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R.G.'s Transformer Basic Hookup Info

Started by RDV, May 11, 2006, 11:35:55 AM

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First of all, tranformers gots polarities. See those dots on the transformer schematic? All the leads with dots go positive (and negative, in turn) together.

Each winding used as a primary needs to be driven with its rated voltage. In the case of the dual 115V ones, you connect dot to dot, non-dot to non-dot, effectively paralleling the primaries.

If you get one winding reversed, you will see major smoke when you power it.

The transformer's rating is based on having enough amperes flowing into the core so the secondaries can take their designed current out. Dual primaries are made half the otherwise-required wire size each, so to get full power out, you must have the designed current flowing in each primary winding. You do this by paralleling for 115, or series for 230. Both situations give you equal current in each winding, and contribute each primary's fair share of ampere-turns to the M-field inside.

If you series two windings, you must connect a dot to a non-dot, and then put voltage across the series setup. again, if you connect it backwards, major smoke pours out, and the transformer is itself tranformed into a Darkness Emitting Diode (also called a DED).

You cannot get more power out of a tranformer than it's designed for by any means except running it at a higher frequency. So, no, hooking each secondary up individually will not help. What the spec is telling you is that each 18V secondary will do 1/2 of the specified 18V current when they are paralleled; this is the same current they will supply in series, which is shown as 36Vct. Same voltages, same currents, hooked in series instead of parallel.

You want primaries parallel, secondaries in series for your amp, since the LM3886 is designed for +/- power supplies.

I prefer a 42Vct 2A transformer for the 3886. Parts express used to sell one of these for $15, but I can't find it at their site now. Even better is a 115/115/22/22 toroid. I got a few of these on ebay.
R. G. Keen