PC power supply - what's missing?

[Dougdeep]

I used to just toss defunct PC power supplies in the trash but the death of CompUSA and Circuit City has dwindled my sources of cheap replacements so I thought about repairing a few. Some fixes are obvious – bulging filter capacitors – but what I really noticed were the parts NOT being used. Schematics of several supplies I found online usually show a fuse, capacitors across the lines and lines to ground, a high value resistor across the lines, and an input choke ahead of the rectifier bridge. Most of power supplies I've pulled apart jumper everything except the fuse so there are no components (just their outlines on the pc board) between the line input and the rectifiers. Aside from saving the manufacturer a few cents per unit, was there a valid reason for eliminating these parts? Assuming I can get a supply to work again, would it be worthwhile to install some of these parts to prevent a repeat failure? Your thoughts on this subject are greatly appreciated.

[sofaspud]

Those missing parts would act to filter the incoming line voltage. Not populating those components on the board could save
the manufacturer millions of $$, so that is likely the only valid reason they need. Most American electrical utilities provide
decent enough 110VAC 60Hz power to their customers that such input filtering isn't a necessity except for special circumstances, eg low noise, high precision, high sensitivity types of circuits. I have serious doubts that the missing
parts had any affect whatsoever on your smps failure. And so, I wouldn't bother installing them "after the fact." Just
make sure the parts you do replace have the proper ratings.
I could be wrong, but those are my thoughts.

[Janitor Tzap]

Yeah, I have too agree with sofaspud.
Putting in the "So Called" missing components will not help make it a better PC Power Supply.

Most of the PC Boards have been modified with jumpers, cut traces,
and use different components then what the board was originally made for.

Since making PC Boards isn't cheap, even in large quantities.
The manufacturer will use up the old PC Boards first, before putting the changes into the next PC Boards.

I have repaired a few PC Power Supplies.
As long as it was something simple like Cap's, or Diode's, or a cheap Voltage Regulator.
Otherwise, I would just get a replacement.

There are several online places that have good prices on PC Power Supplies.
You may want to go that route.
I found the best way of ordering stuff from places like MCM Electronics, or such.
Was to get together with several friends on an order.
Thus, we could divide the shipping costs amongst us.

Well....
There was a good article on PC Power Supply repair on the Nuts&Volts site.
But, it may not of been placed back into the Nuts&Volts archives.
If your interested. I have it in PDF form, and I can send it too you.


Signed: Janitor Tzap

[MrAl]

Hi,

You could also check on the web for various power supplies sold as surplus. Just an idea.

[Bob Scott]

I think those missing filter parts were meant to prevent switching hamonics (RF noise) from leaving the power supply, going down the AC supply, and interfering with radios and TVs. I wouldn't worry about it unless you have some receivers that are bothered by the RFI.

[Bigglez]

Aside from saving the manufacturer a few cents per unit, was there a valid reason for eliminating these parts?

Nope. Just a way to lower costs in a very tough market.

Assuming I can get a supply to work again, would it be worthwhile to install some of these parts to prevent a repeat failure? Your thoughts on this subject are greatly appreciated.

Nope. To improve reliability remove even more parts!
"The most reliable parts are the ones you don't have".

The original purpose of the parts was to prevent noise
form the SIlver Box PSU from radiating out into the AC
line. Its a lawful requirement, varies by country, and can
be done by other means (filters at the AC inlet, choke
torroids on the AC line cord, shielding, etc.) Once a system
has passed a national or regional testing lab, and received
type approval, the circuit can be downgraded as required
by cost-cutting. To plan for the worst the PCB was probably
over-engineered or borrowed from another model with
tougher requirements.

[Dougdeep]

Thanks to all of you that replied. The really telling remark is that putting in the “missing” components won't make it better power supply – if the manufacturer is skimping on absent parts they likely skimped on present parts as well (low temperature ratings on filter caps for instance). Evaluating a supply using the old quick-quality guidelines of weight, wire size and cable bundling doesn't seem to work when they are too easy to fake. I found a supply recently that got its heft through welding two chunks of angle iron inside the shiny chrome top case. Although it seems like a waste, the best action seems to be to bust these things for parts, get some recommendations from Tom's Hardware and then put my credit card to use.