Draining B+ with a multimeter?

[RockinRocket]

After I power down my amp the volts quickly reach the 20 mark. Any reason why it is bad or bad on the multimeter to leave the multimeter on testing the volts leads on the B+ and ground. Its just that much more convenient and only takes a couple minutes to drain the rest of the volts off this way.

[martin manning]

Monitoring the voltage as it drains is a good idea to confirm that is has diminished to the point where it is safe to work on the amp. The only possible risk I can think of is, depending on the power supply configuration, the meter might see a voltage spike when you shut the power off.

[xtian]

If I read you correctly, you are suggesting that the multimeter helps drain the voltage, which I think is incorrect.

[RockinRocket]

If I read you correctly, you are suggesting that the multimeter helps drain the voltage, which I think is incorrect.

With the amp off and the multimeater on set to DC, with the leads on the B+ and ground the volts drain in a few minutes.

[jjman]

The meter places a small load. Very small but it's there.

[RockinRocket]

My mistake 20 volts will take more than "a few minutes"

[R.G.]

It is a Very Good Idea to put bleeder resistors on your main filter caps so that you can be sure that
(1) when AC power goes off, the DC voltage *will* drain out all by itself
and
(2) you can actually compute and rely on what time this takes.

Generally putting something like a 1W resistor set up to dissipate 1/2W at normal operating voltages will drain the caps down to under 40V in a couple of minutes. 40V is roughly where the international safety crowd think voltages start being hazardous unless they can also supply big currents.

Meters are an unreliable way to drain caps. Older analog meters were often rated at something like 20K ohms per volt of voltage setting, so a 300V scale would look like a 6M load. DMMs tend to be either 1M or 10M input resistance. Neither one will help drain down filter caps much.

[roberto]

I always dimension the bleeder resistors to be 5 times the leakage current of the capacitors. Not just to lower the voltage when the amp is turned off, but also to equally divide the voltage ripartition when capacitors are used in series (as often happens on plates and screens).

[JMFahey]

If I read you correctly, you are suggesting that the multimeter helps drain the voltage, which I think is incorrect.

With the amp off and the multimeater on set to DC, with the leads on the B+ and ground the volts drain in a few minutes.

No doubt, but I guess they also drain without the multimeter, simply you don't witness it real time.

There are other loads besides the meter internal resistance.

Now, if you had, say, a .47uF charged polyester or polypropylene capacitor (meaning a high quality dielectric type) on a table, and you applied the meter leads, then yes you can attribute the main discharge to the meter.

Otherwise it would hold its charge for hours or even days, they are that good.

[boots]

The multimeter will drain the charge eventually, but I feel better using a resistor to drain caps, with the meter in parallel across the resistor, as a visual indicator of what is going on.

Once the voltage is down to just a few volts, it's probably a good idea to dead short the filter caps just to make sure they're completely discharged.

[JMFahey]

True.

That said, even after a screwdriver short, sometimes capacitors recharge on their own to a couple of volts.
Not enough to power an amp, of course, but sometimes (Murphy's Law) to make you measure something wrong.
Some call it "memory" , I guess it's some chemical reacommodation inside the cap, anyway it exists.

Oh well.