[georgeburns]

Newbie making a power supply.

I have a 400VA transformer outputting about 50 volts at around 8 amps.

I'm making a case to house it along with the electronics. I'm cooling the motor stepper drivers and the rectifier with a fan cooled aluminum heat sink.

I'm currently planning to have all the airflow be external to the box.

Will the transformer need a fan over it, or is it going to be okay in a sealed enclosure with no fan?

[JoeFin]

Check the specifications of your transformer.

Most are rated to supply 100% load with a 40C rise over ambient temp. Calculate your load requirements, they should be no more then 80% of FLA of the transformer.

If by chance you have other electronics inside this same "box" with the transformer, you will most likely want to use a fan. ICs typically start dropping out around 150F. Manufactures specifications rarely state recommended operating temps above 100F

[MicroMill]

Yes, they can get hot. Sometimes a simple open baffle above the transformer will let enough heat escape in a convection to dissipate unnecessary heat build up. As the previous gentleman suggested, be aware of the other components (diodes, capacitors, etc) that may be adversely affected by generated heat. BTW, the heat generated is produced by the ineffiency of the transformer (not that you can do anything about that), so oversizing (VA) the transformer for the application, is helpful to some degree.

[vulcom1]

Along with the rest that is mentioned, have you actually taken an amp meter and measured the draw on the transformer. You are working at the maximum now and you should be using a larger transformer. The other question I have as I have done this before is how hot did it get. If it got hot on the outside, the inside could have melted the insulation deep on some layers. That will also build your heat and it then just gets worse until you can really smell it or the stuff just oozes out.
John

[MicroMill]

Good Point! Measure the "actual" current draw from the transformer. Of course you will need an AC ammeter to do this. The 50 V @ 8 amp is the rated output for your 400 VA transformer. That is running at something just under 80% duty. If your actual is say 6 amps, then you are using 300 VA (50 * 6). It sounds like you are still in the build stage and haven't really hammered it yet.

[martinw]

Dear georgeburns,

1) If your transformer is a toroid, it may be 90% plus efficient. If laminated, say 85%.

2) If it is "working flat out", it will be delivering about 400 watts of electrical energy. It will also be producing about 10 to 15% of heat as it does so.

3) Without quibbling about the maths, you have about 40 watts plus of heat to deal with.

4) Whatever you imagine your transformer enclosure to be, would you be happy putting a 40 watt light bulb in there? My guess is that a "sealed enclosure" is not a good idea, unless large in comparison with the transformer.

5) Put in ventilation grilles/perforated plates, (like your stereo amp) and you should be OK.

my two cents,

Best wishes,

Martin

[steve323]

We used to sit on large industrial transformers to keep warm in an electronics test floor that was kept at about 60F. They took the chill off.

Think of a transformer as similar to a stepper motor with no moving parts. Stepper motors can get really hot without suffering damage. Transformers can probably operate up to the point of melting the insulation on the wiring. Google any transformer manufacturer and get an idea of rated temperatures.

I agree with Martin, that a few vent holes is probably sufficient.

Steve

[martinw]

Dear Steve,

Actually, IMVVHO, lots and lots of holes. Like perforated metal all round.

Remember about the 40 watt lamp analogy.

Best wishes,

Martin