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Brymen 869s DC offset issue when reading ACmV
Kleinstein:
The AC coupling can do harm when charged to a high voltage. By nature it does not discharge very fast. If you are lucky there is some 10 M directly at the input side to ground and than maybe some 1.1 M as input impedance for the actual AC input. So discharge may take a few seconds. In some cases the resitor to ground at the input is missing and the charge can stay there longer.
It happend to me once with a scope. Look for ripple on to of +-150 V range supplies - because of the low volatage and relative low frequency with an 1:1 probe. The step from the +150 to -150 was than a bit too much for the scope input with specs for some 200 or 250 V peak. Chances are a charged capacitor of the scope or DMM could also damage sensitive parts - not just very sensitive ones.
David Hess:
--- Quote from: Kleinstein on November 12, 2022, 02:54:31 pm ---It happend to me once with a scope. Look for ripple on to of +-150 V range supplies - because of the low volatage and relative low frequency with an 1:1 probe. The step from the +150 to -150 was than a bit too much for the scope input with specs for some 200 or 250 V peak. Chances are a charged capacitor of the scope or DMM could also damage sensitive parts - not just very sensitive ones.
--- End quote ---
On the coupling switch, some older oscilloscopes use the Ground selection between between the AC and DC coupling options to precharge the AC coupling capacitor. Very rarely they have a separate precharge position.
NickKUK:
Just to follow up - I've had the Brymen 896s for a while now and it's great bit of kit. I like the leads have screw fitting for the little banana connectors too but also the plugs also fit my other HV probe kits with big croc clips so no fingers in shock range.
It's valid point on the cap discharge and it helps the inbuilt bleeds that take the B+ & B- plus the DMM back down to zero. Top of the all tube cascode + CCS (+320) to bottom (-320) means with 640Vdc in there I'm not sticking my fingers anywhere near it to reposition whilst in operation! By the time the system has bled down to preposition so has the DMM.
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