Bench AC supply - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: September 09, 2025, 01:06:35 pm
content: I have a small transformer I want to make use of. It was purchased for "Power supply experiments" and served it's purpose at creating a fixed +/-15V supply.... that I don't need.I figured the transformer might be repurposed as a experimental AC supply.It's not exactly a complicated device, the only real components will be the transformer and a mainside switch.It is however a mains device with lethal voltage (240VAC). So respect it required.The transformer I have is center tapped +/-17V and something small like 0.1VA IIRC.I already have it wired to a fused and switched IEC socket at the input. My PSU experiment then followed it with a 100mA fast blow fuse. Then the transformer.If I am to use transformer 3 connections as 4mm banana outputs, I expect I might want some protection:1. On the load side - maybe a basic resetable fuse? Hard fuses for the secondary current rating?2. On the main side - a fuller set of filter caps and movs, possible choke - to stop noise from my experiments getting back onto the mains.I have a project box to put it all in, so it will be at least single insulated.If you could pick a "middle ground" between "perfect" and "lash up" reference circuit, what would you point me to?

author: paulca
author_rank: Super Contributor
timestamp: September 09, 2025, 01:19:49 pm
content: BTW, I have a competing run of thought which says, "The signal gen puts out more power than that transformer and it's easier and safer to use"

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: September 09, 2025, 01:25:13 pm
content: Do you require to be variable voltage and/or frequency?

author: paulca
author_rank: Super Contributor
timestamp: September 09, 2025, 01:35:44 pm
content: Quote from: UnijunctionTransistor on September 09, 2025, 01:25:13 pmI haven't thought that far. I was just going to work with the 34VAC 50Hz and things like capacitors and inductors. There are things I hear people say and show, but nothing clicks, so I figured I'd "do and learn".Again though the signal generator might be better, a sine wave allows me to do "one" thing where as the signal gen lets me send pulses and PWM etc.Sort of a "transformer with no job" problem just.The other option is to just make into a 1980s PSU for my self build 1980s computer.It might power it, needs 160mA at 5V. However if it's linear that means the transformer needs at least 250mVA?160mA at 5VDC is 0.8W. *~1.4 = 1.12VA.Reading the actual transformer is 1.6VA per tap, 100mA secondary current.34V, down to 5V linear is not practical though and asymetric loading is probably not wise.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: September 09, 2025, 02:26:32 pm
content: The simplest and most straightforward solution would be to use a Variac + transformer solution. There are many low cost voltage / current measurement modules that you could permanently add to the project for real time monitoring.For protection, the simplest again would be a small breaker plus a fuse. You could design, if you are so inclined, an electronic interrupter, with different activation times breakpoints depending on the overload level. But regardless of other protections don’t omit the fuse. Those are your last line of defense against a “thermal event”.

author: paulca
author_rank: Super Contributor
timestamp: September 09, 2025, 02:30:08 pm
content: My understanding of fusing is... "The fuse is always the weakest link". In this case it should therefore be a "prompt" fuse around the primary and secondary current limits. So a short (however that can happen) pops the fuse and not the windings.The UK plug will be fused with a 13A, which is not to code, but the cable is a 13A cable. The socket is a 13A fused socket, but I think I put a 3A in it. Against these are to protect the cable and the socket and intended for 3kW kitchen devices. I could probably have done with a figure 8 2 pin connector and a 3A fuse.

author: themadhippy
author_rank: Super Contributor
timestamp: September 09, 2025, 02:36:03 pm
content: Quotewhy not?

author: paulca
author_rank: Super Contributor
timestamp: September 09, 2025, 02:49:21 pm
content: Quote from: themadhippy on September 09, 2025, 02:36:03 pmWhen my spark installed my shower extractor fan it was plugged into a socket. However because it's only a 3A device he insisted in going to the shop to get more 3A fuses and switch out the 13A one it came with the IEC cable.This may not be "code", it may have been that the shower extractor fan stated it should be fused with a 3A fuse.

author: Benta
author_rank: Super Contributor
timestamp: September 09, 2025, 02:54:27 pm
content: I have doubts about this project. Not because it's not feasible, but because the mains sine shape in most locations is quite distorted.You'll get better results with your signal generator.

author: paulca
author_rank: Super Contributor
timestamp: September 09, 2025, 03:01:17 pm
content: Quote from: Benta on September 09, 2025, 02:54:27 pmJust let me plug my scope in and find out.....Maybe the battery powered one eh?

author: themadhippy
author_rank: Super Contributor
timestamp: September 09, 2025, 03:28:32 pm
content: QuoteThat the important bit,in the case of your psu a 13A fuse with a bit of flex rated =>13A is perfectly fine,if ya could cram a bit of 6mm in the plug you wouldn't even need a fuse.

author: Benta
author_rank: Super Contributor
timestamp: September 09, 2025, 04:04:28 pm
content: Quote from: paulca on September 09, 2025, 03:01:17 pmDon't do that. Look at the secondary of a (any) transformer instead. I don't want to be an accomplice to you killing yourself.

author: MrAl
author_rank: Super Contributor
timestamp: September 10, 2025, 02:25:15 am
content: Quote from: paulca on September 09, 2025, 03:01:17 pmIf you know about how powerful a 230vac mains is then yes, a battery powered scope with the right probe like 10:1 or something like that. You have to be careful though. Turn off the AC mains, then hook up your scope, then turn the AC mains back on. That's a safe way to do it. Then turn off the AC mains first when you are done.Also make sure the little transformer is isolated. That is, the secondary is not galvanically connected to the primary.Ground fault interrupters are also good to use.