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I have a simple question, when troubleshooting a SMPS is highly adviced to use an Isolation transformer. But my question is this, if im using a scope to troubleshoot the "hot" side of my SMPS, both the SMPS AND scope AC power should come from the isolation transformer ?

OR

Should i connect the SMPS to the isolation transformer, and my scope to the house (non-isolated) outlets?

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  • \$\begingroup\$ What model of oscilloscope do you have? \$\endgroup\$ – Nick Alexeev Aug 3 '12 at 19:20
  • \$\begingroup\$ Tek 2335 and 2235. \$\endgroup\$ – S.s. Aug 3 '12 at 20:22
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Power the circuit you are testing thru the isolation transformer, and the scope normally (not thru the isolation transformer). It does matter which of the scope or circuit you isolate, since the scope has a chassis and connectors that you can easily touch. You want that stuff grounded so that the dangerous parts are a little harder to touch.

Note however that as soon as you connect the scope ground to some part of the device, all the other parts are now at ground-reference voltages, meaning they are no longer floating. You can get seriously zapped if you touch one of those other parts and ground at the same time.

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  • \$\begingroup\$ That makes sense, since the chassis 3rd pin of the scope will ground the ground of the SMPS, this is exactly why i asked the question, so thanks for your answer! Lets just go crazy for a while, and assume that i connect the SMPS to the isolation transformer, and i connect my scope not thru the isolation transformer BUT (and i know is a safety hazard) i lift the 3rd pin from the scope AC plug. Can this keep the SMPS reference floating? would the end result be the same as connecting both the scope and SMPS to the isolation transformer??? THANKS!!!!! \$\endgroup\$ – S.s. Aug 3 '12 at 20:18
  • \$\begingroup\$ @JoeM: Not connecting the ground prong on the scope is a really bad idea. The neutral will still be essentially ground, so the scope may not float like you think it will. Even if it does, you may hit the insulation limit in the scope between neutral and ground since it wasn't designed for that and you don't know what it is. \$\endgroup\$ – Olin Lathrop Aug 3 '12 at 20:45
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    \$\begingroup\$ @JoeM - if you want to safely measure floating voltages, it may be worth investing in a differential probe. Also, a workable but non-ideal method is to measure both signals (relative to ground) and use CHA-CHB mode (e.g. leave the difference between the channels) \$\endgroup\$ – Oli Glaser Aug 4 '12 at 13:24
  • \$\begingroup\$ Will definitely look into it, ive been wanting one for a long time now, thanks again Oli. P.S. someone suggested using a probe isolator to get by the fact that the scope grounds the smps when you connect the probe ground to the device like you described, thoughts? \$\endgroup\$ – S.s. Aug 17 '12 at 4:14
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    \$\begingroup\$ I just found this youtube video which explains a lot of this very well. youtube.com/watch?v=11Yve2ijWyk&feature=g-vrec \$\endgroup\$ – S.s. Aug 17 '12 at 15:02
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You connect the SMPS through the isolation transformer because the voltages on it are lethal and when directly connected to mains power may zap you hard. The transformer isn't a full proof solution, but at least you are safe when touching a single terminal.

Also when using a isolation transformer, you are more or less free to choose where to connect your scope ground, without short circuiting. If you connect both devices on the transformer (or none of them), then you cannot.

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  • \$\begingroup\$ Thanks for your comments jippie, not having to worry about burning up my scope due to connecting it to the wrong ground is something i like about isolation transformers, however, i always wondered if all the AC powered equipment had to be isolated too while troubleshooting something like a SMPS or old equipment with "Hot" grounds... \$\endgroup\$ – S.s. Aug 3 '12 at 22:38

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