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If a toroidal power supply transformer says for example "15-0-15V 1A" on its label does that mean it's 1 amp for each rail, or 1 amp for total 30 volt output when not using the split? I looked online but couldn't find a definitive answer to this. Thanks.

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    \$\begingroup\$ 1 amp for each rail, or 1 amp for total 30 volt output In my opinion, that is the same. This is a 30 VA transformer so it is 2 x 15 V at 1 A or 1 x 30 V at 1 A. \$\endgroup\$ – Bimpelrekkie Sep 15 '16 at 14:34
  • \$\begingroup\$ Great news to hear, so that means if I use a full wave rectifier (two diodes) to use it as a single rail supply I would have 15 V+ at 2A I believe? \$\endgroup\$ – Kage Sep 15 '16 at 14:50
  • \$\begingroup\$ @Kage if you filter it with a capacitor you will have about 20V at 1A maximum (20W). 2A (40W) will likely burn out your 30VA transformer in fairly short order. \$\endgroup\$ – Spehro Pefhany Sep 15 '16 at 14:55
  • \$\begingroup\$ Yes, that is possible. Just do not exceed the 30 VA. \$\endgroup\$ – Bimpelrekkie Sep 15 '16 at 14:55
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"15-0-15V 1A" means 1 A is the most the wire is good for. You must ensure that no more than 1 A goes thru any of the three leads.

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    \$\begingroup\$ Did you mean 1A, not 15A? \$\endgroup\$ – HandyHowie Sep 15 '16 at 14:40
  • \$\begingroup\$ Olin where the 15A come from. I see only 1 A. An oversight? \$\endgroup\$ – Decapod Sep 15 '16 at 14:40
  • \$\begingroup\$ @Handy: Oops, fixed. \$\endgroup\$ – Olin Lathrop Sep 15 '16 at 14:42
  • \$\begingroup\$ Still one 15 A there \$\endgroup\$ – Bimpelrekkie Sep 15 '16 at 14:42
  • \$\begingroup\$ @Fake: OK, I think I got them all now. \$\endgroup\$ – Olin Lathrop Sep 15 '16 at 14:43
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You are talking about 'rails' which makes me think this is a transformer-rectifier-filter capacitor question.

If you build an audio amplifier supply using that centre-tapped transformer and four diodes (or a bridge) and a filter capacitor you will get about +/-20V.

The maximum DC current you can safely draw is 620mA from +20 to -20 (25W).


If you use two diodes and a filter capacitor you will get a single rail of about 20VDC at 1A maximum (20W).

enter image description here


The transformer rating itself is based on a resistive load and is 1A RMS for the total winding (30VAC) so 30VA. When the currents from each end of the winding are equal, the center tap current is zero. If you use only 1/2 the winding you should not exceed 1A, so in that case you would only be able to get 15VA from the transformer.

In practice you could probably get a bit more because the heating from the second half of the winding is not there, but not a lot since copper heating rises with the square of current. 18VA might be safe. Maximum DC current from a rectifier/capacitor will be commensurately less.

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  • \$\begingroup\$ In my opinion the Idc = 1 x Iac_sec is a bit conservative, there's only a current flowing 50% of the time so if you would average that you could draw 2 x Iac_sec and still not exceed the transformer's maximum power rating. But if you never want to exceed the wire's current capability then indeed it is 1x Iac_sec. It depends on personal preference/safety margin what you prefer. \$\endgroup\$ – Bimpelrekkie Sep 15 '16 at 15:02
  • \$\begingroup\$ @FakeMoustache Current in a rectifier-filter cap circuit is not a simple current, it is drawn in sharp peaks at the top of the AC cycle, so the RMS current is much higher than the output DC current. Copper heating is a result of I^2*R so the copper heating is much higher than the DC output current might imply. This is not conservative. You can reduce the RMS current by using an large inductor in the filter but that is not usually cost-effective because the added core and coil is as costly as using a larger transformer. \$\endgroup\$ – Spehro Pefhany Sep 15 '16 at 15:05

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