# Series Connected Secondary Transformer

I have a multiple-winding transformer with the following configuration:

• primary: 230V
• secondary: 2x 10-12-15V
• frequency: 50Hz
• power : 66VA
• current: 2x 2.2A

As far as I can understand from its specs the connection should be done like this:

• the primary coil should be connected to the main's AC by using the pair 4-7
• for the secondary coils we can connect the pin as following:
• to get a 10VAC we may use the pair 9-12 or pair 2-17
• to get a 12VAC we may use the pair 9-13 or pair 2-18
• to get a 15VAC we may use the pair 9-14 or pair 2-19

I think it's obvious how to get a 10VAC or 2x10VAC from this transformer, right? The same apply for the 12/15 VAC outputs.

I know that in general we can connect these secondaries either in series or in parallel (thus increasing the secondary voltage or the current rate).

So my questions are:

1. I'm wondering if it's safe to assume that by connecting the pair 9-13 in series with 2-18 I could expect to get a 24VAC. The same for 9-14 with 2-19 which should give a 30VAC.
2. It is safe to assume that by connecting the pin 9 to 13 then 13 to pin 2 I would get a 24VAC between pins 9-18 ?

Assuming that the both questions above would get a YES answer, what else should I know before doing that? Any tips?

• You should trust your multimeter when connecting two windings in series. If you can measure 12 V AC between point 9 and 13 as well as between 2 and 18 and 24 V for both windings in series you connected the windings in the right order. If you measure a voltage lower than some 1 V for the series connection, you just have to reverse one winding. If you connect point 13 to point 2 and you measure 24 V between points 9 and 18, all is well. If you measure a much lower voltage between point 9 and 18, try to connect point 13 to point 18 and measure between point 9 and 2.
– Uwe
Jun 1, 2017 at 14:34
• Yes, I already did that after Andy's answer. I can confirm that the 20-24-30VAC outputs obtained by series connecting the alike winding work. Obviously it would work also by connecting different tap points like 10V-12V to get a 22VAC or a 12V-15V to get a 24VAC, etc. Jun 1, 2017 at 15:10

I'm wondering if it's safe to assume that by connecting the pair 9-13 in series with 2-18 I could expect to get a 24VAC. The same for 9-14 with 2-19 which should give a 30VAC.

I think it's a safe bet that if you series connected the windings you get twice the voltage so, for instance 9-13 in series with 2-18 would give you 24 V AC. Ditto for 9-14 and 2-19 to give 30 V AC.

It is safe to assume that by connecting the pin 9 to 13 then 13 to pin 2 I would get a 24VAC between pins 9-18 ?

You cannot connect pin 9 to pin 13 under any circumstances (unless you want to burn out a winding).

• You have right, by connection 9 to 13 the respective winding would be a short circuit. My HUGE mistake. Thanks for pointing this out. The correct path would be 9-13-2-18 which would mean to connect 13-2 and measure the output between 9-18. Is that correct? Jun 1, 2017 at 13:07
• Yes that would be correct and is the right way to go - plenty of folk do this with dual output secondaries. Jun 1, 2017 at 13:13
• Now what if I want to get both, a 24VAC (by series connecting the 12-12VAC secondaries) and a 30VAC (by series connecting the 15-15VAC secondaries) at the same time? Any idea if that's possible? Jun 1, 2017 at 13:31
• No, I don't think that is possible with the transformer you have unless the common link between the two secondaries was from pin 9 to pin 2 (as opposed to pin 14 to pin 2). The label gives a strong indication that the common link is from 14 to 2 but, if you can dig-up a data sheet for it then it might say things differently. Jun 1, 2017 at 14:09
• The datasheet doesn't say much (at least not to me): goo.gl/kVujkV Jun 1, 2017 at 14:18