Modding an isolation transformer, please advise - Page 1
Forum section: electronics / projects

author: BravoV
author_rank: Super Contributor
timestamp: January 01, 2012, 02:17:32 pm
content: Just bought a never used, new in box isolation transformer from local tech surplus shop for about $50 US, the seller claimed its rated 1000 VA. The reason they offered this price level was because it was mishandled during shipping and the unit's case is somehow deformed at the bottom, check photo "02. Current Condtions.JPG' for the detail of the defect.The physical looks very new without any dents or scratches, and no dust at all and came with box. I was allowed only to test it briefly with their cheap dmm for resistance checking between primary and secondary windings just in case there is an internal short, also a very brief 5 minutes load testing on a PC with crt monitor to check the input vs output voltage, thats all.There are no descriptions nor labels at all at the unit's front/rear plate, the only label printed at the box is just the words "Isolation Transformer" and without any brand either, while internally there are only two labels indicating its "1000 VA" and 220 Volt rating on primary & secondary termination, detail on photo "03. Terminations.JPG".I used to have a smaller one rated at 200 watt was used for working with high voltage measurements like probing with scope on switching power supplies and etc, but unfortunately I never did open it up to see how its wired inside, and don't have it with me now.Appreciate any help or advise on modding this into technician isolation transformer, so far I've been reading at the net and also watched the great tutorial like the one from toddforfun's video on youtube. Although the connections are very simple, its just I want to be really sure since this is a high voltage stuff that must be taken very seriously.Made the the schematic below after dismantling the whole unit, traced all connections, and also made few measurements on the transformer windings electrical properties :Questions :1. Is that the right modification just by cutting the ground connection to the Neutral at secondary, to turn this transformer into a technician version of isolation transformer ?2. How about the connection from the Ground to the iron core ? Just leave it as is ?3. You can see for yourself that the unit is just so plain without on/off switch, indicator light and fuse like the one I used to have. Where is the best place in the circuit to put those additional components ?4. At this price and with very minimal labeling or description, from the detail photos below and the measured electrical properties, does this transformer really rated at 1000 VA ?5. I'm planning to do a load test, and I have a killawatt to measure watt consumption, is it safe to use a high wattage load like hair dryer, an iron or even my rice cooker to test loading it.6. What happened if an isolation is overloaded ? Symptoms ? What kind of measurements should I check to see if it reached it's max rating ?....or any other suggestions or considerations that I missed.Pictures below show the current conditions of the unit when I brought it home, planning to hammer that case to straighten it's base, also as you can see the space inside is quite empty even with the transformer in it.Included few closeup shots to show the built quality of that transformer, really like to hear your comment about the transformer built quality as well, one thing for sure, this thing is damn heavy.Thanks in advance & Happy New Year !

author: Zero999
author_rank: Super Contributor
timestamp: January 01, 2012, 03:29:37 pm
content: Quote from: BravoV on January 01, 2012, 02:17:32 pmYes.QuoteDon't cut the earth to shield/iron core connection, it's required for electrical safety.QuoteI'd recommend a slow blow 5A fuse/breaker in series with the primary.QuoteIt seems reasonable to me, more than anything else, mass is normally correlates with the power rating and minimum working frequency of a transformer.QuoteYes, as long as you don't overload it for too long.QuoteIt gets too hot and will start to smoke and catch fire if left overloaded for too long.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 01, 2012, 04:38:34 pm
content: These aren't the droids you're looking for.Ahm, this is not the isolation transformer you are looking for. What you have is an isolation transformer to be used to reduce Neutral-Ground noise, hence the connection of N and G on the secondary site. This is not intended to protect your life when doing repairs on mains-powered equipment. I see a lot of things wrong with modding this thing into an isolation transformer for repair work.E.g. the unknown provenance and the unknown specifications of the device and esp. the transformer worry me. You have absolutely no guarantee or indication that the isolation of the transformer is sufficient for your purpose.The multiple outputs on the secondary worry me. Several local electric codes prohibit the connection of more than one piece of equipment to an isolation transformer (of the kind for repairing stuff), require special precautions when doing so or require only special trained personnel doing such a connection.The way the ground connections are done worry me. Some electric codes prohibit the usage of mechanical elements (case, transformer, lugs) in between to join grounds together. Instead, all ground connections need to be properly joined together.In my opinion this is some stuff I wouldn't use as the start for building an isolation transformer for working on mains equipment.

author: BravoV
author_rank: Super Contributor
timestamp: January 02, 2012, 07:58:16 am
content: Quote from: Hero999 on January 01, 2012, 03:29:37 pmThanks , so a single pole circuit breaker is enough ? Will it be excessive if I used the one with double poles both series on live and neutral lines ?Quote from: Hero999 on January 01, 2012, 03:29:37 pmSo by that 12 Kg in weight, it is close to a ballpark number for 1000 VA rated transformer, is there any simple formula for that ?Quote from: Hero999 on January 01, 2012, 03:29:37 pmWill the secondary voltage sag once it near its rated wattage ? Cause I've been thinking to measure the voltage with dmm and Killawatt at it's output, and another dmm at primary for voltage comparison. Is this method adequate to detect an overloaded condition ?A destructive test by load testing it just to check it's rating until its toasted is not in my plan.lolActually I'm pretty sure that I won't use it at more than 500 watt load.Quote from: BoredAtWork on January 01, 2012, 04:38:34 pmReally appreciate that you've raised this concern, yeah, I'm aware of the risk, its just I can't afford a new one, its cost almost > $500 here, while the one I owned previously its gone, it was given by a friend who is a retired tv serviceman. I guess I have to live with it, again, thanks.Btw, with your experience, specifically about the transformer built it self, any comments ?Quote from: BoredAtWork on January 01, 2012, 04:38:34 pmOk, so its better to have only single output than those 3 in parallel, should I put another fuse or circuit breaker at secondary ?Really curious how are those parallel connections look like if they're done properly ?Quote from: BoredAtWork on January 01, 2012, 04:38:34 pmAppreciate if you could shed some light on how to make a proper ground connections for this purpose.

author: Psi
author_rank: Super Contributor
timestamp: January 02, 2012, 08:49:51 am
content: Transformer safety? meh.. who needs itSeriously though, you do need to be careful with isolating transformers, they can give a false sense of security.If you touch phase and neutral together you will get a shock far worse than a typical phase shock which goes through ground (since there is resistance in the path from your feet through the floor/house and back to neutral, which limits the current somewhat).

author: eternal_noob
author_rank: Regular Contributor
timestamp: January 02, 2012, 09:00:57 am
content: QuoteWeight can give you a ''roughly-rough'' ballpark indication of VA rating, but there are better methods for determine this. One is to measure the cross-sectional core area and use a chart like this:Second is to load the transformer and measure the voltage drop, like this:''A more scientific way to determine VA capability is to power up the transformer on your bench and measure the no-load secondary voltage. Now add some resistance (big power resistors) across the secondary until the voltage drops by 30% for small transformers (<50 VA), 20% for medium sized transformers (50 to 100 VA) and 12-15% for big suckers (>>100VA). For high efficiency semi-toroidal transformers, all voltage drops will be roughly 1/2 of these listed. Make note of the voltage under load and use ohms law to discover VA rating:* Rated secondary current = E/I = voltage under load/resistance in ohms* VA rating in watts = I*R = current from above formula * voltage under load''Pictures and info is taken from this website:http://geek.scorpiorising.ca/lcms/index.php?page=windingtransformersIf you want to know more about transformers I'll suggest you download and read this book:http://www.mediafire.com/?egie7lacm1qe611

author: Unknown
author_rank: Unknown
timestamp: January 02, 2012, 08:29:43 pm
content: A reasonable way to estimate by weight is to compare it with datasheets of similar transformers, if available. Try searching the big distributors (eg. Digikey, Mouser, Newark/Farnell/Element14, RS) for something similar.

author: wkb
author_rank: Super Contributor
timestamp: January 02, 2012, 08:56:17 pm
content: Quote from: Psi on January 02, 2012, 08:49:51 amHealth & Safety Act is a Government plot anyway

author: Ajahn Lambda
author_rank: Regular Contributor
timestamp: January 03, 2012, 05:18:30 pm
content: Quote from: Psi on January 02, 2012, 08:49:51 amOMG what am I looking at?!?Is that cotton-insulated wire wrapped around what looks to be a cardboard (and assuming some iron) core?Ah well, looks like he got some nice booger welds out of it.

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 03, 2012, 06:49:14 pm
content: Quote from: SigEpBlue on January 03, 2012, 05:18:30 pmOfftopic:http://www.africancrisis.co.za/Article.php?ID=15164&crazy...

author: RJSC
author_rank: Regular Contributor
timestamp: January 03, 2012, 10:42:39 pm
content: Quote from: BoredAtWork on January 01, 2012, 04:38:34 pmWhy?As long as you don't hook up CH1 scope probe ground on one device and CH2 probe on the other without checking if the 2 ground points are the same (causing a short circuit trough the scope) or doing the same with your 2 hands, where is the problem?Quote from: BoredAtWork on January 01, 2012, 04:38:34 pmIf its only some of the electrical codes, than it should be that bad.In most cases (with screws threading into bare metal) only if the device itself breaks apart the ground gets interrupted.In the case of the transformer, it can be bad because usually the laminations are coated in varnish insulating them from one another to reduce eddy currents, or else the laminations wouldn't bring any big advantage over using a solid core.This varnish can also insulate the laminations from the screws that cross it and where the grounds are connected.I would connect the 2 grounding wires with a jumper similar to the one joining the ground and neutral, and remove the one joining earth and neutral.As for the multiple sockets, if you really take in to account what I have said earlier, you can leave it, but if you are one of the people hat sometimes makes short-circuits by distraction, then I would remove the extra 2 sockets and find some covers to it or find another box if it as cheap or recovered.About the insulation rating between windings, if you are just connecting the ground of the scope to the output there shouldn't be any problem.However I wouldn't trust it for some exotic uses such as adding some extra 230 V floating on top of an high voltage source, like floating it on top of things like neon sign transformers, microwave oven transformers, Tesla coils, etc, which would put over 1.4 kV of electrical stress on the insulation.On the typical situation of grounding the circuit with the scope the inter winding isolation only has to stand 650 V peak in the winding are in opposite phase and only 325 V if they are in phase.

author: BravoV
author_rank: Super Contributor
timestamp: January 06, 2012, 06:50:14 am
content: Thanks for the great advise and infos, really sorry unable to catch up with this thread, its just I've been so busy traveling lately.Quote from: Psi on January 02, 2012, 08:49:51 amHa..ha.. thats really scary.Quote from: Psi on January 02, 2012, 08:49:51 amYes, I'm fully aware of that, thanks for the reminder. I used to have a really good mentor which is retired now, he was an experienced tv serviceman that worked through his life back there when tv was still using vacuum tube. Actually my 1st isolation transformer was a gift from him, and he teached me alot tricks & tips using it with scope or other measurement tools.Its just that transformer already gone given to good friend of mine, and I never really seriously take a look inside that transformer.Quote from: eternal_noob on January 02, 2012, 09:00:57 amThanks alot for those infos & links, really help a lot, btw my iron core's center area is 4 cm x 10 cm = 40 sq cm = 7 sq inch, its off that chart, pretty convincing that its above 500 watt. Anyway, I believe I will never use it more than 500 watt.Quote from: eternal_noob on January 02, 2012, 09:00:57 amGreat, coupled with watt meter, this should be good enough method to determine it's max rating.Quote from: eternal_noob on January 02, 2012, 09:00:57 amAnother interesting info is about the wire size, this one has about >1.5 mm and should be close to 14 AWG with max current at 5.87 Amps, great.Quote from: RJSC on January 03, 2012, 10:42:39 pmI've been thinking this too, actually the iron core laminations is quite good and that ground connections from input to output I believe is not beefy enough if they're connected just through that iron core as the medium. I will give it another separated ground wire from input to output, rather than rely on the iron core to do the job.About the output multi sockets, I'm aware of the risk and potential problem there, I've been thinking to remove those two sockets and use those two holes for two analog meters for measuring voltages or probably output current.Quote from: RJSC on January 03, 2012, 10:42:39 pmThanks, points & note taken.Other thing, I have bunch of used but still working RFI filters scattered around in my drawer, putting it at the input to primary is fine right ?