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#10757 06/17/02 09:36 PM
Joined: Feb 2002
Posts: 840
Not too long ago, I installed one of those cheesy "puck" light kits from Home Depot for my neighbor. Anyway, I was really suprised to read the specs on the transformer that say it operates at 12 volts at 20 kHz!

Yesterday, I was putting up some low voltage track heads with an MR16 lamp, also with an electronic tranformer. Curiosity got the best of me, so I opened one up and looked inside.

Here's what I found: a cicuit board containing a small magnetic transformer, 4 diodes, some resistors, a couple caps, a coil, and 2 transistors mounted on a heat sink.

So, with my basic knowledge of electronics, I guessed the following: The magnetic transformer steps 120 volts down to 12 volts, and the 4 diodes make up a full-wave rectifier, which converts AC to pulsating DC. After that, I'm lost, and I have no idea what the other components do, even though I know how transistors, caps and coils work. I suspect they may have something to do with increasing the frequency. [Linked Image]


1. How do these work? What do the transistors do?

2.What is the advantage of operating halogen bulbs at such high frequency? More light output?

3.Don't these transformers intoduce undesirable radio frequncies and harmonics into power lines? I'm thinking of the full wave rectifier here (aka switching power supply).

*calling pauluk*

#10758 06/17/02 09:53 PM
Joined: Sep 2001
Posts: 806
What you have there is a "switching power supply". Incoming 60 HZ AC is rectified and filtered to produce a high DC voltage, which is then converted into a high frequency AC by an oscillator (several transistors), and then fed into the primary of a high frequency transformer, and stepped down (or up) to the required level for the load. Most of these supplies then convert the AC back into DC with a rectifier stage, and usually a voltage regulator to ensure stable output. In the case of running a few puck lights, the raw 20 KHz AC is apparently deemed sufficient. [Linked Image] Switching supplies are everywhere today, from PCs to microwave ovens to TV sets. The "electronic ballasts" used for fluorescent lights are specialized forms of switchmode power supplies.

The primary advantage of switching supplies is weight and space savings by eliminating bulky 60 Hz magnetics, and physically large filtering capacitors needed at 60 Hz. Higher overall efficiency is an added benefit. Disadvantages include RF noise emission (both radiated and conducted), increased complexity, and susceptibility to voltage spikes and surges.

These things can be made quite well, with low levels of RF emission. But well-made usually equals high-priced. An el-cheapo device with an AC output fed through zip cord sounds like a sure-fire way to destroy radio reception for a fair distance around. Does the power supply have an FCC part 15 label or something similar on it?

#10759 06/17/02 10:16 PM
Joined: Feb 2002
Posts: 840

Hmmmmm....good question, I didnt notice any labelling on either the puck light xformer of the track head.

These track heads are definetly not well made, as one of them crapped out right away (brand new). I'm curious to see how long the puck light xformer lasts.

I guess I was on the right track. The transistors do increase the frequency.

#10760 06/18/02 04:24 AM
Joined: Aug 2001
Posts: 7,520
Somebody call?? [Linked Image]

Well, NJ has pretty much covered it already, but I'll add my 2 cents.

Yes, these sorts of switch-mode power supplies really are almost everywhere these days. TVs were one of the first domestic appliances to adopt them during the 1970s, although they've been around longer than that in commercial applications. These days you'll find them in VCRs, computers, and even in the charger units for some cordless tools.

The economics have played a big part in consumer goods. Transistors and ICs (integrated circuits) are dirt-cheap to manufacture now, whereas traditional transformers for 50 or 60Hz are relatively expensive. The switch-mode supply just needs a cheap, lightweight transformer which usually has a ferrite core due to the high operating frequency.

The combination of price, size, and weight is what has made them so popular in today's miniature equipment.

In the application of LV lighting, there's no particular advantage in using the higher frequency for the bulbs. It's purely the cost/size/weight factor over using a taditional 60Hz xfmr to feed them.

I agree with NJ on the interference question on the "el-cheapo" units. I've seen some here with harmonics and other hash right up into shortwave frequencies. Terrible.

#10761 06/18/02 06:41 AM
Joined: Feb 2002
Posts: 840
Thanks for the repsonses. Does anyone know of any good websites that describe these in greater detail? Possibly with schematics?

#10762 06/18/02 11:30 AM
Joined: Aug 2001
Posts: 7,520
Haven't had time to search extensively on the web, but this one looks as though it should give you some background:

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