inverter idea

[vivi-d]

Would it make sense to build an inverter with a 5v transformer used in reverse? Possibly driving the (now primary) with a PLL make out of high current op-amps?

[BeBuLamar]

What do you mean by using a 5V transformer in reverse? You mean a stepdown transformer that is designed to step down from 120V or 220V to 5V and you want to use it to step up? I tried and it doesn't work. It draws a lot of current even with open circuit on the output.

[vivi-d]

Yes... looking at the math it seems hopeless.

[golden_labels]

Not only you can, but this is how inverters are built.

However, with 5 V primary and 120 V secondary, the primary will need  to handle current 24× bigger than the secondary. Both supplying and controlling current that big is not trivial. Unless you want to draw milliamps on the high-voltage side, first study schematics of existing inverters. And understand limitations of components invovlved.

Normally inverters start with a higher DC voltage (±12V, ±24V) to reduce the problem of the high current.

[BeBuLamar]

If you have a step up transformer designed to step up from 5V to 120V it would be OK. But when I use a step down in reverse it doesn't work right. It draws very high current even though the output is not connected to anything. At one time I simply want to generate relatively high voltage AC like 120 to 250 etc.. to test a multimeter in that range. The multimeter is essentially a no load but when even when I connect that to the output of a 110W power amplifier that capable of driving down to 2 ohms impedance load the amp had a hard time doing that. So using a transformer in reverse (that is using the secondary as primary and primary as secondary) doesn't work although I really don't know why.

[Analog Kid]

And yet a lot of (presumably working) inverter circuits found online use just that, a step-down transformer used in reverse (usually 12 volt ones, though).

A 5 volt primary sounds like a little too much of a load here.

[Xena E]

It will work, but the caveat is that there's likely to be a lack of primary inductance, and wasted current draw due to core saturation.

IF, you are driving the transformer with a sine wave, and are  able to use a higher frequency things get a lot simpler. Iron laminated power transformers will often be most suited to perhaps 10 times their design frequency when used in reverse.

Also, a high magnetising current can often be tuned out by literally just connecting a capacitor across either the primary or secondary, (or combination of both).

There are several factors that govern the choice of this resonating capacitor, but it is often most satisfactory to just decide on a value empirically for any given frequency. A recent example I was involved with had a 6 Volt supply driving the transformer at 600Hz via a bridge output stage, and that needed about 5μF to compensate the magnetising current of the particular transformer being used. YMMV.

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