Is this transformer suitable for a lab PSU? - Page 1
Forum section: electronics / beginners

author: steve30
author_rank: Frequent Contributor
timestamp: July 28, 2012, 07:15:30 pm
content: Quick question about transformers...I'm trying to build a lab PSU. Nothing particularly special, just a simple design.I have a spare transformer which came out of an old 250 Watt overhead projector. Did this ought to be suitable? Anything to watch out for?ThanksStephen Coates aka steve30

author: IanB
author_rank: Super Contributor
timestamp: July 28, 2012, 07:20:24 pm
content: It's a perfectly fine looking transformer. Quite elegant in fact.But what about the secondary side? At a guess it will be a nominal 24 V at about 10 or 11 A in order to feed a 24 V 250 W halogen lamp.That would make it quite suitable for a variety of power supplies. It's quite large, too large for most electronic purposes, but it would therefore be quite unstressed at low loads, if you don't mind the weight.

author: steve30
author_rank: Frequent Contributor
timestamp: July 28, 2012, 07:28:23 pm
content: Yeah, it does look quite nice.I measured the output voltage with an AvoMeter set to AC Volts, and it showed as 24 Volts. Probably should show that since it is powering a 24V lamp.I'm assuming approx 10A but I didn't measure it.Weight is no issue. Good quality power supplies are supposed to be heavy.

author: steve30
author_rank: Frequent Contributor
timestamp: July 28, 2012, 10:15:46 pm
content: OK, so I plugged it in to the mains (with no load on the secondary) and it blew the 5 amp fuse in the plug and tripped the 30 amp miniature circuit breaker*.Just wondering if I connected the correct side of the transformer. Now I forget which way round it worked.Mains stuff is fun.*but it didn't blow the 13 amp fuse in the extension lead plug.

author: mariush
author_rank: Super Contributor
timestamp: July 28, 2012, 10:47:17 pm
content: Easiest way to check it would be to get a 9v battery or 12v from your computer's power supply.Connect one end to your multimeter set on 750v AC or something around that value, then connect and disconnect the battery/12 to the two wires you pick on the other side. By connecting and disconnecting the battery, you're simulating (really pushing it) AC voltage from the battery. On the multimeter you should see for short moment, the voltage on the other side. If you don't see anything, try the other way around.If you leave the battery connected, you won't see anything, as it will be DC voltage through the transformer.

author: PedroDiogo
author_rank: Regular Contributor
timestamp: July 29, 2012, 12:34:05 am
content: Quote from: mariush on July 28, 2012, 10:47:17 pmIs this common practice? I say this because I can understand why this will work, but I'm not sure if these measures are reliable.

author: David_AVD
author_rank: Super Contributor
timestamp: July 29, 2012, 01:16:29 am
content: Measure the resistance of the two windings. On a step-down transformer like that, the primary (240V) winding should read higher resistance than the secondary (24V) one.

author: mariush
author_rank: Super Contributor
timestamp: July 29, 2012, 01:51:47 am
content: Quote from: PedroDiogo on July 29, 2012, 12:34:05 amIt's not a method designed to give you accurate measurements, it's meant just to show you which side is the primary and which is the secondary. Measuring resistances is also a good method.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2012, 02:11:25 am
content: If, when connecting to the mains, you put a 230v 100W lightbulb in series then it will save you from blowing fuses/breakers. (NOTE: use a normal incandescent lightbulb, not a CFL)Basically, a shorting primary or shorted secondary will cause the light bulb to illuminate and nothing bad will happen.If the transformer is has an internal short somewhere you see light, if the transformer is ok the lightbulb will stay off.Be safe though, don't have exposed mains wires around the place.Side note: With transformers that have multiple windings over a short range, like 220/230/240V, you can safely pick and match your actual mains voltage with any of the windings to adjust the secondary voltage a little.This is ok because the range between 220 - >240 is quite small, its only 10% differenceThe reason for multiple taps so close together (voltage wise) is to get the correct secondary voltage in any country, the primary doesn't care.(Now, if it had 110v / 220v though, that's a different story, don't feed 220V into a 110v winding)

author: steve30
author_rank: Frequent Contributor
timestamp: July 29, 2012, 02:12:10 am
content: One winding does have a higher resistance than the other. I must have wired it up the wrong way round.This should have been obvious as the secondary side has much thicker wire than the primary side, but I managed to overlook it when wiring it up.edit:QuoteThat's a good idea, thanks. I'll remember it for the future. Unfortunately though, I have never used a 100W lamp (and hence don't have any), and am out of 60W lamps. There's an official lamp shortage here in steve30_land.

author: Psi
author_rank: Super Contributor
timestamp: July 29, 2012, 02:21:38 am
content: The lamp wattage doesn't really matter, it's just that some transformers are a bit lossy and cause a low wattage lamp, like a 25W, to glow even when there's no short.

author: steve30
author_rank: Frequent Contributor
timestamp: July 29, 2012, 02:25:52 am
content: This one has several taps on the primary. I will have to play with that.I believe the several taps were used in the OHP as part of its brightness selector switch (either hi or lo).

author: David_AVD
author_rank: Super Contributor
timestamp: July 29, 2012, 02:54:17 am
content: In the picture, the primary looks to be the one closest to the camera. The (heavier) secondary winding is towards the keyboard.

author: IanB
author_rank: Super Contributor
timestamp: July 29, 2012, 03:53:58 am
content: Quote from: Psi on July 29, 2012, 02:21:38 amWhat Steve means is that certain interfering busybodies in government have banned the sale of incandescent bulbs. 100 W bulbs now trade on the black market at £50 each(Though, note to Steve, I think you can still get incan bulbs with a halogen lamp inside the glass envelope?)

author: steve30
author_rank: Frequent Contributor
timestamp: July 29, 2012, 03:57:30 am
content: Yeah, halogen replacements are available. Not tried them though.I never really changed to CFLs due to the slow warm up time etc. But I do plan on coming up with a new lighting system for my lab/bedroom, probably using LEDs. Only just started playing with power LEDs though so that may take some time.

author: SeanB
author_rank: Super Contributor
timestamp: July 29, 2012, 07:58:48 am
content: Primary is the side nearest to camera, with blue/brown/black wires. Secondary is furthest away with only 2 wires. Mains is connected to either blue/brown ( for high) or blue/black ( for low) to give either 24VAC or 20VAC on the secondary.

author: mianchen
author_rank: Regular Contributor
timestamp: July 29, 2012, 09:03:22 am
content: Quote from: IanB on July 29, 2012, 03:53:58 amThere are plenty of them on eBay UK... just saying

author: SeanB
author_rank: Super Contributor
timestamp: July 29, 2012, 09:26:24 am
content: Why fleabay? I just walk into any store and buy them. They now have a carbon tax on them that trebled the price, but they are still half the price in a 2 pack of a single CFL lamp. Probably will last longer than the WinGPanGPong lamp in most enclosed fixtures though. If I go to FonkKongLand I can pick them up for $1 for 10, the CFL lamps are $1 as well there, illegal untaxed imports, but with a receipt in Chinese and the price in Renbinmi.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 29, 2012, 09:44:14 am
content: Quote from: SeanB on July 29, 2012, 09:26:24 amBecause the UK is not part of South Africa? Just saying ...

author: SeanB
author_rank: Super Contributor
timestamp: July 29, 2012, 09:54:28 am
content: And for that I am grateful. no bin police, no council taxes, no BSE.

author: madires
author_rank: Super Contributor
timestamp: July 29, 2012, 10:25:46 am
content: Quote from: IanB on July 29, 2012, 03:53:58 amSpecial lamps, e.g. for fridges and ovens, are still incandescent bulbs.

author: steve30
author_rank: Frequent Contributor
timestamp: July 29, 2012, 03:15:58 pm
content: I think its only the bog standard household 60W/100W lamps that you can't get now. Anything specialised I don't think is affected. e.g. My 60W daylight lamp is still available as far as I'm aware.I definitely had the transformer connected the wrong way round. I have now connected it the correct way round, and I get a nice 25V AC on the output with no load:I think it will need securing down well as it seems to vibrate a bit.

author: SeanB
author_rank: Super Contributor
timestamp: July 29, 2012, 03:40:06 pm
content: Tighten the screws that hold the frame together and it will quieten up a lot.

author: steve30
author_rank: Frequent Contributor
timestamp: July 29, 2012, 04:02:34 pm
content: In case anyone is interested, here it is with some load.With the OHP lamp connected, there is more than 10 Amps.And the voltage with the lamp connected is 22.5V.So, you loose 2.4V with a very large load.

author: T4P
author_rank: Super Contributor
timestamp: July 29, 2012, 04:09:52 pm
content: Quote from: SeanB on July 29, 2012, 09:26:24 amHongkong isn't half as bad as china is! and ... HK is in HK Dollars ...Quote from: steve30 on July 29, 2012, 03:15:58 pmI can find a 100W bulb here ... but that's because they continue selling because some places it's neccesary to use a incandescentYeah, big transformers can rattle almost anything, i have my (That's close to 300W!) lodestar PSU that produces a loud inductive kick sound on start sometimes and can vibrate my plastic top table ... with full load that just stops

author: SeanB
author_rank: Super Contributor
timestamp: July 29, 2012, 04:34:58 pm
content: Love that nice Avo 8 there..........

author: steve30
author_rank: Frequent Contributor
timestamp: July 29, 2012, 06:06:15 pm
content: The Avos are really nice multimeters. I use them regularly.I also have a Model 9 SX (see:http://stevecoates.net/avo/), but that is currently in pieces and is waiting to be put back together.