Transformer Help

[tomchr]

If you want to use a voltage regulator, you can use an LM317 with a handful of external components to create a high-voltage regulator (National Semiconductor App Note (pdf)). You'll probably need more current than this can provide, but that's easy with a bipolar pass transistor (good ol' 2N3055 comes to mind).

Just remember that your voltage regulator needs to be able to handle sqrt(2)*VAC, where VAC is the no-load voltage of the transformer at maximum AC input voltage.

Tom

[pacificblue]

Originally posted by kscharf
You can connect the output of a smaller transformer in series with the primary to reduce the input voltage though.

Remember that a transformer is designed for a certain current. If you reduce the primary voltage, while the current remains the same, you get half the power rating. At the same time you need twice the space for two transformers.
If you buy a new transformer, it makes sense to buy one with the correct output voltage for the amplifier and use the 2x40 V for a different project.

[Redshift187]

Originally posted by kscharf
Since this transformer has a dual primary if you connect the windings in series and run the transformer off 120v you will end up with a 20-0-20 volt secondary (half) which will give you 28 vdc.

Is there any reason this is a bad idea?

[pacificblue]

Originally posted by Redshift187
Is there any reason this is a bad idea?

See post #12

[kscharf]

Originally posted by Redshift187



Is there any reason this is a bad idea?

The VA rating of the transformer core remains the same even if you half the primary voltage. BUT, the current rating of the primary and secondary windings are based on the VA rating at the rated voltage. This means that the VA rating of the transformer is reduced to about half of it's original rating at half the input voltage. You can probably pull a little more than that though, because you won't be saturating the core (which can handle the original VA rating). The limit will be due to heating of the primary and secondary windings as they are asked to handle more than their original current ratings. Monitor the output voltage of the secondary under load, and limit the current to that drawn when the output voltage drops to one half the rated output voltage (20 volts).

[Redshift187]

Ok, now I get it. I was having a slow moment

[Artie]

Originally posted by peranders

We are talking about a PSU for power amps and it's rather unusual to have stabilized supply voltage.

Hmmm . . . thats interesting just from the perspective that I don't think I've ever seen an audio amp, (pro or consumer), that didn't use a regulated PS.

I guess I'll just continue to read and learn.

[robbobnbill]

Thanks for your input guys, think ill stick with keeping the amp simple, ill bite the bullet and get the right transformer