Figuring out wiring of an old transformer - Page 1
Forum section: electronics / projects

author: lausvi
author_rank: Unknown
timestamp: June 09, 2025, 04:15:48 pm
content: Hi all.Years ago I was given a (very self-made) project step-down mains transformer (240->120). The outer casing was falling apart so I discarded the case but kept the transformer hoping to eventually build it a better case. I've now found a very neat 70's (orange!) case with a handle that might work. However now looking at the wiring of the transformer, I am not quite sure I am following how it is wired. (I have another big isolated variable transformer, but I'd like to have a smaller unit for the occasional testing of 120V wired devices instead).The transformer is about 10cm x 10cm and weights several kgs. Old but looks to be in reasonable condition. It has a label (in finnish) that gives a non-informative model number ("M2") and states only that "PRIMARY 110, 110" and "SECONDARY 8 220". Does that mean secondaries of 8vAC and 220vAC? However there are 17 wires coming out and I am not quite following what's going on.The wiring is currently as it was when I got the unit and was only told that it works as a step-down transformer. The mains goes in to the outer yellow wires. The middle two yellow wires are shorted together and used as the 120V output live. And thats indeed what comes out.As the neutrals (blue wires) are connected together, I take it that this is wired as an autotransformer? However, if the label means that there is a secondary winding of 220 and two primary windings of 110, shouldn't it be possible to wire this as a proper 240V->120V isolation transformer instead?Also; I am troubled why the wires currently enter and exit from the same side. If it goes over the transformer, shouldn't the output wiring be on the other side? Or is this simply a matter of wiring and the wires are just put evenly to the two sides for spacing?While wired in the current configuration, the other side of the transformer (4x green, 2x dark blue, 3x orange) shows only small AC voltages (20, 20, 25+25 or 50 over the outer orange wires). Between the two dark blue there is 9vAC, which might be the 8 from the label?The 3x green on the input side give 125+125 between the center tap and outer taps, or 250 between the outer taps. As the pins are more spaced out, would it make sense that this would be the 220 secondary, and then the 4x wires would be the two 110 primaries?

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 09, 2025, 11:15:08 pm
content: Quote from: lausvi on June 09, 2025, 04:15:48 pmI guess that marking is actually for a whole device, but the transformer itself seems to have a bit more windings.Your task is to find primary windings (seems they are two 110VAC in series for 220 VAC), (or those you want to be primary), so others are secondary.

author: moffy
author_rank: Super Contributor
timestamp: June 10, 2025, 12:30:20 am
content: You should be able to use the 220V winding as your primary and the 110V as your secondary, just note that you don't exceed the current ratings of the respective windings.

author: lausvi
author_rank: Unknown
timestamp: June 10, 2025, 04:17:25 pm
content: Thanks. I carefully re-wired and tested (using a variac to first check with lower voltages that everything works as expected) and it's indeed working.Is there an easy way to ballpark the current rating for the output winding?I don't dare to show the wiring yet, I still need to drill some more mount holes to properly mount the transformer and install a fuse holder etc until it's fully presentable. But I do like how it looks. In that orange caseThis used to be a strobe light, a separate unit from some 70's (there is a sticker inside that says 1977) school demo setup. Made by "Esselte Studium" in Sweden. The case is sturdy enough that it should hold the transformer nicely.I re-used the two cable glands and the original power switch on the rear panel. If there is enough space I might get a panel mounted output socket and mount it to the front. Although I quite like the window into the transformer, perhaps I'll need to add some lights

author: prosper
author_rank: Regular Contributor
timestamp: June 10, 2025, 05:39:49 pm
content: you can get a rough estimate of a transformer's power handling capabilities by weighing it. Somewhere in the ballpark of 50VA per kilo is a reasonable guess.

author: moffy
author_rank: Super Contributor
timestamp: June 10, 2025, 11:17:24 pm
content: Quote from: lausvi on June 10, 2025, 04:17:25 pmYou need to estimate/measure the conductor cross sectional area for each winding you are using and its current capacity, but transformers are limited among other things by temperature rise and insulation breakdown.

author: lausvi
author_rank: Unknown
timestamp: June 14, 2025, 08:30:57 pm
content: The wiring coming in/out of the transformer seems to be 0.75-1mm (diameter, copper itself measured from a bit that was wisible between the posts and the sheath, didn't want to desolder) so it should be pretty beefy? I added a primary and secondary side fuses, I put in a 1.75A for now, as I intended this to be only a smaller, more portable 220/110 unit (vs. my bigger variac).I drilled new holes for better mounting the transformer and re-did the wiring. I found from my parts and old 220V neon-lamp and added that as a power indicator. The thing with orange continuing, and I like the glow