Discharge Cap

[_ar_]

Hello All,

I need to change a large cap in a switching mode power supply. AFAIK it's a 400V 220uf one. Can I discharge it safely with a Brymen 235 in AutoV LoZ mode without killing myself? It's been off for months now, but better safe then sorry. The Brymen's case says 300V CATIV certified, while the manual says the top of the range is 1000V. I'm a bit confused, hence the topic. I know the probes should keep me alive (1kV certified).

Thanks!

[ataradov]

It will be discharged after months. But just a screwdriver will do the trick, it is not that big of a capacitor.

[TimNJ]

A capacitor will be (almost) completely discharged after applying a resistor across it for 5RC time constants. In this case, if you want your capacitor to be discharged in 3 seconds, solve for 5*(220E-6)*(R) = 3. R = 2.7K. If you have a resistor in the 500ohm to 5K range, those should work fine. Just remember to discharge for longer if you are going to use a higher resistance.

Based on Brymen 235 datasheet,

In low-Z mode: "Impedance increases abruptly within a fraction of a second   as   display   voltage   is   above   8V   (typical)". So if you put 400V on it, your resistance is going to go way up..not doing much good to discharge it. But your probes are not going to kill you. Just check the voltage across the cap with that same multi-meter before  you pull it out! Make sure it's like 1V or less.

Screwdriver would work too since you're planning on tossing the cap anyway. I'm just not fan of poking around with screwdrivers.

[_ar_]

Thanks guys for your answers! So you're saying the BM235 not gonna do me much good discharging caps. Would it survive taking a measurement on 400V 220uf cap without blowing a fuse, tho? I'm confused about the CATIV 300V rating and the 1000V upper range of measurement. Thanks!

[Zero999]

Thanks guys for your answers! So you're saying the BM235 not gonna do me much good discharging caps. Would it survive taking a measurement on 400V 220uf cap without blowing a fuse, tho? I'm confused about the CATIV 300V rating and the 1000V upper range of measurement. Thanks!

No, it's no good for discharging a capacitor.

Yes, it will survive 400VDC, without blowing the fuse. The CAT rating is what over-voltages it can withstand.
http://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

[_ar_]

Awesome, thank you all for your help!

[_ar_]

So it turns out it's a 400v 33uf cap, not a 220uf. Does this change anything you guys wrote above drastically? Thanks!

[ataradov]

There is even less chance that it is changed after a month. Nothing else changes.

[kripton2035]

you can use a tool like this one to discharge the capacitor safely :
http://kripton2035.free.fr/LabTools/lbt-sparkpen.html


or http://kripton2035clone.free.fr/LabTools/lbt-sparkpen.html

[CatalinaWOW]

A capacitor will be (almost) completely discharged after applying a resistor across it for 5RC time constants. In this case, if you want your capacitor to be discharged in 3 seconds, solve for 5*(220E-6)*(R) = 3. R = 2.7K. If you have a resistor in the 500ohm to 5K range, those should work fine. Just remember to discharge for longer if you are going to use a higher resistance.

It is also worth being aware of the power rating of the discharge resistor.   In this case, although it is unlikely that there is full voltage across the capacitor you will want to use the full possible 400 V charge for a safety evaluation.  The power dissipated in the resistor could initially be V^2/R or 60W.  This drops rapidly, after about half a second it will be down to 15W.

You don't need a resistor rated at the peak power, but I would be worried about using one of the eighth watt resistors that are commonly used today for this application.  You can reconcile this by measuring the voltage first so you know the real power that must be dissipated, or by raising the resistance value appropriately and just waiting longer for the discharge to occur.

[_ar_]

Awesome, thanks everyone!

[_ar_]

Maybe one more Q. I always read that touching a hv cap without discharging will shock you. What they mean by that is touching the _terminals_ of those caps, right? I mean, touching the packaging or even the metal top will never give a shock, right? Or one really must keep a 'safe distance' when taking say a PSU board out from its case while the caps may hold a charge (holding the board by the edges, for example)?  Thanks again!

[ataradov]

Just touching the terminals. And depending on the construction of the capacitor, touching any exposed metal parts while touching something else in the device may be bad as well (this does not apply for most typical modern consumer capacitors).

And even without discharging, the shock is not very pleasant, but not deadly, unless you have some other medical conditions, or fall off the chair from surprise. Just don't touch two terminals with fingers of each hand, which is a good idea when working with any other high-energy circuits.