Electronics > Beginners

General questions about Isolation i.e. how NOT to blow up your 'scope follow-up?

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FakeDave:
Hi! I've been tinkering with electronics for several years now, and getting familiar with an old analog scope while saving for a new digital one, and I want to take it to the next level by repairing some devices that might require looking at signals at higher voltages than just 5v logic. I've been doing a lot of research about how to avoid doing damage to scopes, DUTs, and myself when testing power circuits. I'm all about safety so I looked up what Tektronix had to say about it and they don't really recommend an isolation transformer anymore, but if you *must* use one that you use something like their A6901 Ground Isolation Monitor in conjunction with it, and I can see why. Windings fail, $tuff happens and your life is worth a little more investment. Then I looked into scope isolators, like the Tek A6902B Isolator and wound up getting an OK deal on one, the trade-off being it was missing the probe for the second channel (if anyone has a spare p850 probe, holla!). I also found what some of you had to say about it in a very old post here, and some advocated for using both an isolation transformer for the DUT and separate protection for the scope! Lower amperage isolation transformers like B&K's are cute, but getting anything over 3 amps gets expensive in a hurry. So what are your thoughts on the matter? And please be gentle, this is my first post.  Thanks!  :-+

helius:
First of all, welcome to the forum.
You aren't real specific about what your needs are: if you are probing devices that live behind the secondary side of a power transformer, you don't need any special isolation. This includes the vast majority of electronics outside their power supply modules.

FakeDave:

--- Quote from: helius on April 27, 2019, 06:23:50 pm ---First of all, welcome to the forum.
You aren't real specific about what your needs are: if you are probing devices that live behind the secondary side of a power transformer, you don't need any special isolation. This includes the vast majority of electronics outside their power supply modules.

--- End quote ---
Thanks! I was a bit vague about that 'cause I felt like the post was getting long-winded. I want to be able to do it all, really (OK within reason). I want to go from looking at things like serial signals one day to big power relays and switches the next.  I'm a broadcast engineer who didn't get their start with EE like all the "old-school" analog guys did, and I find that I've learned things through experimenting at home (on cheap things) that have applied to work (and much more expensive things) and vice versa. So I'm trying to make up for lost time by cramming as much as I can into my brain. My poor brain.  |O

Ian.M:
It doesn't matter whether you use an external battery + inverter or an isolating transformer, *ANY*  form of 'floating your scope' is strongly discouraged, unless the scope was designed for floating operation.   A scope is *NOT* a multimeter - a multimeter typically has operating controls with insulation designed to keep the user safe when probing a 1000V circuit, no matter which lead is 'hot' - a mains powered scope doesn't have that grade of insulation, and is designed with the expectation that its chassis will be grounded.   Competent engineers have died making measurements with floating scopes, so don't join them!   

bdunham7:
A couple of general observations for you:

1.  It is a mistake, IMO, to overthink the general case while underthinking the specific one.  For example, you worry about your isolation transformer not being big enough or failing.  I find very few cases where a bigger isolation transformer would be needed.  You should give an example of DUT where you feel you need a bigger transformer and then we can discuss how you would approach that specific case.  Buying super-safe equipment so that you can go poking around randomly is not a very good plan.  You have to understand each DUT and how to safely approach it. 

2.  I don't think of isolation equipment primarily as personal safety equipment.  Of course, in the case of old hot-chassis tube electronics, perhaps it is.  But for the most part, there are different reasons for isolation circuits.  I have a scope isolator in the same line as yours (A6909) and an old Scopemeter.  A typical use would be to scope a component, such as an amplifier output device, where neither side can be referenced to ground.  I can essentially use the scope in the same manner, with no more or less care, as a DMM.  This works great up to about 600VAC (300 for the Scopemeter) and has a significant advantage over differential probes in terms of circuit loading and isolation. 

3.  If you go to true high voltage--lets say anything over 600VAC, but I'm thinking about things like CRT circuits, microwave ovens, etc--then you need to shift gears entirely.  Because of the energies and frequencies involved, you need a ground-referenced high voltage probe, or  you need to be an experienced expert that has thought the situation through.  There's no "doing it all" in this realm nor any place for vague generalities.  And you need to understand dV/dt and why it matters.

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