transformers & dimmers

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  #1  
Old 04-04-05, 06:47 PM
reglilly
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transformers & dimmers

I'm stepping down 120v to 12v for 4 50w low voltage lamps. I've always been told that you have to have a special dimmer switch with transformers (or you'll eventually burn them out). I call one web-sales place that said that you could use a standard dimmer because their transformers are electronic. Yet, I've heard of special electronic l.v. dimmer switches.

Is it possible to use a standard dimmer in this circumstances, and would it be on the line side or load side of the transfomer?

Thanks,
Reg
 
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  #2  
Old 04-04-05, 10:09 PM
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WOW!! That must be one heavy transformer.... 120 to 12V @ 17A minimum for 4 X 50W 12V lamps.

I'm no electrician but I work with transformers all the time in guitar amps and I've never heard of an 'electronic' transformer.

FIRST check the manufacturer's instructions for ratings and warnings regarding dimmers and low voltage conditions, if any.

IF the 'electronic' transformer is simply a transformer or transformer/rectifier combo that is often used for low voltage lighting, and you purchased it as a unit, I will venture that you could use a regular dimmer on the LINE side. The draw there calculates to be about 1.7A + losses. Check the manufacturer's instructions and choose a dimmer that exceeds the line or supply requirements of the 'transformer' unit. Dimmers are usually rated in watts on the package - and based on what you said, 300W will probably work a long time.

The load side is 17 amps. Way too much for a regular dimmer - even though the watts calculate the same - dimmers are rated at 125V which means 2 or 3 amps or so. They're normally just wire wound variable resistors and all resistors are derated with heat. Current flow = heat and too much heat = failure. Just like wire. That's where the warnings you heard about come from.
 
  #3  
Old 04-05-05, 04:43 AM
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Originally Posted by rodek01
WOW!! That must be one heavy transformer.... 120 to 12V @ 17A minimum for 4 X 50W 12V lamps..
A 200W 12V transformer for low voltage lighting is pretty common.

Originally Posted by rodek01
I'm no electrician but I work with transformers all the time in guitar amps and I've never heard of an 'electronic' transformer.
An "electronic" transformer is basically a switch-mode power supply - the incoming AC is rectified to DC, chopped at a much higer frequency than the 60 Hz line frequency, run through a transformer (which can be quite small due to the higher frequency) and rectified again to DC. Not the same as the sort of transformer you would find in a guitar amp.

It is true that many (but not all) electronic transformers can be used with regular dimmers.

Originally Posted by rodek01
FIRST check the manufacturer's instructions for ratings and warnings regarding dimmers and low voltage conditions, if any.
This is excellent advice.

Originally Posted by rodek01
The load side is 17 amps. Way too much for a regular dimmer - even though the watts calculate the same - dimmers are rated at 125V which means 2 or 3 amps or so.
Yes, the dimmer goes on the line side.
Originally Posted by rodek01
They're normally just wire wound variable resistors and all resistors are derated with heat. Current flow = heat and too much heat = failure. Just like wire. That's where the warnings you heard about come from.
Dimmers are not variable resistors. They are electronic devices which vary the (RMS) output voltage by interrupting the sinusoidal AC signal at an (adjustable) point during its cycle. They are much more efficient than a variable resistor.
 
  #4  
Old 04-05-05, 06:41 AM
reglilly
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Thanks rodk01 and especially mikewu99. I spokle with the salesperson, who seemed to know what he was talking about, but I'll check the manufacturer's specs. I just had a hard time believing what he told me was true.

Thanks again.

Reg
 
  #5  
Old 04-05-05, 08:33 AM
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I've seen quite a few LV lighting transformers that are designed for and work just fine on a standard dimmer. You're probably okay.
 
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