Electronics > Beginners
General questions about Isolation i.e. how NOT to blow up your 'scope follow-up?
FakeDave:
--- Quote from: bdunham7 on April 27, 2019, 08:44:22 pm ---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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So that's your idea of gentle, huh? I kid. OK, let's start with this last bit first, I kept it general not because I'm willfully ignorant of the finer details of the subject but because I see myself working on a lot of different things in the future, and I don't want to have to buy something new in a year or two because what I use now isn't capable.
Yes I know that 'true' high voltage is much higher than average people think and it is something I have no interest being around, in fact I leave some projects that are say CAT3 to a bonded electrician. So please don't confuse my adventurous nature for bravado (I did say "within reason" after all.) That being said, eventually I want to experiment with Nixies and other tubes like geiger tubes so yes 300 volts or more isn't out of the question. And I know that most of what I'm talking about is usually multimeter territory but I want to see all the signals! so I do appreciate you mentioning a scope meter, they look very handy indeed and I've been reading a lot about them. So yeah, thanks for your 'observations'.
I would like to hear more about other people's favorite test gear while I'm on the subject. I saved an old Tek TM504 from the recycle heap and I've had a lot of fun tinkering with some modules, looking at all the different ones online and getting outbid on every single one I've ever bid on. |O It is the perfect system for me, compact and easily reconfigurable for a variety of roles. I love it, I'm even already thinking about a TM5006 for my rack. Yeah, I know. I have a rack. I had to sneak it inside while my wife was away. I'll see you in TEA shortly.
rstofer:
--- Quote from: bdunham7 on April 27, 2019, 09:35:57 pm ---The issue with differential probes that I think maybe is overlooked a bit is that they introduce circuit loading that may be more than some anticipate.
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I didn't watch the entirety of the 2d video I linked above but she is talking about differential probes having an order or magnitude more bandwidth than single ended probes. If this is true, I suspect they aren't hobby grade in terms of cost!
FakeDave:
--- Quote from: rstofer on April 28, 2019, 05:07:13 am ---
--- Quote from: bdunham7 on April 27, 2019, 09:35:57 pm ---The issue with differential probes that I think maybe is overlooked a bit is that they introduce circuit loading that may be more than some anticipate.
--- End quote ---
I didn't watch the entirety of the 2d video I linked above but she is talking about differential probes having an order or magnitude more bandwidth than single ended probes. If this is true, I suspect they aren't hobby grade in terms of cost!
--- End quote ---
No, they aren't, there's a link in another comment to the one that the EEV blog's own Dave Jones designed, that's the one I would buy if I needed one. But like many other things in the realm of test gear, it's one of those things where the term "low cost" is not as low as you might think. But in fairness they are a very specialized piece of equipment that is in demand and surely aren't that cheap to develop or make. Honestly one of those isn't even on my RADAR right now. My next several-hundred dollar purchase will be a Rigol 1054Z with all those additional pricey software keys now included. It's even gone down in price since I started watching them. In that case at least, it pays to be 4 or 5 years late to the party.
0culus:
--- Quote from: FakeDave on April 28, 2019, 01:49:10 am ---I thought maybe a 100:1 probe might be enough but I wasn't sure, so that answers your question.
--- End quote ---
Probes for truly high voltage tend to stop looking like standard probes, and be quite a bit more expensive too:
http://w140.com/tekwiki/wiki/P6009
http://w140.com/tekwiki/wiki/P6013
http://w140.com/tekwiki/wiki/P6015
That last one is pretty cool because it uses a dialectric fluid (freon) to achieve it's colossal rating. The later P6015A switched to a silicone based oil fill after freon got banned.
Personally, I'm going to to try to stay far away from anything that requires a probe like that. The only possible thing I might work with once I have room for a larger lab and can implement a strict safety protocol is CRTs in my old test equipment.
BravoV:
--- Quote from: FakeDave on April 27, 2019, 06:07:41 pm ---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! :-+
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Few advice from a fellow enthusiast too, I had very similar path with yours on probing mains line related devices, yes, I've read & researched a lot too similar to your journey.
I've walked thru the path of using isolation transformer, Tektronix Isolator A6902B (yes, I have two :-[) and finally ended up with Differential HV Probe, and its the probe I used whenever I'm going to probe mains line connected/related DUT.
On A6902A/B subject, beware, its huge, here the shot to give you the idea on it's size compared to the scope, definitely not comfortable to use compared to current modern HV Diff probe.
Also on A6902A/B again, its all about the probes, it uses proprietary connector ::), if you really want to buy one, simple, no probes, no deal, as you will face with the trouble finding the right probe for it.
I was lucky able to score with complete probes, here to give you the idea how they look like.
As you mentioned and fully aware of your self that it will be dangerous voltage, again, reminder, one mistake is enough to toast your scope ... or your life.
Consider to save money for HV differential probe, even the crappy one, still better than none, but buy new, imo, its worth the investment.
Once you owned it, probing mains related devices is very comfortable, an example scope that was probing the mains line that powered the scope it self, using HV Diff. Probe. Mine is Fluke, but current Chinese made are also pretty decent and "relatively" cheap, search around this forum, there are plenty of review/teardown etc.
My 2x1/4 watt resistors worth.
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