How to identify primary transformer windings for 120/240V? - Page 1
Forum section: electronics / beginners

author: Lomax
author_rank: Frequent Contributor
timestamp: June 26, 2016, 09:11:34 am
content: Hello,So I got this device which has a 24V transformer wired to run off 120V with no voltage selector. The transformer has multiple primary windings, but only one of them is connected up, with two additional wires left unconnected. It seems reasonable to assume these are for alternative primary voltages, and that it should be possible to switch it over to 240V operation, but I have no idea how to determinewhichof these wires to switch. Some photos:I have measured the DC resistance between the wire pairs and these are the results:Black-White: 73 OhmsBlack-Red: 175 OhmsBlack-Orange: 188 OhmsWhite-Red: 248 OhmsWhite-Orange: 60 OhmsRed-Orange: 188 OhmsThe white and black wires are the ones currently connected (for 120V operation), with the white wire being the switched & fused one. I have tried to locate a datasheet for the transformer, but it has an OEM specific model number and was made by PowerVolt for a company called EFD, so no such information is available. Appreciate any pointers!

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 26, 2016, 09:30:04 am
content: Put 24V on the secondary and see where you can measure 240V.

author: Lomax
author_rank: Frequent Contributor
timestamp: June 26, 2016, 09:32:52 am
content: That's... just... whooaaa. Too easy!Man I'm such an idiot.

author: Ian.M
author_rank: Super Contributor
timestamp: June 26, 2016, 09:41:36 am
content: It doesn't have to be 24V applied to the secondary. Any 50p or 60 Hz AC voltage from 24V down to about a third of that will do - just measure the voltage across the known primary, and scale all the others proportionately.

author: Lomax
author_rank: Frequent Contributor
timestamp: June 26, 2016, 10:13:25 am
content: Quote from: Ian.M on June 26, 2016, 09:41:36 amYeah, I just had the same thought. Although I do have a second 24V transformer to hand, I think I prefer to do it with 6V instead, for safety reasons. I'm assuming I should expect to see 60V on the 240V primary (1/4 of 240V)?

author: Ian.M
author_rank: Super Contributor
timestamp: June 26, 2016, 10:27:54 am
content: Not exactly. Small transformers usually have the Pri:Sec winding ration adjusted to partially compensate for losses, and when driven in reverse, both the losses and this difference from the ideal ratio reduce the voltage. That's why I said measure the known primary. If its a nominal 115V, and lets say you get 25V across it, you'll be looking for a pair of wires with double that, i.e, 50V across them to use as a 230V primary.

author: Gyro
author_rank: Super Contributor
timestamp: June 26, 2016, 10:49:51 am
content: QuoteDid you remove insulation crimps or anything to access those wires, or were they just 'terminated' that way?

author: Seekonk
author_rank: Super Contributor
timestamp: June 26, 2016, 11:13:46 am
content: I think you will find:white......neutralorange....100Vblack......120Vred.........240VNormally I would expect to see 120, 208 & 240 in this type of transformer. It looks like this was intended for also Japan.

author: MarkF
author_rank: Super Contributor
timestamp: June 26, 2016, 11:39:39 am
content: The OP already said the black and white are connected for 120V operation.I would suspect that:Black = neutralWhite = 120VRed = 208VOrange = 240V

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 26, 2016, 12:48:07 pm
content: any chance you could measure again? esp Black Orange?

author: StillTrying
author_rank: Super Contributor
timestamp: June 26, 2016, 01:25:30 pm
content: As none of us can find the solution to a relatively simple problem, one or more of the resistance measurements must be incorrect.eg. orange-black would be one of the lowest resistances, but instead it's one of the highest @188Rwhite......neutralorange....100Vblack......120Vred.........240V

author: Seekonk
author_rank: Super Contributor
timestamp: June 26, 2016, 06:29:20 pm
content: Intrigued by the possibility that it might be both.

author: Aeternam
author_rank: Unknown
timestamp: June 26, 2016, 06:40:35 pm
content: Won't there be problems because of different mains frequencies (50 Hz vs. 60 Hz)?

author: edpalmer42
author_rank: Super Contributor
timestamp: June 26, 2016, 07:44:09 pm
content: Those resistance values don't seem to work.The highest value (white-red) must be the end points, and the sum of the intermediate points (white-black-red) or (white-orange-red) add up correctly. But when you look at the final combination (black-orange) it just doesn't fit. Even if you correct the black-orange value, the numbers still don't work. Either there is an error in the values or the unit has some parallel connections that are distorting the measurements.My best guess of what the values might be is:white - 73 ohms - black - 115 ohms - orange - 60 ohms - redThis gives the rather odd result:100V = orange-red120V = black-white220V = black-red240V = white-orangeBut until those measurements are confirmed, I wouldn't consider this correct.Ed

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2016, 07:47:40 pm
content: The different mains frequency has only little effect. At 60 Hz it is possible to use the same transformer for a 20% higher voltage without saturation. But It's perfectly fine to use the same voltage, this just uses less of the possible magnetization, thus slightly lower loss and less hum, but the same voltage ratio.The other way around, at 50 Hz for a transformer made for 60 Hz the magnetization will be higher. Thus higher loss, strong hum and possibly saturation peaks are possible, unless a lower voltage is used. 100 V at 50 Hz is of for the 120 V 60 Hz windings - just the output voltages will be lower too.

author: StillTrying
author_rank: Super Contributor
timestamp: June 26, 2016, 09:58:49 pm
content: Quote from: Seekonk on June 26, 2016, 06:29:20 pmI don't know what you mean by both.But as you'd already mentioned Japan, I'd looked to see if they were 50 or 60 Hz. I didn't know they had the country split in the middle with one half 50 Hz and the other 60 Hz!

author: Lomax
author_rank: Frequent Contributor
timestamp: June 27, 2016, 10:50:31 pm
content: Quote from: Gyro on June 26, 2016, 10:49:51 amHehe, I was wondering if anyone would comment on that. Don't worry, they had heatshrinked insulation "teats" on them - I had to remove these to take measurements. This insulation will be replaced later.

author: Lomax
author_rank: Frequent Contributor
timestamp: June 27, 2016, 11:01:23 pm
content: Hmm, maybe I did make a mistake when taking notes - I'll repeat the measurements when I have time, which might be in a few days. The power switch is a lit type - could it be that the neon bulb might have affected the white-black measurement? I thought these bulbs require a high-ish voltage to conduct? I think I'll disconnect the transformer completely and take it out of the chassis so I can do the reverse voltage check more easily. Need to get some spade crimp terminals as well.

author: Seekonk
author_rank: Super Contributor
timestamp: June 28, 2016, 02:03:12 pm
content: I'd say it was safe to connect it up to the mains, white - red for the highest voltage. Then read voltages between windings in reference to both white and black.By both I meant that it would be able to supply 100 and 208/220V.

author: Lomax
author_rank: Frequent Contributor
timestamp: July 04, 2016, 09:19:44 am
content: Quote from: Seekonk on June 28, 2016, 02:03:12 pmI'd say you're spot on there! I've just measured the voltages when supplying 24V in reverse on the secondary:ORG-BLK: 17VORG-WHT: 87VORG-RED: 121VBLK-WHT: 104VBLK-RED: 104VRED-WHT: 208VThat looks to me as if RED-WHT is the primary for 220/240V operation. Agree?

author: StillTrying
author_rank: Super Contributor
timestamp: July 04, 2016, 12:35:42 pm
content: Quote from: Lomax on July 04, 2016, 09:19:44 amYes, but check that the winding is isolated from everything else.These all add up perfectly.WHT 0 VORG 87BLK 104RED 208How come the original ohms are so far out.ORG-BLK: 17V 188 OhmsORG-WHT: 87V 60 OhmsORG-RED: 121V 188 OhmsBLK-WHT: 104V 73 OhmsBLK-RED: 104V 175 OhmsRED-WHT: 208V 248 Ohms

author: Ian.M
author_rank: Super Contributor
timestamp: July 04, 2016, 03:06:30 pm
content: Looking good. As the Red ad Orange wires had a fairly low DC resistance to White and Black, they must be on the primary side, either one tapped winding, or two interconnected windings.The next thing I'd do is power up the transformer using the white and orange primary wires, from your 240V mains supply, WITH NO LOAD ON IT's SECONDARIES, and a 60W 240V incandescent bulb in series with the primary as a current limiter. If the bulb glows or the transformer gets hot with no load on it, you've got a problem (e.g. core saturation because iis underrated for 240V, 50Hz operation). Check the secondary voltage is as expected, or at least within the range your application can tolerate. If its still running cool after half an hour or so, remove the bulb from the circuit and connect the normal load to the secondary.

author: Seekonk
author_rank: Super Contributor
timestamp: July 04, 2016, 03:09:48 pm
content: Identical voltage windings can have different resistances because they are further from the core and much longer for the same number of turns. With multiple series windings they will use different gauge wire because lower voltage taps draw more current. Multiply those read voltages by 1.15% and you get 100, 120 & 240V. Nothing more to discuss.

author: Lomax
author_rank: Frequent Contributor
timestamp: July 04, 2016, 05:34:44 pm
content: Thanks everyone! I'm happy to report that having switched the black & red primary inputs I measured 23.8V on the secondary, and after leaving it plugged in for half an hour the transformer did not get warm. Been using the device for a while now and all is well; even the neon bulb in the power switch seems to have survived the voltage increase. So I am now the happy owner of an EFD 10VAC vacuum pick up tool operating on 240V

author: StillTrying
author_rank: Super Contributor
timestamp: July 04, 2016, 08:40:14 pm
content: Quote from: Seekonk on July 04, 2016, 03:09:48 pmBut now we have your theory of different gauges of wire used on the primary to discuss.I'm still going for the theory that the OP made mistakes in the DC resistance readings as they don't add up.

author: Lomax
author_rank: Frequent Contributor
timestamp: July 04, 2016, 10:11:53 pm
content: Quote from: StillTrying on July 04, 2016, 08:40:14 pmI have to admit this is a very likely possibility!