# Transformer: Center-tap out of two windings + amperage

#### Ticala

Joined Apr 16, 2019
3
Hi,

My first post here!

I know I can utilize two secondary windings in series to create a center-tap. What is the advantage of this over a single winding with a CT? I'm asking because I'm planning to replace a 120 V transformer (alongside the step-down one) with a 230 V in an audio effects processor and I can easily buy 2-winding transformer while I'd have to have the one with a CT made for me.

Also, how much should I care for amperage of the new transformer? The unit takes 8 W and the fuse is 0.5 A / 250 V.

#### drc_567

Joined Dec 29, 2008
1,156
... two transformers ... secondarys in series sounds reasonable ... just be sure to verify that the two secondary side voltages are in fact the same numerical voltage as that which the original center tapped transformer provided. That is, you must actually take a voltmeter and measure the new assembly to be sure that everything is correct. ... A previous post described small wattage transformers as bi-filar wound, so the polarity of the transformers should not be an issue. ... Still, it would be a good idea to check the over-all secondary voltage as well as the individual coil voltages before actually attaching the load circuitry.
... The wattage rating should not be an issue at that level.

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#### Ticala

Joined Apr 16, 2019
3
Thank you! Yes, I'm going to check things meticulously - I don't want to burn my whole unit.
As far as wattage rating is concerned, what is the overhear that I should be aiming at for the transformer? 10 W, 15 W, 20 W?
And what about the amperage? How can I find out?

#### drc_567

Joined Dec 29, 2008
1,156
The secondary amps is determined by the ratio of watts to load voltage ... e.g. if your load is 8 watts, and the required load voltage is maybe 12 volts AC, then the arithmetic amp value is 8/12= 0.67 amps. However, it is customary design practice to allow a larger wattage transformer than necessary in order to avoid heating and any related problem. A nominal safely rated transformer for your 8 watt load might be 50 volt-amps ... essentially 50 watts, which should be easy to find at a store or on-line. This is more capacity than necessary, but should produce little excess heat.
... One of the fundamental transformer principles is that the product of volts * amps on the primary side should always equal the product of volts * amps on the secondary side. As a result, the amps on the high voltage primary side will be much less than the amps value on the low voltage secondary side ... using RMS AC measurements.
... Placing a fast-acting fuse on the secondary side, its size determined by the voltage, should help to protect your audio processor if there is an unforseen short circuit. The 0.5 amp fuse mentioned previously is probably located on the primary side.

#### Ticala

Joined Apr 16, 2019
3
@drc_567 Thanks a lot! You've been very helpful.