Electronics > Beginners
Differential measurements using isolated oscilloscope
capt bullshot:
--- Quote from: tggzzz on December 13, 2019, 04:25:20 pm ---
--- Quote from: GeorgeOfTheJungle on December 13, 2019, 02:19:11 pm ---
@tggzzz, have you never ever floated your 'scope? Everybody else: raise your hand if you've never done it.
--- End quote ---
No, I have never floated a scope. I've used my skill and imagination to find ways not to float a scope.
--- End quote ---
Not that difficult. Just keep your scopes away from trees and rivers.
magic:
Love those scope floating threads :popcorn:
--- Quote from: Aners on December 13, 2019, 02:09:40 pm ---Ok, when you say it is shunted by 100pF what do you mean then? Because I have been looking at simplified probe schematics for single ended probes like the one below, and I dont see what you mean.
--- End quote ---
Your picture is wrong, it shows a conventional scope. Cut the connection from input probe ground to oscilloscope ground. Replace with a bit of parasitic capacitance. That's a scope with isolated input channels.
Aners:
--- Quote from: magic on December 13, 2019, 05:36:52 pm ---Your picture is wrong, it shows a conventional scope. Cut the connection from input probe ground to oscilloscope ground. Replace with a bit of parasitic capacitance. That's a scope with isolated input channels.
--- End quote ---
Oh yeah, I just assumed the ground is floating so the symbol in it self could be ignored, but still the same idea. I was more wondering about the 100pf shunt, and what that is.
David Hess:
--- Quote from: Aners on December 13, 2019, 02:09:40 pm ---Ok, when you say it is shunted by 100pF what do you mean then? Because I have been looking at simplified probe schematics for single ended probes like the one below, and I dont see what you mean.
--- End quote ---
In your example, an isolated probe has the ground connection on the right hand side cut. The shunt capacitance is then the capacitance from the probe "ground", more properly referred to as common, and the environment including the isolation barriers it uses for power and signal. This also includes the entire length of the probe cable.
If an isolation transformer is used, then it has its own capacitance from the primary to the secondary or from the secondary to an electrostatic shield and it will likely be considerably more than 100 picofarads although they do make expensive "low capacitance high isolation" transformers. An isolated probe has a lot less "surface area" contributing to its common mode capacitance.
not1xor1:
--- Quote from: tggzzz on December 12, 2019, 10:35:38 pm ---Sigh.
The OP, who is also new to this forum, would be well advised to read, learn, and inwardly digest this: https://www.eevblog.com/forum/projects/isolation-transformer-for-scope/msg2259465/#msg2259465 Especially the anecdote including "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."
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I agree about the dangers of high voltage, but I want to stress that a random quote from a post forum has absolutely no value as a proof.... that smells so much of urban legend...
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