Clearly someone has nil experience working inside a modern (90's forward) PSUs.
gdjacobs wrote:mdog69 wrote:
Leave the PSU disconnected from the main. Wait 10 minutes, then carefully measure the voltage across the input electrolytic capacitors (the ones rated at 400v) to verify that the unit is safe to work on. I would consider 50v or less as safe.
The 20 to 30V range is more widely considered safe, assuming dry skin conditions, no probe stabbing, etc. 50V isn't particularly dangerous, but in certain circumstances it could pose some cardiac threat. However, any PSU worth it's salt will have bleeders in shunt with the bulk caps that will deplete any residual charge very quickly.
CORRECT
luckybob wrote:If its unplugged, its fine.
If it does bite you when its unplugged, DESTROY IT; a PSU that does not discharge its primary caps is a death trap and shouldn't be used. PERIOD. If they cut cost by the 2 PENNIES those bleeder resistors cost, they cut costs EVERYWHERE and it's a trash unit.
That said, I've been nailed by 240V on multiple occasions, and as long as you aren't grabbing it with both hands, even a cap bite is a non-issue. This level of caution is appropriate for CRT's, and I appreciate being cautious.
CORRECT
[Comparing scars]
In the early 80's I got hit by 400 vdc from a Radar unit.
We had matting down and I ground my arm so I only took it from my wrist to my forearm.
Once was more than enough, thank you very much!
SirNickity wrote:Handling a PSU is never restricted to the secondary side. I don't think I've ever done anything in a PSU case without bumping up against a heatsink (which may or may not be connected to a pin on a transistor / diode / regulator package, or ground), or a wire connected to the power inlet or switch, or the through-hole bits on the bottom of the PCB.
IMO, it's either safe to handle, or it isn't, and you shouldn't be futzing with it if it isn't. 😀 It only takes a second to probe the reservoir cap pins, or at least cross them with a screwdriver. By the time you get the thing undressed, they should be at zero (or nearly zero) volts. If not, I would also be suspicious about what other corners they cut. Lingering HVDC is just bad design.
CORRECT
SirNickity wrote:Oh get over yourself. It's cramped quarters in there, and sooner or later you're going to bump a knuckle into something metal. If your safety practice is "don't do that" without regard to whether the rest of the supply is safe, then you're going to get away with it right until you don't. It's a moot point anyway unless you're not working on a supply with the mains plugged in.
CORRECT
retardware wrote:LIve PSU handling is not recommendable unless you have an insulation transformer.
As I don't like getting shocked, I short the p […]
Show full quote
LIve PSU handling is not recommendable unless you have an insulation transformer.
As I don't like getting shocked, I short the primary caps using a wire with two alligator clamps.
This is better than accidentally short and damage some components.
If there is a noticeable zap, then be careful for the reasons the others already mentioned.
CORRECT, but you need more than just an insulation transformer.
Tiido wrote:I have a 150ohm ceramic block power resistor with two pins on one side for safely discharging the big capacitors without damaging them or causing huge sparks.
That's a good plan.
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The wet rag thing is more red-neck than I am, and that's pretty red-neck.
Yes, when I first heard of it I tried it a few times.
Not only does it not work well, if the caps are charged it's MORE likely to get you shocked than just leaving the caps alone.
The wet rag is a conductor and you would be holding a conductor with your fingers and pushing it against a live voltage source.
If fully charged the primary caps (the two in series) are at around 338 vdc.
This is why they are 200-250 v caps and why PSUs with single primaries use 400-450 v caps.
If poor choices have you using a simulated sine-wave BBU and a PFC PSU that 338 is closer to 380-390 vdc.
If you are going to do the wet rag you should be wearing electrical safety gloves rated for at least 500 volts.
Just a screw driver is not a great idea either but it's certainly safer (to the human) than a wet rag.
A proper shorting device is basically a screwdriver like thing with a lead that has a high wattage resistor in it and either a probe or a clip at the end of the wire. (The clip gets put on first when you use it.)
You can DIY one or amazon/ebay have scads of 'economy' (hobbyist?) variations for under $20.
The ones we used in power plants were about 3' long with a 8" to 10" long ¼"-3/8" copper alloy rod, the resistor was embedded in the glass handle and the clip lead was 8 gauge (IIRC) thin strand copper welding type wire.
Bear in mind the newer larger plants used an internal 4160v 3-phase grid while the older or smaller plants still used 440v 3-phase.
I have only once ever seen PSU primaries hold a charge and that one was in operation when lightning struck a power pole 50 feet from the guy's house.
If you check the voltage on the primaries and find a charge then PSU either has too much damage or it is too cheap to even bother with so just throw it out.
You should never see voltage on any cap anywhere in a modern PC because it's a design consideration.
When AT/ATX PSUs start-up they have timing sequences based on how fast the voltages come up.
Too slow a rise and the PSU thinks there is a short and shuts itself back off to protect downstream.
Too fast a rise and it thinks there is no load so it shuts itself off to protect itself.
Pre-charged caps when the PSU starts would muck the timing all up so PCs are designed to bleed the caps down when there is no power. Low end PSUs don't always do that right.
Where you can get into trouble working in a PSU is by powering it up with the cover off.
Those exposed heatsinks usually have power on them when power is applied and can it be at that 338-390 volts that is on the primary caps.
So, if you have to power it up to check something don't go randomly poking around in there.
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