Scored A Nice Iso Off Of Ebay - Now To Wire It Up - Page 1
Forum section: electronics / projects

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: August 30, 2012, 10:51:05 pm
content: I just picked up a huge 1500W isolation transformer off of ebay (this one:http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&item=290765236993&nma=true&rt=nc&si=QTKqC4i0B9UusK2TJvX0e0ElvJk%253D&sspagename=STRK%3AMEWNX%3AIT&orig_cvip=true&rt=nc).I work on DC stuff all the time but don't mess with mains wiring much. The L,N and Ground on the left side of this transformer are obviously going to the corresponding connections to the mains. The two wires on the right are obviously the isolated Live and Neutral. Where I get a little confused: the isolated side has no earth so I would assume I run a ground (from the DUT's corresponding earth prong) to whatever metal chassis I fit this thing into (no?) but then shouldn't the other side of the transformer (the mains side) also be grounded to the chassis? This defeats the whole isolation principle so I'm obviously not thinking this out properly...

author: KTP
author_rank: Frequent Contributor
timestamp: August 30, 2012, 11:21:28 pm
content: Heh, evidently the seller was or is doing what I used to do...get free medical lasers and strip them apart. I never thought about selling the isolation transformers though....I just threw them in the recycle bin at the local dump.With the price of copper now though, that transformer you bought sold for practically the copper cost. $29 for 45 pounds of transformer? I know the steel core is a large part of it, but there is a bunch of copper in there too.If you find that that one is not large enough, I still have a few 5000VA ones out of 100 megawatt ruby laser systems. Only weight about 120 pounds!

author: gxti
author_rank: Frequent Contributor
timestamp: August 31, 2012, 12:24:16 am
content: For general use you should wire the downstream outlet's earth to the upstream earth. This is partly for safety but also for EMI suppression -- oscilloscopes for example don't work very well without mains earth. This doesn't defeat the purpose of isolation because the hot and neutral wires are still floating with respect to earth, so if the ground clip of your scope grazes something it won't leave a smoking crater. You can also leave the earth unconnected but then you should only connect devices under test, not test equipment. You should also keep in mind that normally safe Class I devices will not have their chassis earthed in this scenario and so a fault could connect the hot wire to the chassis. However, because of the isolation transformer it will require other circumstances to receive a shock.

author: Rerouter
author_rank: Super Contributor
timestamp: August 31, 2012, 02:53:00 am
content: for an isolation transformer earth is tied to neutral at the output of the transformer, similar to how your house has them terminated together at its entry point,

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: August 31, 2012, 04:16:58 am
content: Quote from: KTP on August 30, 2012, 11:21:28 pmHa, 5000VA is way more than anything my bench is likely to see anytime soon, hek the transformer I just picked up is way more than enough in itself. Thanks though.Quote from: gxti on August 31, 2012, 12:24:16 amI'm having a tough time grasping this. What does it matter if the hot and neutral are floating if earth is still tied to earth? Isn't this what causes the (safety) issue to begin with (since the scope's bnc connector is directly tied to earth - potentially causing a loop)? forgive the ignorance.Quote from: Rerouter on August 31, 2012, 02:53:00 amI see, so in this case, you're grounding earth to neutral and NOT to the chassis (on the secondary side). And I assume... its ok to ground the incoming earth connection to the chassis (or is that even necessary in this case)?EDIT:Something else, I'm definitely going to wire a switch into this but how about a fuse or breaker (12-15 Amp maybe to gain the full potential of the transformer - good/bad idea)?

author: amyk
author_rank: Super Contributor
timestamp: August 31, 2012, 05:59:42 am
content: Quote from: StubbornGreek on August 31, 2012, 04:16:58 amThe neutral line is called such because it is tied to earth and should be at earth potential, whereas hot is going to be at full line voltage relative to it --- and the earth. The safety issue arises because anything live has this potential relative to the earth.On the secondary of the isolation transformer, the two terminals (ends of the coil) are going to have the line voltage onlybetweenthem, and are symmetrical: there's no distinction between one being at earth potential or not, because they're both floating. The earth terminal can be connected to mains earth, but still no current will flow if only one of the two terminals were to contact it, because there does not exist a potential difference there. This is the safety benefit: you need to have a path between both the output terminals for current to flow, instead of between one (the live) and the much bigger ground, as is the case without isolation.

author: madires
author_rank: Super Contributor
timestamp: August 31, 2012, 12:02:33 pm
content: Quote from: gxti on August 31, 2012, 12:24:16 amPlease don't power any test equipment via an isolation transformer unless you really have to. The advice about connecting the downstream earth to the upstream earth might decrease safety. If the DUT has an issue regarding power line isolation earth could be connected to one of the floating outputs of the isolation transformer. If you start probing with your scope (ground connected to earth!!!) you'll close the circuit. Bingo!

author: Rerouter
author_rank: Super Contributor
timestamp: August 31, 2012, 12:11:11 pm
content: yes the whole point of isolation transformers is to not connect the grounds, mains earth is tied to the casing, and isolated earth to isolated nuetral, as you can touch either side of the isolated output safely, just not both at the same time, as both are truely isolated circuits,so your mains referenced scope can then be tied onto near any mains potential without issue, just you have to be aware the case of your scope then floats at that potential,

author: nctnico
author_rank: Super Contributor
timestamp: August 31, 2012, 05:19:19 pm
content: IMHO its better to use a GFCI (ground fault circuit interruptor). It covers much more fault modes.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: August 31, 2012, 05:43:32 pm
content: Ok, so my understanding is this: Connect primary earth to chassis and mains earth, connect secondary neutral to secondary earth and call it a day.I understand that I'll need to be careful not to create a new loop with my scope (especially when using multiple probes).Quote from: nctnico on August 31, 2012, 05:19:19 pmI'm thinking that you'd still have a safety issue as your scope could still potentially create the loop this way, no?

author: IanB
author_rank: Super Contributor
timestamp: August 31, 2012, 06:24:09 pm
content: Quote from: StubbornGreek on August 31, 2012, 05:43:32 pmReading through this thread, this is all complicated enough to make one's head explodeThere is no "secondary neutral" of course. Both secondary outputs are symmetrical and equally live respect to each other, equally isolated from everything else.I feel that it's fundamentally bad to create two different earths in the same work space (e.g. a "mains earth" and a "secondary earth"). It's bad because these two earths could develop different potentials and you might easily get confused or forgetful about which is which.Better I think to say earth is earth and make sure it is always the properly grounded mains earth. Keep test equipment like oscilloscopes powered from the mains and properly connected to mains earth.If the device under test is to be isolated, make sure it really is isolated. Power it from the floating outputs of the isolation transformer and don't ground it. Let its chassis float. Bear in mind however, that as soon as you connect a scope ground probe to something you will reference that point to mains earth and now any other part of the circuit could have a dangerous potential.In short there is no way to completely eliminate risk when working with dangerous voltages. While the voltages exist you can get a shock and the only way to reduce the risk is to use appropriate care and handling when working with the circuit.

author: madires
author_rank: Super Contributor
timestamp: August 31, 2012, 07:41:20 pm
content: Quote from: StubbornGreek on August 31, 2012, 05:43:32 pmKeep the secondaries floating! If the chassis of the DUT is connected to one of the secondaries you got some problems more.QuoteYep! And also take care that you don't create a new loop with your body.QuoteA GFCI has no idea about the isolated secondaries. Only helps with directly connected stuff.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: August 31, 2012, 08:01:26 pm
content: OK, got it now: float the secondaries completely - no earth.I really appreciate the input from everyone; I'll make sure to post some pics of it all assembled.

author: SeanB
author_rank: Super Contributor
timestamp: August 31, 2012, 08:20:11 pm
content: As well remember that there will be some capacitive coupling between the windings, Connect a multimeter set to the highest AC voltage range between the PE terminal and one of the secondary leads with nothing plugged in, then you will probably get a reading of about half the mains voltage. This can only be reduced by having a separate bobbin for the secondary on a separate transformer limb, and an earthed core.

author: nctnico
author_rank: Super Contributor
timestamp: August 31, 2012, 11:37:32 pm
content: QuoteFor clarification: don't use an isolation transformer. Before GFCIs became common an isolation transformer was often used to provide some means of 'protection' against accidentally touching mains voltages in repair workshops. In some really old TVs the chassis was directly connected to the mains.If you ever need to measure in circuits connected to the mains use two probes and the subtract function of your scope or get a decent differential probe.

author: Berni
author_rank: Super Contributor
timestamp: September 01, 2012, 08:08:49 am
content: Well you didn't say for what you actually want to use this isolation transformer.I have one laying around for when i work on anything that is connected to mains (Most recently that was a night activated switch for 220V lights that has a transformeless PSU). I always power the circuit im working on from the isolation transformer because i can touch any wire inside the circuit without getting a shock (As long as i dont touch any other part of the circuit of cruse). This just makes me much less nervus to poke around the thing while its powered on. Another beneficial thing is that the transformer somewhat limits the current you get from mains and so turning a short circuit in to a spark rather than a loud wire vaporising bang as perhaps 500A or even more rush in to your circuit.