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AN8008 US $19, 9999count, 1uV, 0.01uA, 0.01Ohm, 1pF resolution meter

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bobale:
I've just tried removing one of the 10M resistor, and it is not measuring voltage at all. So definitely they are part of a voltage divider.

After mucking about with different resistors, I've managed to get it to show 9.990V on 10V reference, and 4.995V on 5V refence. I don't think I can make it any better, so I'll leave it as is.

TurboTom:
The failure of your DMM doesn't appear to be related to the 2*5MOhms dropper resistors directly, if so they both would have been charred. It rather looks like the quite high striking voltage of the CCFL inverter just caused an arc to form from the "hot" terminal of the first resistor to ground, resulting in the underlying FR4 to dissociate/carbonize. In order to repair the instrument properly, you would have to grind away all of the affected FR4 so there won't be any bare carbon residues left in this area. Otherwise, there will always be a low(ish) impedance path to ground, making measuring modelstly high voltages VERY dangerous (provided the source is capable of delivering high energy).

Fungus:

--- Quote from: bobale on January 15, 2020, 02:47:45 pm ---I've just tried removing one of the 10M resistor, and it is not measuring voltage at all. So definitely they are part of a voltage divider.

--- End quote ---

Yep. Those resistors are the input protection for the voltage range. There's two of them in series to make it tougher. If you remove one then the volts can't get through to get a reading.


--- Quote from: bobale on January 15, 2020, 02:47:45 pm ---After mucking about with different resistors, I've managed to get it to show 9.990V on 10V reference, and 4.995V on 5V refence. I don't think I can make it any better, so I'll leave it as is.

--- End quote ---

Any high-ish values will do but 10M total is the standard.

bobale:

--- Quote from: TurboTom on January 15, 2020, 05:09:54 pm --- Otherwise, there will always be a low(ish) impedance path to ground, making measuring modelstly high voltages VERY dangerous (provided the source is capable of delivering high energy).

--- End quote ---
I've dug around the board as best as as coud. How high voltages are we talking about? I've tried measuring 230VAC, and it worked fine. Also, just measured input filter cap on switchmode psu (~325VDC), and it was fine. That's the highest voltage I usually measure.

Fungus:

--- Quote from: bobale on January 15, 2020, 08:03:21 pm ---I've tried measuring 230VAC, and it worked fine. Also, just measured input filter cap on switchmode psu (~325VDC), and it was fine. That's the highest voltage I usually measure.

--- End quote ---

If you "usually" measure voltages like that then it's time to get a Fluke 101.

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