Electronics > Beginners

Classic don't-blow-up-my-scope question

<< < (3/4) > >>

IanMacdonald:
If working on the primary side of a SMPS you absolutely do need an isolating transformer in its feed. This is because they use a bridge rectifier, so no part of the circuitry is at 0v potential, no matter which way round the supply is connected.  The same applies for some vintage TV sets which use a non-isolated bridge rectifier.

Vintage radios that are 'live chassis' (no mains transformer) likewise, although some people are happy to work on them after checking that the chassis is connected to the neutral, rather than the live wire. The problem here is that if the neutral were to come off the chassis would become live. Thus, not advisable. The isolating transformer arrangement is safer for these

On high voltage equipment such as valve guitar amps, which usually have a mains transformer anyway, there is no real advantage in using isolation, and I would just check that the chassis is grounded before working live on it.

The important point in all such cases is not to place your resting hand (or any body part) on grounded metal whilst doing high voltage tests. The temptation to do this on a  guitar amp chassis, vintage scope case or the like is ever-present, and should be resisted. Any mistake you make in touching a live part whilst one hand is earthed will have far more serious consequences.

I cringed at seeing Mr Carlson holding the metal case of a signal probe when doing live tests on valve gear. Much as he is a brilliant vintage equipment restorer, I would disagree on this point. Put some heatshrink on it. Or if nothing else is available wrap some tape round it. Just don't use it bare. Please!

Should you isolate your scope? Good question, and the problem of the case becoming live if the probe outer does, is a very good point. It could also be unwise on vintage CRT scopes with some whacking voltages inside, since it could allow such voltages to find their way onto the case. On modern scopes this is less of a concern though, and putting an earthed test lead onto a point which you thought was at 0v but isn't, is a good way to damage the DUT, let alone the scope.

The classic trap for young players here is audio amps with bridge speaker drive. Especially car audio. Need to scope the output, so put the probe croc clip on the 'negative' speaker terminal, and 'BZZZZT-POP!' as you fry the putput stage. Reason; in this configuration both sides of the speaker have power on them.  Of course isolating the scope won't help if you put two probe grounds on different places. In that case you just create a short between the probe outers.

Roll on the day when scope inputs are like multimeters.

exe:
Concerning modern scopes, the bnc connector is grounded. It's not that difficult to touch it.

DDunfield:

--- Quote from: exe on February 20, 2019, 05:16:16 pm ---Concerning modern scopes, the bnc connector is grounded. It's not that difficult to touch it.

--- End quote ---

There are little caps you can get specifically for the purpose of insulating unused BNCs, and there are probes readily available which have the scope end insulated as well (or you can tape it).
My point being that unlike the days of large metal boxes, exposed grounds on modern scopes can often be more easily insulated than whatever it is you are testing.
Neither (floating scope or DUT) is a good solution. I personally keep an old Fluke97 which is my go to when I need to scope something hot. Battery powered, well insulated even the BNCs and probes are insulated to reduce the chance of unintended contact.

exe:

--- Quote from: DDunfield on February 20, 2019, 05:45:28 pm ---My point being that unlike the days of large metal boxes, exposed grounds on modern scopes can often be more easily insulated than whatever it is you are testing.

--- End quote ---

I'm not arguing :). I'm the opposite, just adding to what you said.

I know these topics were discussed quite a bit, but I like we have a condensed version of all typical techniques on a single page.

Question: is it worth wearing gloves or something? Is it worth putting on rubber slippers?

Ian.M:
A reasonably thick rubber or plastic mat on the floor at the bench is a useful addition for safety if you work with mains or other high voltages and your workshop has a concrete floor or if there's any chance of damp flooring.    Rubber soled shoes on their own are less useful because the combination of a cracked sole, moisture and dirt, or simply treading on a drawing pin can cause them to become conductive with very little warning.  They are still far better than leather or fabric soles or bare feet.  However insulation under your feet or chair is useless if you come in contact with any other ground, so exposed and possibly grounded metalwork in your work area must be minimised.

Floating a scope, even a modern one with minimal visible exposed metalwork is still a *LETHALLY* *DUMB* idea when working with any voltage above 50V, as you don't know the insulation rating of the front panel controls.  All it takes is a scope floating at HV with one metal grubscrew in a small plastic knob to electrocute you.  This is not hypothetical - people have died floating scopes.  Also modern scopes with SMPSUs are likely to have excessive noise levels if not grounded as per manufacturer's recommendations.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod