Modifying an ATX SMPS main transformer - Page 1
Forum section: electronics / projects

author: AQAE
author_rank: Newbie
timestamp: September 20, 2025, 06:15:18 am
content: Hello everybody. First posting here.First of all, I'm glad being here discussing electronics with you all electronicers!Jumping to the question real quick, I'm modifying an ATX PSU to make it work like a bench PSU using attached schema. But I don't feel good about how it wired up the secondary side of the main transformer. My transformer picture and schema is also attached. I'm going to use the 5 volts winding but don't know what to do with the not using 12v. Leaving it floating is not a good idea I believe neither hard or soft shot it. I was thinking to rewind it to eliminate the 12v completely but I don't wanna break the transformer.

author: magic
author_rank: Super Contributor
timestamp: September 20, 2025, 06:38:05 am
content: The schematic seems to show using two 12V windings in series for a 24V output. Then output voltage is further increased to 30V by manipulating TL494 feedback, but this is dangerous because it increases stress on Q1, Q2.Switching to 5V windings would make sense if the PSU will have lower maximum output voltage than 30V, but high current. Then 12V windings can be left floating, I guess, what's wrong with it?

author: AQAE
author_rank: Newbie
timestamp: September 20, 2025, 09:04:12 am
content: So you say I go through the two 12 volts for the maximum 24 volts output and leave the 5 volts winding not connected right? That makes sense though.

author: AQAE
author_rank: Newbie
timestamp: September 20, 2025, 09:17:10 am
content: Do you have any advice on the Schematic?

author: Ian.M
author_rank: Super Contributor
timestamp: September 20, 2025, 11:09:19 am
content: Would such a PSU (old enough to use a TL494 controller) even have two high current 12V windings? -12V was always a fairly low current rail.The schematic clearly shows the modification to three secondaries wired in series - presumably the high current ones feeding the +12V, +5V and +3.3V rails, for about 21V at nominal duty cycle. If you only use the 5V one you will seriously limit the max output voltage you can get, probably to around 10V.Also an ATX PSU using a TL494 is probably so old its in serious need of full recapping with new electrolytics if you want it to last . . .

author: AQAE
author_rank: Newbie
timestamp: September 20, 2025, 11:29:49 am
content: Quote from: Ian.M on September 20, 2025, 11:09:19 amYes it has. Two windings for 5v and 12. Other rails are driven from these two.QuoteThe 5v and 12v are already in series. First I thought I should use 5v. But turned out that it is 12v windings to use.QuoteI'm going to use 12vQuoteIt's already done.

author: AQAE
author_rank: Newbie
timestamp: September 20, 2025, 11:35:14 am
content: But out of curiosity, if I wanted to use 5v winding, was it ok to leave the 12v windings floating?

author: Ian.M
author_rank: Super Contributor
timestamp: September 20, 2025, 11:46:07 am
content: Except in the cases of current transformers and very high voltage transformers, which both may require a minimum load, you can always leave totally unused transformer secondaries open-circuit.