Any technical reason not to use isolation transformer to make DC?

Nope. You can also use the two primaries of any 120/240 sort of power transformer as input and output windings.

Check that the isolation transformer is really 1:1. It's posible that some might not be.

John

Thanks. Just to follow up, I'm trying to avoid being second-guessed over this on technical grounds. Given the requirements for isolation transformers, it seems if anything that they might be over-designed rather than under-designed for use in the way I'm proposing, but I'm hoping my question will smoke out (bad choice of words?!) any gotchas.

Mike

One possible reason...Money! It depends on your application. For example, there is little reason to use an Isolation Transformer for driving a DC Motor. Such a transformer would add significantly to the cost.

Like all transformers the unloaded output will be higher, and some isolation transformers use a bit higher than 1:1, like 1:1.1 or something like that.

greg

e,

Money actually is my reason. I do need the isolation of a transformer, and isolation transformers in my VA range are much cheaper than any other type with about the same ratings. I don't know if that's just an issue of volume or what.

Mike

Nominal output unloaded will be around 170VDC, maybe a fair bit more if the isolation transformer is wound to give nominal output at full output current, so I hope you are flexible on your 160VDC upper limit. You can always buck the input or output voltage down with another (low voltage) transfomer (eg. 12VAC or 24VAC). Best regards, Spehro Pefhany

Any 'off-line switcher' supply has a bridge-diode-cap input to the hi freq chopper and hi freq transformer, so you do get magnetic isolation by using a switcher power supply.

Hi, Mike -

As for gotchas:

Use full wave rectification to minimize DC in the transformer.

Note that the transformer RMS current can be about twice the DC output current value depending on your selection of filter capacitor value, transformer impedance, and other factors. Peak currents can be several times the DC value. Do a SPICE simulation to see these effects.

Cheers, John

Yes, definitely. Diodes are cheap!

es

Right you are. I've been using these figures

Mike

mple,

Yes, I'm flexible as to the exact output voltage. The transformers I've looked at also all have a step-up ratio to compensate for losses. Turn them around and you get only around 100V at full load.

Mike

If this connects to mains you should use one unless the application is totally encapsulated and on its own (no external connections). One reason is that even in the US it happens that electricians miswire an outlet. IOW what is supposed to be neutral is hot, you connect something ... KABOOM.

The designs of the isolation transformer and the "purpose-designed" power transformer are different. The power transformer is designed to accommodate the kinds of peak short current conduction phase angles that you have with rectification while the isolation transformer is not. Therefore, if you intend to drive the usual rectifier filter combination for a DC output, the power transformer is the one to use.

What exactly are the internal differences? Suppose I choose the isolation transformer such that it stays under a reasonable maximum temperature (the same temperature it would run at in isolation duty) - any reason to avoid it in that case?

So what happens if he buys the 'power transformer' and then decided to work on the equipment with the 'isolation transformer' in front of it for safety?

Will those current peaks be seen by the isolation transformer in front, or are they contained to the secondary of the 'power transformer' ?

Even if it doesn't seem them in this situation, wouldn't the isolation transformer see them if it were used to troubleshoot a system that really needed it - something that put the rectifier right on the inputs with no isolation?

If you are indirectly asking if it is okay to use an isolation transformer that you have on hand, then sure, if it does the job without overheating, it is okay, the safety performance should be the same. The technical differences are profound and complicated, with the goal of getting an optimum design to the customer.

Yeah-that's why you would need to use an isolation transformer of a slightly higher rating than the power transformer...increase its VA rating enough and the high crest factor is a non-issue.

You misunderstand the question, it pertains to using an isolation versus a rectifier transformer.

Oh, right. I guess a transformer is a transformer, whether it's called iso transformer or regular power transformer.