Proper use of an isolating transformer

Discussion in 'General Electronics Chat' started by tom66, Dec 8, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I bought a 500VA isolating transformer so I can test a power supply's primary side with an oscilloscope.

    I need to fit a plug and a socket on the other end, but it's otherwise complete. It looks to be ex-MOD (ministry of defence) equipment.

    As I understand it, I connect the equipment under test, in my case usually a TV as I repair them, to the transformer. This then allows me to touch the primary side with an oscilloscope which is earthed, of course with a 10X probe and a high voltage rated scope (in this case 400V.) I can probe relative to the hot ground which is usually the -ve terminal of the main filter cap. I must be careful not to touch both live and neutral at the same time but that normally applies when working on mains wired equipment.

    Have I missed anything? Safety precautions etc.? Should I fit a GFCI/RCD on the output of the transformer?
     
  2. Jotto

    Member

    Apr 1, 2011
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    Never touch the isolation transformer and the metal chassis your working on. I take it your working on CRT tv's. I still use a isolation transformer working on LCD units, not because I need one, just because I have a couple of them.

    I use a Sencore PR570, which you can check hot to external earth ground, hot to safety ground, hot to neutral, and isolated output.

    You can also set the trip so you don't do anymore damage, I usually set it for a bit less then the actual fuse in the unit. It will display current draw in amps or watts, show hot chassis ground, and leakage. A/C voltage can be adjusted.

    What I do now is plug my power supply into the PR570 and then run the power to the unit under test. I also use this unit for troubleshooting SMPS. I also use the filter cap, no need trying to find a better spot, its faster this way.

    I would recommend this unit to anyone that wants a well built unit. I am not sure it is manufactured anymore. Sencore sold out to another company, I can't remember the name but they are more into video and audio calibration. So, the test equipment is taking a back seat and some is being discontinued.

    Nice unit you have there too.
     
  3. debe

    Well-Known Member

    Sep 21, 2010
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    No need for a RCD on output side of transformer as its isolated & wont work. I use a 150W Isolating T/F salvaged out of an old TV for working on TVs & SMPS, works fine.
     
  4. tom66

    Thread Starter Senior Member

    May 9, 2009
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    I'm actually working on LCD and plasma TVs. I've decided it's not really worth working on CRTs. I've tried to give one away - even offered to deliver it - but no one would take it (not even the charity shop) so it went to the tip :(.

    I'm encountering many TVs now with bad power supplies, but not the usual bad caps or diodes - more complex primary side faults. In one case, I've found bad poly caps on the primary side, which didn't provide enough drive to the transformer to power the backlight. (Half bridge topology.)

    I disassembled the transformer case. Inside there is a huge torodial core transformer, encased in epoxy. The live/neutral goes to the transformer and red/black is the isolated output. I assume live/neutral here is irrelevant as it's isolated, it can go either way around. The isolation transformer's metal case is connected to the mains earth. There's a lead that goes from the metal case which is an earth tag. The output cable has an earth wire, but it seems to be optional, as the currently fitted connector (some kind of odd proprietary one) has a screw lug to remove the earth. The case is painted, and appears to have practically infinite resistance (>66Mohm on my meter.)

    In many power supplies, the earth is connected to the PSU common, but not to any part of the hot side, except maybe through some class-Y caps, so in theory it shouldn't matter if the earth is connected. Am I correct in assuming this?

    It was a bargain too for what it is, only £30. Probably worth at least that in copper and iron alone! (it weighs ~10-15kg)

    The PR570 looks nice, but for now is a bit out of my price range.
     
    Last edited: Dec 8, 2011
  5. Jotto

    Member

    Apr 1, 2011
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    Very unusual to have a lot of problems on the primary side. Guess its those parts made in China.

    Yes the ground is optional on my unit also. Some of mine are also as you have described. I have never had a problem using the isolation transformer in that manner.

    I have a crazy isolation transformer. Its 100 vac input, 120 vac output. I can push it higher, but what is the point of that. I picked this one up for 50 cents or maybe it was 25 cents. Load capacity is 2000 va, freq 50-60 hz, 16.7 amps, 333 va for trans cap.

    Are you recharging the plasma tvs?
     
  6. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Look up the 17PW22 series of power supplies... these are satan's spawn. They NEVER seem to work properly and it's never an easy fix. And these are Turkish power supplies, but made with whatever components were cheapest on the day, it seems they change from day to day. I've seen four such supplies so far and they are all different in their own "special" way.

    That's good to know. I'm going to give it a try later today.

    Wow. Nice find. Where did you get it?

    Recharging? Not sure what that means. I have fixed about 4 plasmas so far. They are usually major faults like no picture, in all cases a shorted high power component on the sustain boards. I can't usually go further than testing the fuse and a few basic primary side components if I've got a power supply that doesn't work properly. I recently got an ESR meter so I can check the caps on the PSU, too.
     
  7. Jotto

    Member

    Apr 1, 2011
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    I don't know how to add quotes like you have. I know its a "+ and something".

    Recharging is the organic material that is used in the screen. They degrade over time and must be recharged. Its not hard to do, start reading about it and you will be ready when the time comes. Taking your time is the most important part of recharging a plasma screen. I made a mistake calling it recharging, its actually replacing coolant and its on the CRT/projection type TV. It turns milky after a while. I just remembered a friend talking about it, So forget what I said.

    That isolation transformer was built in Japan by Sugand Electric Laboratory. It was at a yard sale. I also got a TV7 tube tester like that. Get a lot of offers on that one.

    Just got done repairing a couple of Lilo LCD units, hardest part is getting them apart.

    The ESR meter is a great item to have. Which one are you using? There is also a site that has a nice one that you can build fairly cheap. I think they use a pic for that also. I use mine every day instead of setting up the LC103, its quick and fast.
     
    Last edited: Dec 9, 2011
  8. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Ah, just add [ /QUOTE ] and [ QUOTE=Jotto;430307 ] between each part. Without spaces, of course.

    Nice. This transformer was made in the UK, I think.

    Some newer TVs and LCD monitors I am seeing have less and less screws; they rely on press-fitting. Very difficult to get apart.

    I'm using the Atlas ESR70. Works very nicely; one touch test, small enough to fit in a pocket, and does capacitance as well as ESR.
     
  9. Jotto

    Member

    Apr 1, 2011
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  10. tom66

    Thread Starter Senior Member

    May 9, 2009
    2,613
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    Well, I fitted the plug & socket and was able to get several devices to work on it. I found turning it on occasionally trips the shed breaker which is a 16A rated unit, but resetting it fixes it. I'm suspecting inrush current is too high, maybe I should change the MCB for a higher rated unit.

    However, yesterday I tried to probe the filter cap of a PSU but I seem to be measuring a 50 Hz ripple waveform several hundred volts p-p, not an almost flat DC voltage.

    The earth terminal of the socket is disconnected. Isolated live/neutral. Scope GND was connected to -ve terminal of cap. Probed the positive end. Scope was running on the non-isolated circuit (was this the problem?)

    Also, I just picked up two very nice 15A variacs from eBay. £52 for two, not bad at all. They need a bit of maintenance, will have to fiddle with them tomorrow.
     
  11. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Does anyone have a solution for my problems?

    Got one variac working already. Discovered an LCD TV will operate perfectly happily on 60Vac (rated 100V - 265Vac.) Starts going a bit crazy below that, picture flickers on/off rapidly...
     
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