Frosty_47

Hello, I would like to know what alternatives there are to the use of step down transformer. Say, I needed to transform 120VAC to 24VAC and I have very high current requirement (30Amps as an example). The use of such transformer will be highly expensive + the overall weight and size of such transformer would be 2 impractical. What alternatives are there?

Thanks

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dknguyen

Not many really...much else would be more expensive and lower capacity...but possibly lighter or smaller like a large boost converter. Though the size of the inductor might be very large anyways.

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crutschow

If you don't need isolation between the 120vac and the 24vac you might consider an autotransformer. They are smaller and cheaper since they only have one winding (hence the lack of isolation). The voltage is tapped off the winding, sort of like a potentiometer.

Fixed autotransformers are not common but a transformer manufacturer may have them or can build one for you if you have the budget. A common variable autotransformer is a Variac.

gerty

One thing that I've done in the past was to use a 480/120 power step down transformer. Yes it's still bulky/heavy, but I needed a high current low voltage source like you. With 120v on the primary (480 terminals)you should get about 30v on the secondary...I don't know what the voltages are in your area, so this may not be possible...

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RODALCO

You need to look at a switch mode power supply, although these will be very expensive, in larger sizes.

I fitted a 750 watts 240 / 24 Volts SMPS yesterday in one of our substations to support the scada system.
That one is rated at 30 Amps. load was around 25 Amps initially, then tapered off to a steady load of 15 Amps.

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Leftyretro

Except he specified needing 24v AC, not aware of any SMPS that provides 60/50 Hz AC output

I can't think of an effcient method of generating 23vac at 30 amps that would not involve a transformer either in it's input stage or output stage if an invertor is tried.

Lefty

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crutschow

If here were a cheaper alternate to a transformer for converting AC voltages, the power companies would be using it.

Papabravo

There must be some kind of educational deficiency out there because this question keeps coming up and the answer is almost always the same.

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Frosty_47

I agree with PapaBravo on "Education Deficiency" as really none of my professors or any electrical people I know seem to have an answer for this. I tried studying switching PSU's however; I found no mention of this topic. Switch mode are basically more efficient but they still require a stable DC input voltage in order to work thus the need for transformer again.

What about Computer PSU's ? Mine is ratted for almost 80A combined and hell, there are no bulky transformers inside of it. Just a few bifilar chokes, inductors, and one very weird, multi-winding, tiny transformer that I would give no more than 1A ratting...

Any more ideas any one? I am seriously fed up with google giving me bunch of useless crap that’s non-related to the desired topic...

Thanks

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Frosty_47

Thank you for that info, I think electrosonic has some variac's. I will definatly do more search on that....

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mneary

At the high frequencies that they use, these 'tiny' transformers may provide 500W++ at any voltage that they are designed for.

Frosty_47

Like look at this S41t ! This is the kind of garbage I get when I type in "Transformer Alternatives" in google. Here is the very first result:

http://thetransformer.us/

These morons don't even know what a transformer is yet they name their site with it. Wish those people who registered the above domain, get hooked up to the secondary of a microvave oven transformer
for a period T --> ∞

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Frosty_47

500Watts for that transformer ? WOW! But how do they step up or generate high frequency without stepping voltage down in the first place ? And even if they do use HV components, wouldn't they require a DC source to operate ? and those timers or w/e they using to generate High Frequency be limmitted to very little current ?

Sorry, I am not trying to go down hard on you, I just realy whish to know more... If you know any good articles, can you please provide a link...

Thanks

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Nigel Goodwin

Search for 'switch more power supply'.

Essentially they rectify and smooth the mains, giving 320V DC in the UK, in the USA they usually use a voltage doubler to give a similar DC voltage. This voltage is then chopped at a high frequency (between roughly 15KHz to 100KHz) and fed to a very small transformer, the output of the transformer is rectified and smoothed, where you can use MUCH smaller capacitors because of the higher frequency. The output voltage is monitored, and correction signals sent back to the chopper to alter it's mark/space ratio and keep the output constant.

Incidently, the worlds first domestic use of a SMPSU was in the Thorn 3000 colour TV about 1972 or so?.

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Optikon

The physical size of a transformer is governed by its losses, not the power that is "transformed" from input to output. Losses typically consist of resistive, eddy current & core (magnetic). Controlling these in your design will determine your maximum power rating.



A typical "offline" switchmode converter will first rectify and smooth the line thus creating a high voltage DC. From there, a PWM method is used on a flyback transformer to generate the right output (typically)