Electronic Transformers

[Bone]

Has anyone successfully used one of the now common electronic transformers for halogen lights to drive an amplifier.
They appear to be a mains driven switching regulator giving an AC output of about 12v at 28-30kHz.

Tony.

[star882]

Much better to start with a PC power supply.
It could be useful for developing an amplifier based on a "cycloconverter" instead of an inverter, though.

[leadbelly]

The real issue is isolation. I suspect they do not isolate, and so are unsafe to use.

[star882]

I remember reading that low voltage halogen supplies do isolate. Fluorescent supplies generally do not, however.

[Steve Dunlap]

Originally Posted by star882

I remember reading that low voltage halogen supplies do isolate. Fluorescent supplies generally do not, however.

That is mostly correct. Off line switching supplies still use a transformer to step the output voltage up or down. The high switching frequency allows for a pretty small and light transformer.

Some fluorescent ballasts do not isolate. Usually the smaller ones. At the company I worked for, all ballasts were isolated, whether transformer or electronic.

I should emphasize that isolation doesn't protect you when you get hold of a few hundred volts at 200 ma.

[star882]

A few fluorescent supplies are isolated to reduce capacitance losses. Isolation is irrelevant for safety as the output is still high voltage at a significant current.

[Eva]

Fluorescent lamps have been traditionally not isolated, and electronic ballasts take advantage of that, at least the ones used for big lamps, 18W, 36W, 56W, etc...

12V halogen lighting has been traditionally isolated, and all the electronic transformers that I have seen are isolated, but sometimes creepages and clearances are not met.

For example, a few months ago I bought several electronic transformers from China on ebay at a very good price (for a new house), and interestingly construction was very good, including a metal case as a heatsink, and design was not bad resulting in little temperature rise, but clearances and crrepages were 2 to 3mm at best, not 6-7mm. That was not a problem in my opinion, since lamp and wiring provide additional isolation.

The main problem when using these electronic transformers for other purposes is that they are self oscillating, switching frequency depends on instantaneous input voltage, and they don't use any storage capacitance or regulation, the output just follows the envelope of rectified mains input, and it's not rectified either, it's usually 10 to 50Khz AC, since this does not make any difference for the lamp.

Old AT PC power supplies have much more potential for modifications, they are not very hard to convert into symmetrical supplies for amplifiers. They primary side is usually always ok (with EMI filtering and all in place and proper clearances), and the TL494 control IC is in place too. Only transformer windings, output inductor windings and output section of PCB need changes.