220V to 110V converter - Page 1
Forum section: electronics / projects

author: artag
author_rank: Super Contributor
timestamp: August 11, 2025, 06:23:14 pm
content: I bought some test equipment in an auction and realised later that some of it is 110V and can't be trivially converted to 230 for use in the UK. It's a purchase option but uses a different transformer. Odd decision, must have saved all of a dollar.I can use them with an external transformer fairly easily but it rather ties them to the bench and makes portable use awkward, so I'm exploring whether I can fit a converter internally. They're rated at 120VA but the DC rating is only 50W so that's probably an overestimate. The smallest I can find is a 45mm high toroid that weighs 1kg. Might work but seems a lot.I'm wondering whether I can find a transformerless solution. A fixed phase-controlled dimmer switch is a crude possibility but requires careful checking of DC offset to drive the transformer. Maybe an alternative would be a greatly simplified VFD - chopping the AC waveform at a much higher frequency with an IGBT or MOSFET to provide a simulated half-voltage output.that's then smoothed away by the inductance of the transformer.Could this work ? Does anybody make such a thing already ?

author: jonpaul
author_rank: Super Contributor
timestamp: August 11, 2025, 07:07:01 pm
content: No a 50 VA stepup autotransformer is cheap and smallEasy wayj

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2025, 07:13:05 pm
content: Since you are plugging them in anyway, could you add fixed 110 outlets where necessary?In the past in that situation, I used a couple of rolling carts.Just because it was a factory option does not mean the instruments cannot be reconfigured. If they have a transformer input, did you check it for dual primary windings?

author: artag
author_rank: Super Contributor
timestamp: August 11, 2025, 08:54:53 pm
content: Quote from: jonpaul on August 11, 2025, 07:07:01 pmEasy if there's enough room inside, but most transformers are cubes. Toroids help and might be a possibility.It's easy, yes. I'm tempted to echo the quote 'We did it not because it was easy, but because we thought it would be easy'. A converter that was only a bit bigger than a couple of power mosfgets would be pretty nice, woiuldn't it ? Especially as I need to step down, not up.

author: artag
author_rank: Super Contributor
timestamp: August 11, 2025, 09:00:09 pm
content: Quote from: David Hess on August 11, 2025, 07:13:05 pmYeah, but these are nice neat units in the HP half-width 2U package (HP3610A, 3611A). A transformer would easily go under the bench but these puss are well suited to taking to a job. The transformer would be a nuisance.They're not useless as they are, I'd just like better.No to changing jumpers. First thing I did was take the lid off. The two primary wires from the switch go straight under the transformer insulation and the transformer itself has a part number that references the 110V option. The schematic shows no mains option wiring.

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2025, 09:06:55 pm
content: Quote from: artag on August 11, 2025, 09:00:09 pmWhat about removing the original transformer to make room for a 220 volt transformer?

author: artag
author_rank: Super Contributor
timestamp: August 11, 2025, 09:20:29 pm
content: It's got 3 secondary windings, some with taps (two aux windings and a low/high range switch). Unless I find a cache of HP 220V ones it's unlikely to be easy or cheap.

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2025, 09:22:02 pm
content: Quote from: artag on August 11, 2025, 09:20:29 pmIt might be a common configuration.I have sometimes used more than one transformer as a replacement to get the proper secondary windings.

author: artag
author_rank: Super Contributor
timestamp: August 11, 2025, 09:26:15 pm
content: That's a more realistic option. It's a fair sized transformer so it would make a bit of space. But a 220/110 would be easier.Does anyone know .. If I used the two windings of a dual-primary transformer as an autotransformer, what rating would it need to be ? Could I take 120VA from one of the primaries of a nominally 120VA transformer if it had no other secondary load ? Or do I need to use double the rating ?

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2025, 10:04:54 pm
content: Quote from: artag on August 11, 2025, 09:26:15 pm120VA is about right for a 50W linear PSU. If you are using a transformer, you'll need 120VA.QuoteIf you have a 240V center-tapped winding (or two 120V windings wired in series) and you are taking 120V at 1A from the center tap in an autotransformer arrangement then you'll just need a 120VA transformer. The center tap current will be 1A, the current of each primary winding will be 0.5A.QuoteThat sounds noisy and unstable, although possibly doable. Replacing the linear PSU entirely with an SMPS would be more straightforward but probably would have noise issues as well.

author: artag
author_rank: Super Contributor
timestamp: August 11, 2025, 10:59:47 pm
content: Quote from: bdunham7 on August 11, 2025, 10:04:54 pmFair enough. I expected some overhead but not as much as that so thought it might be just startup surge.Quote from: bdunham7 on August 11, 2025, 10:04:54 pmExtremely helpful, thank you. Couldn't get my head around what the currents in an autotransformer would be. That opens up a lot more ebay optionsQuote from: bdunham7 on August 11, 2025, 10:04:54 pmWell, yes. I'd be keeping the case and the display at that point.It's just the small size and low cost of 'electronic' transformers appeals to me and I wondered if I could steal ideas. In terms of development cost it's insane. In terms of neat ideas it's great.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 12, 2025, 12:22:31 am
content: Quote from: artag on August 11, 2025, 09:26:15 pmAutotransformers have less loss than an isolating transformer.The bottom winding draws magnetizing current levels, and the top winding has load current + magnetizing current.With only one winding giving I^2R losses, it will run a bit cooler at the rated current.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2025, 01:54:15 am
content: Quote from: artag on August 11, 2025, 09:26:15 pmI have the perfect solution for you, shown below. It will require a cart, or two people, to move.Shown is three 1500+ watt transformers with their split primaries wired in parallel to operate as a whole house 120 to 240 converter. I used three transformers because four was too heavy.

author: artag
author_rank: Super Contributor
timestamp: August 12, 2025, 09:50:06 am
content: I do have some 'tool transformers' up to 5kVA - cheap and robust 220-to-110 transformers used on UK building sites, but I'm reminded of Tom van Baak's 'Most accurate wristwatch'http://www.leapsecond.com/pages/atomic-bill/