Transformer outputs too high a value - Page 1
Forum section: electronics / projects

author: briesmith
author_rank: Newbie
timestamp: November 27, 2023, 05:18:58 pm
content: I bought a 240vac to 24vac transformer to drive a bench power supply I am building using a 30vdc circuit from Ali Express. (Based, I believe, on a Hiland design).The transformer, as you can see from the attached pix, is labelled 24vac 30w but outputs 57vac or 28vac from the centre tap. I don't know if I am measuring the output incorrectly or the transformer is mis-labelled and is actually a 57vac device?If anyone can help me with this puzzle I would be grateful.Best wishesBrian

author: ataradov
author_rank: Super Contributor
timestamp: November 27, 2023, 05:39:06 pm
content: This is a dual output transformer. You can use it for generating negative and positive supplies, for example.If the windings were separate, you could connect them in parallel and get full output power. But it is impossible with a center-tap winding, since winding direction matters for parallel configuration.I would assume that 30 W rating is for both windings, so if you only need one output voltage, then you will have to use only one winding at 15 W max.And as far as 24 V vs 28 V, it is normal, the output would vary with the variation of the input voltage and it often varies a lot.

author: bdunham7
author_rank: Super Contributor
timestamp: November 27, 2023, 05:50:55 pm
content: Quote from: ataradov on November 27, 2023, 05:39:06 pmOr use both sides with 2 individual rectifiers instead of a bridge. Not quite as good as 2 parallel windings, but there's no other good choice here.

author: MathWizard
author_rank: Super Contributor
timestamp: November 27, 2023, 06:06:11 pm
content: So what does it do with some load resistances? Like at minimum, a power on LED, so smaller than 2k load say for ~20mA. What's the output voltage then from 28VAC ? A lot of IC's I've seen, have max voltage ratings of 36V. 28VAC =39.6Vp, but with some rectifier drop and idle load, it might never be a problem.I made a roughly 0-20V 1A CV-CC all BJT PSU. At 1st I had to use 3 old transformers I had, then I got a big stereo transformer that had all 3 rails I needed. Plus I added connectors to the transformer, so it can be plugged into other projects still.It has something like a +/-70V main rail, and IDK what the safe current is, but I think with a half-wave rectifier I pulled 4-5A from it and it's voltage was still around 30Vdc down from some 50Vdc unloaded.It was a pain too drop some voltages enough to run some 36V rated parts, like a 7812 regulator (so not an all BJT PSU, with an LED meter too)

author: Benta
author_rank: Super Contributor
timestamp: November 27, 2023, 08:06:39 pm
content: Small transformers always exhibit higher output voltage when unloaded. The nominal output voltage is only correct with a nominal resistive load. In your case 77 ohms (don't try it with a carbon resistor, you need at least 30 W rating).But almost 20% too high is unusual for a 30 W type. I'd accept 20% for a 1...5 W one.Looking at your photos, the transformer seems small for a 30 W type. My guess would be around 20 W.In short: it's a cheap, overspecced Chinese thingy containing as little iron and copper as possible. It will run very hot and is most likely not suitable for continuous duty.Try comparing size and weight with a transformer from a reputable company.

author: Gyro
author_rank: Super Contributor
timestamp: November 27, 2023, 10:17:21 pm
content: Doing a quick search of the thread, nobody seems to have mentioned that the label on the transformer say that it has a 220V primary. This will further increase the output voltage when run on typical UK 230-240V (I'm surprised the unloaded output voltage isn't higher) and push the core even closer to saturation.

author: tunk
author_rank: Super Contributor
timestamp: November 27, 2023, 11:14:18 pm
content: Seem to remember that from Big Clive's videos that the UK voltage is above 240V.

author: themadhippy
author_rank: Super Contributor
timestamp: November 27, 2023, 11:35:14 pm
content: Quoteofficially its 230v, -6% +10% . so can be anywhere between 216 to 253 volts