Oh that tickled... how long till complete power discharge?

[jwillifixit]

I was putting the power supply board from a Vizio e701i-a3 in a box and got one heck of a tickle from something. I don't know exactly what i touched but am wondering how much power is present where and how long should I expect the board to be charged after removing the power plug? Attached images of the board.

Thanks

[vinceroger69]

Re: Oh that tickled... how long till complete power discharge?

maybe from the three long primary side capacitors not sure how long they store a charge for but i always check these with a multi meter to make sure there discharged

[retiredcaps]

Re: Oh that tickled... how long till complete power discharge?

I don't know exactly what i touched but am wondering how much power is present where and how long should I expect the board to be charged after removing the power plug?

Most SMPS have a discharge resistor, but if that discharge resistor is open, then the large filter capacitor will be charged.

Always use a multimeter to check and never assume that even unplugging the device for several hours or days is enough to discharge.

I ran into this myself. See

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

https://www.badcaps.net/forum/showthread.php?t=31740

[Topcat]

Re: Oh that tickled... how long till complete power discharge?

Anyone who worked on CRT chassis' will be LOL'ing at this little tingly tickle!

[PeteS in CA]

Re: Oh that tickled... how long till complete power discharge?

Power line filter circuits are supposed to have resistors that discharge those X-caps in a few seconds. I don't see resistors that would serve that function on that PCB.

[stj]

Re: Oh that tickled... how long till complete power discharge?

Anyone who worked on CRT chassis' will be LOL'ing at this little tingly tickle!

yep, got hit by a 14" tube once, about 15,000v - hate to think what a 25" would feel like!

[momaka]

Re: Oh that tickled... how long till complete power discharge?

maybe from the three long primary side capacitors not sure how long they store a charge for but i always check these with a multi meter to make sure there discharged

+1

Always check high-voltage caps with a multimeter before handling the board. If there is no way to measure the voltage on the caps (i.e. on a board where you don't have access to any test points, then only try and hold the board by the edges or corners, and preferably where there are no solder joints and exposed traces.

That said, as far as how long the caps will hold a charger - that really depends on the PSU. Pretty much all modern PSUs have discharge resistors for those caps, but it can take at least a few minutes for them to fully discharge the caps. However, beware that some older PSUs may not have those resistors and can hold a full charge even for weeks. I had a CRT monitor like that myself. The primary cap sat charged for over 2 weeks. It had only about 120-130V in it, and I didn't get zapped at all (I was careful and knew better). However, I thought about discharging the cap with a screw driver just so I can check resistances on the primary side and.... BAD IDEA. When I shorted the cap, it made a huge pop. Scared the crap out of me. Ever since then, I know better and use a 1 to 10 KOhm resistor to discharge the cap.

[goontron]

Re: Oh that tickled... how long till complete power discharge?

Ive done that.... It was a CRT monitor and the primary DC filter cap was charged 3 days after removing power. The CRT had lost vacuum and the board shutdown with a flash code after powerup. 3 days later i go to take it to bits and the dan thing was still charged! After it bit me i grabbed my soda can and smashed it on the board where the cap was. Let out a hell of a bang!

[PeteS in CA]

Re: Oh that tickled... how long till complete power discharge?

momaka is not exaggerating about film dielectric X-caps holding a charge. They are REALLY low leakage parts. I got lazy one time building a prototype power line filter, and left out the bleeder resistors. Each phase had three 2.2uF X-caps to neutral. An hour or two after performing a conducted noise test I was visited by a very upset EMI engineer ...

[momaka]

Re: Oh that tickled... how long till complete power discharge?

Ive done that.... It was a CRT monitor and the primary DC filter cap was charged 3 days after removing power.

Yep, quite common for CRT monitors when they have a PSU fault, actually.

But it is not just CRTs, though. I also repaired a family friends' Panasonic 5.1 surround sound system, which had a shorted multi-layer ceramic cap on one of the PSU outputs. As such, the PSU would shut down and crowbar as soon as it was plugged in. The primary cap remained charged to full 160V even 2 days after they gave it to me.

momaka is not exaggerating about film dielectric X-caps holding a charge. They are REALLY low leakage parts.

Well, my story was about a primary electrolytic cap... but you are right about the film X2 caps too - while they won't hold enough energy to make a huge pop, they can still sting you a bit and destroy unprotected equipment.

But in SMPS PSUs, these will usually get discharged, even if there is no resistor across them (through the bridge rectifier and main filter cap).

I got lazy one time building a prototype power line filter, and left out the bleeder resistors. Each phase had three 2.2uF X-caps to neutral. An hour or two after performing a conducted noise test I was visited by a very upset EMI engineer ...

That's funny! I can only imagine two reasons why that is: either he got bit or destroyed some test equipment.

On that note, microwave ovens and modern washers tend to have power line filters with big X-caps like that as well - typically 1 to 4.7 uF. So be careful with those.
... though when it comes to microwave ovens, I think any line filter X-caps will be the least of your problem if you are not careful with the main cap on the inverter/high-voltage side for the magnetron - those are actually lethal when charged.

[PeteS in CA]

Re: Oh that tickled... how long till complete power discharge?

...
That's funny! I can only imagine two reasons why that is: either he got bit or destroyed some test equipment.

On that note, microwave ovens and modern washers tend to have power line filters with big X-caps like that as well - typically 1 to 4.7 uF. So be careful with those.
... though when it comes to microwave ovens, I think any line filter X-caps will be the least of your problem if you are not careful with the main cap on the inverter/high-voltage side for the magnetron - those are actually lethal when charged.

The engineer unplugged the computer - a refrigerator-sized minicomputer. The AC plug was a 5-wire, 20A Twistlok and he got bit from the exposed pins. Had I included the bleeder resistor the 6.6uF per phase of X-Caps would have been discharged, but ...

Magnetrons? Or travelling wave tubes? Either way, they operate with several KV, and 800W-1000W of power means those filter caps are storing a lot of energy.

[goontron]

Re: Oh that tickled... how long till complete power discharge?

^ 4.4 joules roughly.... Thats a lot when being discharged within a microsecond timespan....

[redwire]

Re: Oh that tickled... how long till complete power discharge?

Modern safety standards like IEC 61010 allow 10 seconds for caps to discharge.
After disconnecting power, exposed terminals to be less than 70VDC.

[mattbatt]

Re: Oh that tickled... how long till complete power discharge?

I got a tickle days after unplugging my TV powersupply so i discharged it with my screwdriver and left skid marks on the PCB. Needless to say i learned how to build a Cap discharge device.

[jiroy]

Re: Oh that tickled... how long till complete power discharge?

I Always differ between fully working circuits and partially or faulty circuits . When dealing with fully working circuits , it's safe to assume that the charging-discharging cycle has completed successfully , and there are very minimum dangers to deal with such circuitry . Nontheless , safety precautions are always needed .

When dealing with partially or faulty circuits , risks of getting a heavy shock by some undischarged capacitors are very high , as it didn't complete a normally working cycle . In such case , precautions and attention are highly needed , as big capacitors can hold lethal and deadly charges for a very long time . If you want to be on a safe side , you'll have to discharge big capacitors using a 50 kohms , 5 watts resistor . Shorting a capacitor leads isn't a safe method , as it may damage nearby components .

[stj]

Re: Oh that tickled... how long till complete power discharge?

not every psu has discharge resistors,
i'v seen a few that could stay charged for weeks or more from new.

[jiroy]

Re: Oh that tickled... how long till complete power discharge?

Since it's turning educative , we must warn from cameras equipped with flash capacitors . Those are the highest dangerous in my opinion .

[kc8adu]

Re: Oh that tickled... how long till complete power discharge?

stick your fingers on these caps and report back...

[stj]

Re: Oh that tickled... how long till complete power discharge?

i think when you see 4 discharge resistors in parallel, it's not a good place to be.

[jiroy]

Re: Oh that tickled... how long till complete power discharge?

stick your fingers on these caps and report back...

I have a meeting now , I'll leave you that ultimate pleasure .