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Brymen BM789

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joeqsmith:

--- Quote from: AndrewBCN on September 16, 2021, 07:27:50 am ---@Joe

That external blocking capacitor looks like it could hold a lethal amount of energy!

The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

--- End quote ---
 
Assuming worse case,  1/2CV^2,  0.005uF @ 20KV is a Joule.   This is the upper end of Class 3.2b.  The part would normally be used with Class 3.1d (< 0.25J).    There's some small risks when experimenting but I'm not too concerned about discharges from this part.   

I don't see any need to discharge the internal block before changing measurements. What would be your concerns?  Seems like having the low voltage clamps engaged and connecting to a +1kVDC source, then instantly measuring -1kV would provide the most stress to the meter.    Well, then again, rotating the switch with a 1kVDC source my light it up pretty good.  Outside of my rebuttal videos for Dave,  I've never tried it for real.  I assume people making such measurements are well educated, use the proper tools and think before they act ....   

Then again, the beginner hooks their new meter to the output of a MOT.  Surely it can handle a couple kV as I saw this guy Joe put 6kV into his.    :-DD 

DOE standard
https://www.standards.doe.gov/standards-documents/1000/1092-BHdbk-2013/@@images/file

Datasheet for part in question
https://www.plasticcapacitors.com/images/com_sellacious/products/attachments/57/OF200.pdf

AndrewBCN:

--- Quote from: joeqsmith on September 16, 2021, 01:48:05 pm ---...

--- Quote from: AndrewBCN on September 16, 2021, 07:27:50 am ---The question is, on the DMMs that use an internal blocking capacitor, how do they discharge it before you go on to the next measurement?

--- End quote ---
...
I don't see any need to discharge the internal block before changing measurements. What would be your concerns?
...

--- End quote ---
Joe, thank you very much for your detailed and perfectly documented (as always) answers. My concern is not really with an internal blocking capacitor damaging the high impedance (10MOhm?) DMM input circuit, but what would happen in the following measurements, if it could damage the circuit under measurement (presumably at a different DC potential), in other words, if the blocking capacitor inside the DMM would discharge back through the leads into the circuit under measurement?

joeqsmith:
Oh, now there's something I had not considered.   :-DD   Say low voltage clamp is active, you make your kVDC noise measurement like shown, then immediately attach the probe to your 3.3V CMOS input....  10nF with a 2kohm source at a kV.... Interesting...

Kleinstein:
The voltage in the DC blocking cap can add the external voltage. Especially bad when you change sign of the DC part.  This way I once killed the input stage of a scope:  first measure ripply (rather low) at a +150 V supply with AC coupling and a x 1 probe, and than the same with a negative -150 V. This was too much for a scope that showed 200 V max.  :-BROKE  Afterwards you are smarter, but still a bit surprised that so little extra voltage from the internal cap can damage the input.

I think the AC input behind the coupling capacitor could be more like 1 M (maybe even 10 M in a handheld) input resistance and I don't expect a clamp between the resistor and cap.

Fungus:

--- Quote from: joeqsmith on September 16, 2021, 03:23:55 pm ---10nF with a 2kohm source at a kV.... Interesting...

--- End quote ---

Doesn't seem like it could damage much ... but your grill starter has killed a few meters that have some input protection on them (high speed clamps after a protective resistor).

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