using transformer 2nd primary as secondary? - Page 1
Forum section: electronics / beginners

author: phums
author_rank: Newbie
timestamp: April 03, 2014, 05:27:49 pm
content: Hi all,(First time poster.)I have a question about power transformers. Specifically, if a transformer has two primary windings, is it possible to connect one of them to use as a secondary?The transformer i'm looking at (Triad vps36-3600) has two primaries (115, 115) and two secondaries (18, 18). Can i connect mains 115v to one primary and get 115v out of the other primary, and also get 18v out of the secondaries?The datasheet says "windings are not intended to be used independently," but is this a "valid" usage in the real world?It would make things simpler and smaller if i could get away with using a single transformer instead of separate ones for 115 and 18v.Thanks!

author: mariush
author_rank: Super Contributor
timestamp: April 03, 2014, 05:36:49 pm
content: No. Learn about how transformers work.If you connect just one 115v winding, you'll get bad output on the secondary side. You won't get the proper voltage, or not as much current as the transformer is rated and other issues. Connect those windings in parallel.

author: Holograph
author_rank: Regular Contributor
timestamp: April 03, 2014, 05:47:59 pm
content: Yes, you can, but you may not be able to get the full amount of power the transformer was designed for. Usually dual primaries are there so you can either run 115V or 230V depending on how you use them. If the wire gauge of the primary coils is enough such that one coil, powered by 115V, can handle as much or more current than the transformer can power, you'll be fine as long as you stay within the transformer's power handling (else you'll saturate it and your voltage will drop).If the primary coils are specced such that you need to use both coils to supply the power the transformer you can handle AND you want to actually use that amount of power, then this transformer will not work for this purpose. Since you say it's specced for using both primary coils, this is probably the case. You would need a transformer designed to handle at least twice the amount of power you want, or you need to find a transformer which can handle its full amount of power over just one of two primary coils (again, this just means the wire gauge has to be thick enough).BTW, check out Antek transformers. Inexpensive, if you're in the US. Not sure about shipping costs to elsewhere.

author: N2IXK
author_rank: Frequent Contributor
timestamp: April 03, 2014, 05:54:59 pm
content: In addition to needing to severely derate the transformer, the 2 primary windings are probably wound together on the same side of the insulating bobbin. So the insulation breakdown voltage/creepage distance between the 2 halves will likely be much lower than between the primary and a proper secondary.

author: Holograph
author_rank: Regular Contributor
timestamp: April 03, 2014, 06:15:06 pm
content: Quote from: N2IXK on April 03, 2014, 05:54:59 pmWill somewhat depend on the construction of the specific transformer - I usually deal with toroids which are a bit simpler construction than EI and such. However, touche overall - this should also be considered.