Where can I source small step-down transformers?

[krivx]

I have some 110V gear with a custom power transformer (8 output windings). I would like to run this on 230V without using my bulky step-down transformer. The neatest solution seems to be a small additional transformer stepping down 230V to 110V-ish, mounted inside the enclosure. I can't seem to find any source for this. I want something in a 4-5cm cube or smaller. Power rating is only about 12VA. Does this exist?

[Ian.M]

I suspect that's going to be really difficult.  Digikey has one possible candidate {my search} as its possible to use a normal 6VA transformer wired as an autotransformer for a 12VA load, because the primary current adds to the former secondary current so it only 'sees' half the load current in that configuration. 

[krivx]

None of those are stocked parts or available in singles though. Autotransformer wiring is a good idea though, I'll consider that.

[David Hess]

Any transformer with a dual primary for 120V/240V can be wired as an autotransformer using only the primary leaving lots of potential options; this avoids the problem of looking for a 120V secondary.  Size the transformer based on the current rating of the primary to avoid surprises.

Sometimes this sort of thing can get out of hand though as shown in my photograph.

[langwadt]

you should be able to use almost any trafo of sufficient size that has dual or split primary as an auto transformer

[mariush]

Another solution would be to determine which of the 8 windings use the most of those 12va and then get a toroidal transformer with one secondary winding designed to output slightly more than the sum of the voltages of those major windings. 
Then unwind the secondary winding of that transformer, and cut the single wire in several wires to form those 8 separate windings.
Here's an example of a transformer that may fit your needs, it's 28mm x 60mm 15VA with 2 x 6v AC secondary windings : http://uk.farnell.com/multicomp/vtx-146-015-106/transformer-15va-2x-6v/dp/1675043

[Ian.M]

The problem with using the split primaries of ordinary transformers as autotransformers is the secondary sitting idle doesn't contribute to the VA rating.  Therfore you can only run them up to their nominal VA rating as half the copper is sitting idle, unlike the doubled VA rating you get in the case of a 120V:120V transformer wired as a 2:1 autotransformer. As size is a real problem for you that may be a show-stopper.

Modding toroidial transformers is not exactly a good option.  They are almost invariably varnish impregnated and you'll have to be really careful not to damage the primary insulation.  Instead if you can identify the secondary with the highest VA load, and its close to equal or more than the total VA load of the other secondaries, you can probably drive it as a primary with about 10% more than its normal voltage. You'll need  a transformer 240V:<Vsec*1.1>, 12VA to do that.  You'll need to check the circuit carefully and see if its going to stand that unreg rail being 10% high or if you need to add a dropper resistor. (or string of antiparallel diodes).  Taps on the same secondary will also be 10% high.    Carefully disconnect and insulate the 120V primary as it will have 120VAC on it by transformer action!

These guys do some empty BS1363 plug wallwart cases:
https://www.jprelec.co.uk/store.asp/c=549/Plug-in-PSU-Enclosures-Vented
If space inside the equipment is too tight, it may be possible to put the autotransformer on the wallplug end of the lead!

[Signal Simon]

You can get small wall-wart autotransformers, such as this 45VA type. £5.02 from CPC (part of Farnell)
Have a look at http://www.farnell.com/datasheets/27822.pdf
That's a UK plug version, the same thing will be available with a Euro plug elsewhere I expect.

Simon.

[ebclr]

If you prefer order in english



https://www.aliexpress.com/item/AC-Adapters-New-50W-US-AC-Power-220V-To-110V-Voltage-For-Travel-Converter-Adapter/32656972830.html?spm=2114.01020208.3.20.epLThJ&ws_ab_test=searchweb201556_0,searchweb201602_3_10057_10065_10056_10068_10037_10055_10054_10069_301_10059_10033_10058_10032_418_10073_10017_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_6&btsid=207555c0-64da-487c-846e-3ccf8622887a

[Zero999]

What about taking the transformer out of a UK shaver socket? It will have an isolating transformer with a 0-115V-230V secondary and will be higher quality than those cheap autotransformer wall warts.

http://www.mr-resistor.co.uk/item.aspx?v=1&i=14983&gclid=CMKllq6tr88CFUkq0wodZNoA9Q

[krivx]

What about taking the transformer out of a UK shaver socket? It will have an isolating transformer with a 0-115V-230V secondary and will be higher quality than those cheap autotransformer wall warts.

http://www.mr-resistor.co.uk/item.aspx?v=1&i=14983&gclid=CMKllq6tr88CFUkq0wodZNoA9Q

These would be fantastic, except I can't seem to find out any specs on them :/ What is the rated power?

e: it seems some of these sockets are rated for 20VA, which is more than enough. This could be a winner.

[Ian.M]

Unfortunately most shaver socket transformers are not rated for  continuous use.  Leaving electric toothbruch chargers plugged in to one often causes overheating to the point of discolouration and cracking of the faceplate!  This is an issue even with reputable brands like MK. http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=28791

[Zero999]

Unfortunately most shaver socket transformers are not rated for  continuous use.  Leaving electric toothbruch chargers plugged in to one often causes overheating to the point of discolouration and cracking of the faceplate!  This is an issue even with reputable brands like MK. http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=28791

Oh I didn't know that. Fortunately I do the same: regularly charge my shaver/toothbrush and don't have any problems: the socket doesn't get very warm. I wonder if the toothbrush charger could be at fault here? If the power factor is poor the VA rating could exceed 20VA and if the crest factor is high it would cause even more heating.

I doubt this would be a problem in the original poster's application, as they need less than 12VA and a shaver socket is rated to 20VA. It's also possible it might not need to be plugged in continuously.

[krivx]

I need continuous use, hours at a time. The enclosure is metal and is fairly small and full so I wouldn't trust it to perform better than a shaver socket cavity box. I think the current plan is to get try some small transformers with split supplies and one of those cheap autotransformer-based converters from Farnell and see what will function and what will fit.

[Zero999]

I've done a bit more digging. Hopefully you'll be able to get them in your area.
http://uk.farnell.com/triad-magnetics/vps230-110/transformer-2-x-115v-25va/dp/1610413
http://www.mouser.co.uk/ProductDetail/Bel-Signal-Transformer/DPC-120-85/?qs=sGAEpiMZZMvwUzoUXIIvyZ8riR3Pta286UEWmtE%2fye%252b8cBHTIfL%252bZA%3d%3d
http://www.mouser.co.uk/ProductDetail/Hammond-Manufacturing/186C120/?qs=qVxIICNKHAE2IzlWuibkQg%3d%3d
http://www.mouser.co.uk/ProductDetail/Hammond-Manufacturing/160G120/?qs=sGAEpiMZZMvwUzoUXIIvyW8HfC4QLhUeHIIZL6ozVnc%3d
http://www.mouser.co.uk/ProductDetail/Bel-Signal-Transformer/DST-5-120/?qs=sGAEpiMZZMvwUzoUXIIvyQnFX5Me0WuQAJqsV0wJI6jWNh6L0gVY3Q%3d%3d

[Siwastaja]

The shittiest quality transformer will work if enough derating is applied.

Which may not be practical, if derating the output power to zero is not enough, but derating input (primary) voltage is also needed; it's not uncommon at all that the shittiest transformers overheat even with nearly zero load, because of the magnetization current in the primary wound in extremely thin wire.

Transformers for low-duty usage, such as for shavers, are often designed for high zero load consumption because they are not energized for a long time.