
141228-2248 EST

I believe the 3 wire Lutron dimmer I have is an SF-10P. Any packaging or the plastic back cover are long gone. No specific model number on any of the internal structure. I dis-assembled this unit when I first purchased it to look at the internal circuitry.

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141229-0909 EST

That is correct, but that does not mean it is not useful for other applications.

As a fluorescent dimmer it means that there is a mechanically switched full line voltage output to supply constant voltage for filament excitation. For applications not requiring this output one simply ignores it.

The really important feature of this dimmer is that a neutral is required. This makes it a three wire dimmer, and means the dimmer can be built to supply constant power for the electronics that controls the Triac. Thus, in theory the dimmer could be built for any phase shift from 0 to 180 degrees for turn on, and when power was removed and restored that the dimmer would operate again at the same phase shift as before loss of power.

Lutron restricts their adjustment range to less than 0 to 180 degrees.

If you operate incandescents at very low light levels, as I do in my hallways, then it is desirable to have the dimmer restore to its previous setting upon a power interruption without having to change the setting to a higher level to restart the dimmer as occurs in many two wire dimmers.

The Lutron dimmer uses a Triac. This means there is a delay to turn on in the cycle, and turn off occurs at the next current zero crossing. Further excitation is maintained thru the on part of the cycle, or internally there is sufficient load to exceed the holding current of the Triac. Which ever is the case the dimmer works as expected with no external load.

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