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Capacitor Safety In Power Supplies

We've all heard it at least once. "Dude, don't open up that power supply, the capacitors will shock you and you'll die! You have to unplug it, flip its switch twenty times, leave it out overnight, then you can open it up; but don't forget to wear rubber gloves and to say the special shock-warding chant!"

It's a tiresome myth, perpetuated by CRT repair technicians and overzealous nervous ninnies. While capacitor safety is very important, when it comes to consumer power supplies life is a lot simpler than what this myth would lead you to believe.


capacitors.jpg

A capacitor (or "cap" for short) is an electrical component that stores an electrical charge between two metal surfaces, and may or may not have some sort of fluid or solid material between them. Capacitors come in many types, shapes, and sizes; the most common by far in consumer electronics are electrolytic capacitors, and solid polymer capacitors. However, there are other types as well, mostly found in the power supply, such as foil capacitors, X capacitors, Y capacitors, and of course numerous types of small signalling capacitor all throughout the PC.

Now, the capacitor's ability to store a charge raises a safety concern that other electrical components don't share. That is, a capacitor can store a charge for a period of time after the circuit itself is no longer charged. While diodes, transistors, and resistors become inert instantaneously or within nanoseconds of voltage difference being lost, the capacitors in consumer electronics can hold onto charge for up to whole seconds.

Whole seconds. Consider a moment, The primary capacitors of even the most powerful ATX power supply, 1200W and up, would drain to safe levels (voltage too low to overcome your body's natural resistance) in less than two or three seconds when fully charged, even if left entirely alone, due to residual draw in the circuit. With lower wattage PSUs the discharge is much quicker; under 500W, a tenth of a second is not unlikely. The charge held by electrolytic and polymer capacitors in PSUs is not that extreme, and the natural resistance in these circuits is low. The capacitors discharge quickly and present little danger of shocking unless you're mucking around with the PSU while it's on (in which case you have more problems than just lax capacitor safety).

However, there is one type of capacitor that can retain a charge for longer and should be treated with some care: X capacitors.

Capacitor_X2_358.jpg

An X capacitor is usually used in a power supply's transient filter stage, "across the line" of the AC input, to help filter surges and peaks (aided by Y capacitors, inductors, MOVs, and other devices). Part of the X capacitor's specification is that it not represent any danger of shocking if damaged, whether that be mechanically or electrically. While that's a very useful feature to have in this application, to limit the damage caused by catastrophic failures, it doesn't forbid these capacitors from holding onto a charge on their own for relatively long periods of time. Left alone an X capacitor can hold a charge capable of shocking a human for several minutes.

Fortunately, two things limit the danger posed by these caps. First is their design; it's almost impossible to shock yourself with one. The entire top of the capacitor is non-conductive, only the leads can pass voltage. To get to the leads you'd have to either stick a conductive object like a paperclip underneath the capacitor, or else remove the entire PSU circuit board from its enclosure and mess around with the leads on the back. The former of those actions is very stupid, and as for the latter, I can tell you from experience that it can take several minutes to properly remove a PSU PCB from its case, in which case I'm probably safe; although I usually do check those leads with a multimeter to make sure.

The second thing limiting the danger posed by these capacitors is something called a bleed resistor. This is a high wattage, 1-2kOhm resistor in the power supply's primary circuit. The purpose of this resistor is to bleed all capacitors in the PSU's primary circuit, including the X capacitors. Generally speaking the primary caps drain in milliseconds, and the X capacitors on a similar scale. In a power supply with a bleed resistor, it's usually safe to say that all notable capacitors will be drained within a second of the PSU being unplugged.

But what if there isn't a bleed resistor? A valid concern. All quality power supplies, and 99.9%+ of even mediocre and poor quality power supplies, have a bleed resistor. Even most ultra-crappy junk units I open up have a bleed resistor. However, I have seen a few without. But these are a level of crap unto their own. Think, $15 ebay special, shipped straight from Shenzhen or Guangzhou.

How do you know if a PSU has a bleed resistor or not? Just look for it. It (or they) should look something like the ones in this PSU:
bleed_resistor_by_phaedrus2401-d4ft7c1.png
In close to the same location.

As long as a power supply has a bleed resistor it should be safe to handle its internals almost immediately after unplugging it from the wall. If you're paranoid, or if you can't find a bleed resistor, leave the PSU for 5-10 minutes (15-20 if it's >1kW) and it should be safe by then.





So where did all the nonsense about leaving the PSU overnight and having to discharge the caps manually come from? CRT monitors. You see, CRT monitors use very, very large bulk capacitors to store charge for the electron gun. These capacitors can hold a charge an order of magnitude greater than any in even the most powerful PC power supply, and are designed to retain that charge for much longer. These capacitors are truly dangerous, and have killed CRT technicians in the past. Don't joke around with the internals of a CRT monitor unless you know what you're doing; and if you do know what you're doing then you probably already knew this, but it can't hurt to ram it home again: ALWAYS safely discharge the CRT's main capacitor before doing anything else.

But while the safety lessons learned by CRT technicians are great and essential to working on CRTs, they are way overkill and overly cautious for PC power supplies, almost to the point of absurdity.

In short, don't be overly obsessed with capacitor safety in PC PSUs. When you open up the PSU, look for a bleed resistor near the primary capacitor. If you see it, you're golden. If you can't find one, leave the PSU alone for a few minutes, grab a drink or something, come back and it should be fine.


Good luck, and happy modding!

Comments (6)

I shocked myself on a capacitor once (not one from a power supply, but it was similar in size). It didn't even hurt all that much, but I wouldn't want to do it again. It was odd, my whole body and mind froze for a second, except my arm, which was shaking much faster than I could ever normally shake it. Fingers stung a little, but it was no more painful than a papercut. The circuit it was in almost definitely had no drain, and the capacitor was charged about 5 minutes before i touched it.

As for crt tv's, yeah don't go messing with them unless you really know what you are doing. There is often over 30KV in there, which is easily enough to melt the handle of your screwdriver and blow you across the room. I know people who have had that happen to them and believe me, it isn't pleasant. Its not as bad in oscilloscopes and black and white screens, but i still wouldn't touch one.
good info fhaey-druus
Capacitors in PSU's are relatively safe, the really dangerous common household component with dangerous capacitors is a microwave. Always make sure to short out the capacitors in a microwave before working on it. (salvaging parts etc... microwaves have some awesome parts in them which can be very useful) To short it out I recommend just using a plastic handled screwdriver and touching the contacts together.
Decades ago I worked in a TV repair shop as a teenage apprentice. Back in those days there were no bleed resistors or X-ray legislation and the CRTs really ran up to 32KV. One day I used the discharge probe on the CRT like normal, but was distracted for 5 minutes. I preceded to remove the CRT and WHAM, I flew backwards. It seems that CRT glass is a high resistance and it can take time for the static charge to travel to the anode, this secondary charge up can till pack a punch. Bleed resistors are a good thing, but I don't trust my life to them. I double check it with a grounding probe or good size plastic handle screwdriver with a aligator clip ground lead.
Interesting. I was always afraid of ever opening up a power supply to clean it out. I might try opening mine if I can easily see a lot of dust through the vents.
I took apart my old tru power trio 550w (it was dead) and salvaged the fan. it was just a standard 120mm fan. I changed the connector and now I use it to bring in extra air into my case.
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