Discharging with DMM ?

[bianchi77]

Guys,

I try to discharge high voltage caps with DMM ?
Is it finished already ?
I can see reading is going down from 140V to 10 mV...

Or I still need to do screw short ? and 47K 2W resistor short ?

Cheers

[retiredcaps]

Re: Discharging with DMM ?

I try to discharge high voltage caps with DMM ?

With 99% of the multimeters, you cannot discharge high voltage caps with them in any short period of time. (*notes below*)

I can see reading is going down from 140V to 10 mV...

Any reading below 3VDC is pretty safe. The reason for your capacitor discharging is that most, but not all power supplies have a bleeder resistor.

https://en.wikipedia.org/wiki/Bleeder_resistor

Most people, incorrectly, assume that once power is removed, the power supply will discharge itself over a few minutes. While most do, I have run into consumer products, at least 2, that do not discharge even over several weeks. See

https://www.badcaps.net/forum/showthread.php?t=41319

In my 2 scenarios, either the power supply had no bleeder resistor or it went open.

Or I still need to do screw short ? and 47K 2W resistor short ?

The safest way to discharge a high voltage capacitor is with a high wattage resistor across the legs of the capacitor while monitoring with a multimeter. At Fluke's website

http://en-us.fluke.com/training/trai...ultimeter.html

they write

"To safely discharge a capacitor: After power is removed, connect a 20,000 Ω, 5-watt resistor across the capacitor terminals for five seconds. Use your multimeter to confirm the capacitor is fully discharged."

Note:

1) Your fellow countryman, Dave Jones, sells an eevblog branded multimeter

http://www.eevblog.com/product/bm235-multimeter/

that when used with the LoZ functionality can be used to discharge high voltage capacitors. However, since 99% of multimeters do NOT have this function, I wouldn't make it a habit to do so.

2) If this online calculator is correct and I'm inputting the correct values, it would take about 137 hours for a typical 10M ohm input impedance multimeter to discharge a 820uF 165V large filter capacitor.

http://planetcalc.com/1979/