Power Supply Woes

[DewmanSNK]

HELP!!

I am building my very first supergun and am using a PC power supply


I have removed it from the metal housing and have mounted it in a plastic project box with my JROK board

The problem is - If I hook my multimeter from the heatsink to gound I read 50+ volts AC. I cant think what the problem is.

I thought maybe grounding, because in its original config, the 4 screw mounts all connect to a metal case, which is then grounded. So I took the grounding wire and soldered it to one screw point. Then I soldered wires to all the 3 screw mounts. So now they all ground the same. Still no luck

One heatsink will read like 4 -5 volts AC - the other heatsink reads 50+
I get about ~7 volts if I read across the 2 'sinks.

I am lost, please help. I can post pics if needed.

THanks
-Andrew


I can post pics if needed.

 

[channelmaniac]

A lot of those power supplies have heatsinks that carry voltage. Make sure nothing can short to them and you are OK.

 

[DewmanSNK]

I have a replacement Power-supply that I can use


I have tested it and it carries <10 volts across the heatsinks


I like 10 alot more than 50+
Same size and bolts, just means more soldering


Yee Haw.

 

[Hewitson]

Why on earth is the heatsink live?

 

[DewmanSNK]

OK...

Now im confused.

I replaced the Power Supply and soldered it all up.

Im still getting 60 Volts AC across the heat sink to the ground


Guess I should listen to channelmaniac

Whats weird is it registered less in the metal housiung.

Now the groud goes right to the edge of the board, before it went through the whole metal case. Maby that explains the increase in voltage, better ground?

I dont know its too late, and channelmaniac says it cool - So im moving on. I will just make sure not to touch it while its on. That would suck, again, LoL

 

[channelmaniac]

One thing to check and make sure of is that the power supply wasn't using the case as a ground plane.

Check the screw holes that mounted it to the case to see if they are all connected to the circuits via traces... if they are you'll want to connect a wire between all 4 of them.

 

[DewmanSNK]

Yes, it does look like the metal case was used as a ground.

All the 4 mounting holes look to make contact with the metal mounting posts in the original case. I have since wired all the metal posts together and then wired to the ground.

I am still reading 50+ volts A/C from ground to heat-sink; but I have checked all my gounds and nothing is getting hot.

Besides, it is making 5Volts and 12Volts D/C like I want - guess I will have to do a long burn in process to verify functionality.

Also, as a side note - The PS did have a switch for 110/220. I simply unsoldered the siwtch as in 110 mode it looks like its just an open circuit and in 220 its a closed circuit. I dont think this would cause power to go to the heatsink, but I thought Id throw it out there.

It just sucks that I tore out the old PS and install a new one - and the condition still exists - Worse yet, is it looks like thats how it is SUPPOSE to function.

Will keep y'all posted

-Andrew

 

[channelmaniac]

Electrically hot heatsinks are very common in monitors and power supplies. They are designed with function in mind. Safety concerns are minor as with normal operation they are inside an enclosure where other things such as wires, screwdrivers, and human hands should never come in contact with them.

RJ

 

[Hewitson]

channelmaniac would you mind explaining what exactly is the "function in mind" they are designed for?

 

[channelmaniac]

Uh... what else would their function be? To draw heat off of components so the can work at higher current ratings than their package limitations allow.

In other words, if you take a TO-220 cased transistor you can run it at a fraction of its current rating without an adequate heatsink but you can run it a full current rating with the heatsink.

Duh...

If you run it in an enclosure then you can get away with not using insulators between the package and the heatsink. That saves pennies per insulator but when you make 10,000 or more power supplies then it translates into real dollars.

 

[Hewitson]

I meant why are they live, not why are they used!

 

[channelmaniac]

Then check the last paragraph...

Sometimes they are cheap and are saving money... other times the heatsink is used as a conductor to increase current capacity when their isn't enough room for a good sized trace to the middle leg of the device.

RJ

 

[DewmanSNK]

I noticed that the heatsinks have legs that solder to the board and connect to some pretty wide traces.


Well, got the new PS mounted and installed. I installed a copper plate below the PS in the project box. All 4 of my slandoffs sit atop this plate, which is connected to the ground wire of my power cord.

I am still reading 50 volts across the heatsinks with this grounding block inplace.

Nothing is getting hot, and the PS seems to be providing adequate power.
I am going to leave well enough alone and move on.

My JROK board should arrive any day now - and I can finally wrap up this Supergun.

Hopefully I will have pics and a small writeup to post.

-Andrew