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Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Since I've already fried one channel on my scope I figured I should make a simple 10X probe.
The scope has an impedance of 1Mohm and a voltage rating of 250V, making a voltage divider for this is simple enough. But what about some surge protection? With a maximum input of 2,5KV a spark gap before the divider should work, right? But what about something like this:
Can diodes break down fast enough and conduct reliably enough to give any real surge protection? The last zeners would be a backup in case the diodes fail, perhaps those would be enough? This design would limit the input to appr 1KV, more than enough for my work and leaving me with a generous safety margin.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im thinking a DIY spark gap would be erratic and not of much use, unless you mean the glass cased ones, no one uses those to often. (gas tube surge arrestors)
better would be a bi-directional TVS at the probe-output, scope-input location. their self capacitcene is negligible, speed is fast and there meant for clamping voltage surges at very high currents.
i wouldnt waste your time with zeners in measurement circuits, they're noisy little cowards.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
A bi-directional TVS diode seems like the perfect solution, I didn't even know they existed. Looks like they're more or less a special version of a zener, right? Guess I have to find a couple of those.
As for zeners, will they really produce much noise below the breakdown voltage?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Fulmen wrote ...
A bi-directional TVS diode seems like the perfect solution, I didn't even know they existed. Looks like they're more or less a special version of a zener, right? Guess I have to find a couple of those.
they're silicon doping is different, and they function on some different principles. use mouser.com i think they ship world wide.
Fulmen wrote ...
As for zeners, will they really produce much noise below the breakdown voltage?
thats mostly a current driven condidtion, so im tempted to say below breakdown your fine. however you shoul connect one to a scope and see... also Zeners change thier capacitence sort of with varying signal. so id really study zeners carfully, or just use TVS's.
based on your schematic i dont see any attempt to include the capacitence of the scope input, nor any frequency dependent effects... ???
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Patrick wrote ... thats mostly a current driven condidtion, so im tempted to say below breakdown your fine. however you shoul connect one to a scope and see...
Yeah, I was planning on scoping things first to see how it behaves. Pretty much academic now really, since I have to order anyhow I'd might as well order TVS's.
based on your schematic i dont see any attempt to include the capacitence of the scope input, nor any frequency dependent effects... ???
That's because I don't have the first clue how to do that
I've tried the divider, albeit only at 1KHz low voltage, and it seemed to perform fine. But I'd be happy to learn more, if there is a thread that describes this please let me know.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Patrick wrote ...
i suspect your reasoning may be faulty here...
I reject your reality and substitute my own.
I know it's not cost efficient to make these things myself, but I had some free time and was looking for something to make. Besides, this is so much more educational than buying, I'd never learned about the TVS's if not for this thread.
Youll be unconcious and bleeding from the ears, and prolly seizing in 4 minutes...
Great, haven't had one of those since last time I tried reading up on quantum physics.
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Building a simple 10X probe
