40 volt 10 amp transformer -where to find one - Page 1
Forum section: electronics / rf-microwave

author: Dom13c
author_rank: Regular Contributor
timestamp: June 17, 2018, 11:16:40 am
content: I live in a shit country India. Finding stuffs for cheap outta junk here is damn tough. I'm building a zvs driver for running both of a flyback transformer and an induction heater. Beefed up the driver to handle high voltage eg.40 volt and high current eg.10 amps but I'm unable to find a suitable power supply. Tried and also fried a few atx power supplys. So I'm trying to build a linear power supply for the zvs driver . thus even if the rectifier semiconductors get fried I can replace them. Can any one suggest from where I can get one of those bulky transformers for cheap.( cant afford to pay more than 10$.

author: Ian.M
author_rank: Super Contributor
timestamp: June 17, 2018, 02:16:30 pm
content: The cheapest option is probably going to be build your own transformer. Warning: its *NOT* easy and you may not be able to source magnet wire for the secondary within your budget unless you can sweet talk a local electric motor or alternator rewind shop to let you buy a part spool at trade prices.Suitable 'donor' cores and pre-wound primaries can be found in microwave ovens. If you don't have any high voltage experience start by reading the SAFETY section ofhttps://www.repairfaq.org/sam/micfaq.htmso you don't kill yourself if you encounter a HV capacitor that hasn't fully discharged.Scavenge the mains transformer from a dead microwave (not a high end inverter model). Ideally choose a 800W upwards microwave, as you need 400VA and that should give plenty of core aperture area for your new winding. A 700W microwave probably has a big enough transformer. A 600W one probably isn't going to work well - although it can handle the VA required, the restricted area will make it much more difficult to wind, and as microwave transformers are not intended for continuous operation, you need to over-rate it if you are going to re-purpose it for continuous use.It needs to be a line frequency transformer, that's in physically good condition with no signs of serious overheating or burning. Secondary to core or secondary to secondary insulation failures are acceptable as you are going to be cutting away the old secondary. Primary to core, or primary to secondary insulation failures are *NOT* acceptable. A quick test with a Megger or other high voltage insulation tester will save you a lot of time and effort if the transformer is defective.Carefully cut away the high voltage secondary and filament supply secondary, taking great care not to abrade or otherwise damage the primary. Knock out the remains of the secondaries from the core apertures, knock out the magnetic shunts, and check there is no damage to the primary.At this point you should power it up and check that it doesn't draw an excessive no load current or heat up excessively before you invest more time effort and materials. If it looks like its OK, leave it on for several hours and monitor the temperature as Microwave primaries often have barely enough turns to keep out of hard saturation as copper is expensive and the extra losses are acceptable if the duty cycle is limited and/or it can rely on extra forced air cooling from the microwave's fan. Worst case: you may have to use an extra 'buck' transformer*to feed it, dropping the mains voltage 10% or so, but if it overheats you'd really be better trying a different model of microwave transformer rather than adding an extra transformer.Now add a 10 turn temporary secondary using hookup wire so you can measure the volts/turn so you can calculate how many turns you need to cram in there for your application and from the aperture area, what's the maximum wire gauge you can use, allowing enough margin for the wires not packing perfectly and to make it possible to thread the winding in there and to be able to add 10% more turns to correct the loaded output voltage. N.B. that will give you the NO LOAD volts/turn - you can expect about 10% less fully loaded.Get suitable wire, and prepare the core by taping the edges of the apertures so you don't scuff the wire insulation on the sharp corners. This is also a good time to add extra insulation between the primary and your new secondary if the primary doesn't already have reinforced insulation. If you've got Kapton tape, great, use it, but worst case you can do a good job with cartridge paper and ordinary clear varnish, if you fully saturate the paper with varnish,build it up in enough layers, squeezing out excess varnish and hold it in place with waxed paper and sponge rubber to keep the pressure on till the varnish has fully cured. Don't use PVC electrical tape for transformer insulation - it melts too easily and the adhesive turns into slippery goo when hot.You then have the really tedious task of threading the wire through the core apertures to lay down the new winding so it helps to have assistance to handle the loop as you pull the wire through for each turn. You are going to be handling a *LONG* loop of wire that its critical not to kink.Start by calculating the length and allow 1/3 extra - running short is a disaster and you may need to add more turns when you load test it. If you need two identical windings, (or a center tap), double the wire over and start from the middle treating it as if it was a single wire, but taking care not to twist them as you lay down the turns. Take the end and tape it in place to the face of the core in line with the center leg - make sure you've got enough spare end length to terminate it properly. Coil up the end and secure with tape so it doesn't get caught on anything as you wind. Lay down an even winding one layer at a time. It may help to tie the wire down with cotton thread at the end of a layer before starting the next. *DONT* lose count of the number of turns.When you've got the calculated number of turns on the core, temporarily secure the free end, (*DON'T* cut it), and strip the tip of the inside end you previously coiled up and taped out of the way, and the tip of the excess wire of the outside end.You can now power up the transformer again and check the no load current hasn't increased (which would mean a shorted turn), check the no load secondary voltage, then load it to your max design current and check the load voltage so you can calculate how many turns to add or remove to trim it to the voltage you need.Its then just a matter of adding or removing the final turns and making a good job of securing the outer winding end, and terminating the windings. If you need a center tap, cut the folded end to separate the windings and connect the inner end of one wire to the outer end of the other. Magnet wire ends should be protected by thin sleeving slipped over them which must be properly secured to the winding and/or core *after* the outer winding end has been secured so it cant unwind at all.* The buck transformer secondary only has to carry the microwave transformer primary current, which will be about 1.7A for 400VA at 240V so it only needs a 2A rating. Even for a 9% drop you only need a 50VA 24V transformer. If you only need to drop 5%, a 25VA 12V transformer will do the job. Wire it as an autotransformer with the secondary in series with the primary, initially phased so the secondary voltage adds to the primary. Move the Line input connection to the free end of the secondary and take the reduced line voltage output from the old Line input terminal. If the secondary is center-tapped you can alternatively take the output from the secondary center tap for half the drop.

author: Dom13c
author_rank: Regular Contributor
timestamp: June 18, 2018, 05:24:53 am
content: Wow. Nice instruction man, love it. I've watched a lot of youtube videos on this topic but none were so much detailed. I'll definetly give it a try. But I was looking for something that could be more handy. My college will start two weeks later and I'll not be able to pay much time on my high voltage hobby.So if you can suggest me something more handier than rewinding a MOT I'll be highly obliged. Last but not the least, nice to see that you care about others as you put the link of hv safety page. I appreciate it.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 18, 2018, 05:40:01 am
content: Doubt you could get one for $10, but ask these guys:http://www.miracle.net.in/Major cities have smaller hole-in-the-wall shops that do transformer winding.

author: johnwa
author_rank: Frequent Contributor
timestamp: June 18, 2018, 08:14:23 am
content: Quote from: Ian.M on June 17, 2018, 02:16:30 pmOr, you could wind the 'buck' secondary on the MOT core, if there is enough room - which is really just the same thing as adding some extra turns to the primary... Use the same thickness wire as the original primary, and make sure it is well insulated.

author: Dom13c
author_rank: Regular Contributor
timestamp: June 18, 2018, 11:06:14 am
content: Can you give an example of any junk part from which I can obtain one of those bad boys??

author: Ian.M
author_rank: Super Contributor
timestamp: June 18, 2018, 11:15:43 am
content: Quote from: johnwa on June 18, 2018, 08:14:23 amTechnically you are correct, but insulating it well enough not to compromise the primary to secondary isolation is *EXTREMELY* difficult, especially considering you are working with a closed welded core.It would be a completely different matter with an interleaved E & I lamination core (or a clamped E & I core) transformer as both are dismantleable so you could build a narrow insulating bobbin for the extension to the primary, wind it, terminate it and fully insulate it before sliding it over the center leg of the core and shimming and gluing it in place next to the primary bobbin.Cutting the welds to open the core isn't a good option either. Its *VERY hard to get a microwave transformer core back together without increasing the gap which has drastic consequences for its no-load current.I'm personally not willing to take the risk of hand winding any part of a primary in-situ on a closed core transfomer that needs to remain isolated, and I wouldn't advise Dom13c or any other novice with no formal training in the art of transformer design and construction to do so either.

author: Dom13c
author_rank: Regular Contributor
timestamp: June 18, 2018, 11:20:51 am
content: Well, I totally agree with you. Can you PLEASE suggest me something from which I can get one of those transformers

author: Ian.M
author_rank: Super Contributor
timestamp: June 18, 2018, 11:41:50 am
content: Quote from: Dom13c on June 18, 2018, 11:06:14 amQuote from: Dom13c on June 18, 2018, 11:20:51 amYou just aren't going to find 400VA 40V 10A transformers at all easily. Old audio amps are probably the best bet, but a 10A rated transformer will generally go with a higher secondary voltage as the DC rail voltages and currents required for a particular wattage amplifier are pretty much set by the nominal speaker impedance. Also they retain their value if working or easily repairable, as there's always *someone* who wants a sound system.If however you are talking about the buck transformer:Stuff with 24V 2A transformers in is fairly rare nowadays. Most devices needing power in that range would use a SMPSU. Even 12V 2A transformers aren't very common.However a low cost Lead Acid battery charger will probably have what you need - the lo-tech type with an analog charging current meter and a switch for 6V/12V and a Normal/Fast charge switch, (not the modern slimline sort with indicator LEDs and claims of 'digital technology' as that will use a SMPSU and you need a line frequency transformer). Its only going to be about 12V RMS for the full secondary.Unfortunately even beat-up old battery chargers are going to be rare in a developing or semi-developed country, as there is always *someone* trying to keep an ageing vehicle on the road who cant afford to replace a failing battery so needs to charge it regularly.Remember, the buck transformer is *ONLY* needed if the microwave transformer overheats on no load. Rather than trying to scavenge a suitable buck transformer to fix a problem with a crappy microwave transformer, you'd be better off putting your time into finding a higher wattage better quality microwave to scavenge a transformer from that doesn't overheat.

author: Dom13c
author_rank: Regular Contributor
timestamp: June 19, 2018, 05:46:10 am
content: Would it be wise to use multi threaded house wiring wire to wind the secondary. As the place where I live lacks the shops which sell single core wire with thick insulation. And I'm pretty sure that using thick gauge magnet wire to wind the secondary is not a good idea due to the overheating stuff as Ian.M mentioned. And I swear that * someone* in India is really a problem to a hobbyist who wants to acquire something for cheap. And in such a developing country there exists almost nothing that can be classified as junk (especially electronic goods expect chinese stuffs)as there is alaways *someone* who wanna reuse something broken after getting it repaired

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2018, 07:11:01 am
content: No. You *NEED* magnet wire. Ordinary insulated wire wont do because the insulation forms too high a proportion of its cross section, cannot withstand high temperatures, is too soft, and doesn't conduct heat well laterally. As a result,you wont be able to get enough turns of thick enough wire through the core apertures, and whatever winding you do manage to get in there will be far more susceptible to overheating, insulation breakdown and failure.

author: johnwa
author_rank: Frequent Contributor
timestamp: June 19, 2018, 09:02:57 am
content: Quote from: Ian.M on June 18, 2018, 11:15:43 amYes, that's probably fair enough, especially if you are aiming for "double insulated" between primary and secondary. PVC insulated wire is obviously unsuitable given the operating temperature, and enamelled copper is very susceptible to being scratched by the core. I guess if you had to do it, it would probably be best to thread some high temperature, insulating, flexible tube once through the core, then thread the wire through it. But the OP is probably better off with an external transformer.Quote from: Dom13c on June 19, 2018, 05:46:10 amOne possible technique(onlyfor the secondary, where insulation is not quite so critical) is a flat strip of aluminium, insulated with adhesive tape on one side (Kapton tape would be good, but paper masking tape is probably a better choice than PVC electrical tape.) You can either fold the tape over the edges to insulate them (which does waste space in the core), or make up some separate U-shaped insulators. I used this method to rewind a couple of MOTs to make a welder:http://www.loopgain.net/welder/

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2018, 10:45:43 am
content: *EXCELLENT* idea johnwa!If my maths is correct, 50mm wide aluminum tape, with 0.1mm metal thickness would dissipate about half a watt per meter carrying 10A. Assuming a total thickness of 0.13mm + 0.15mm masking tape, it will be something like 0.3mm/turn, so a 50 turn winding should fit comfortably. 50mm x 0.1mm aluminum tape should have the same current carrying capacity as 12 AWG copper wire.You may even be able to fit in two layers of aluminum tape (in parallel), which if you cut one narrower, lets you fold the masking tape round the edges maintaining a near-constant thickness so there are no significant voids in the middle.However terminating it is going to be a PITA. It can be carefully concertinaed down to a more or less square bundle which can then be covered by heatsink, but you'd need to use solvent to strip the adhesive layer from the very ends beforehand, lightly smear with noalox (or other metallic zinc powder loaded electrical grease) then crimp in tin plated copper terminals, or use appropriate saddle clamp terminals.

author: tautech
author_rank: Super Contributor
timestamp: June 19, 2018, 10:55:50 am
content: The OP might find this transformer winding guide useful:https://ludens.cl/Electron/trafos/trafos.html

author: Ian.M
author_rank: Super Contributor
timestamp: June 19, 2018, 11:05:58 am
content: Quote from: tautech on June 19, 2018, 10:55:50 amBookmarked!

author: Dom13c
author_rank: Regular Contributor
timestamp: June 19, 2018, 04:12:46 pm
content: Man, you might be a thought reader! Just neede this one. Well, just for information I've studied science in my high school so I'm not a completely novice in electronics. NOT to be thought me as a boast😋

author: johnwa
author_rank: Frequent Contributor
timestamp: June 20, 2018, 08:05:08 am
content: Quote from: Ian.M on June 19, 2018, 10:45:43 amThanks!I reckon the aluminium strip I used would have been about 0.5mm think, but this was for a lower voltage, higher current application. Agreed on the termination, if you look closely at my photos you can probably see the bodge job I did of forcing the strip into some DIN rail terminals. That was quite along time ago, the Al strip has probably cold flowed and oxidised by now. But it doesn't matter, I have since upgraded to a 'real' welder (which, funnily enough, suffered a failure in the termination ofitsaluminium secondary winding a couple of years ago.)

author: james_s
author_rank: Super Contributor
timestamp: July 10, 2018, 03:56:27 pm
content: I know very little about India however I would expect that microwave ovens are probably the most likely item you'll likely find scraps of which will contain a suitable transformer core. An old style arc welder is another item containing a big transformer core which I expect would be found in virtually all parts of the world however you might not find a scrap unit. If there are any metal recycling scrapyards around you could go there and see if they have any large transformers, they are popular scrap items due to the copper windings.Another thing you can do if you need a lot of low voltage power is to find some hot swap server PSUs, most can be modified to allow two or more to run in series, so you can have 24, 36 or 48V. They will often be available anywhere that has datacenters nearby, so I'd expect at least some of the more tech-centric areas of India would have some.

author: Dom13c
author_rank: Regular Contributor
timestamp: August 01, 2018, 10:19:54 am
content: Quote from: james_s on July 10, 2018, 03:56:27 pmMaybe I'll find one someday.