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What a capacitor on the power connector of ATX PSU does?

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krairach

Registered
Joined
Nov 25, 2002
Location
Bangkok, Thailand
I have 2 ATX PSUs: one is high quality and the other is a cheap one.

Upon opening the case, I notice the difference that on the power connector of a high quailty PSU, there is a capacitor connecting L to N. But I don't see any in a cheap PSU.

May I ask what does this capacitor do?
Does this have something to do with Power Factor Correction?
Should I do the same with the cheap one?
 
This ATX PSU diagram shows that capacitor as C4 (upper left, 0.22 uF or 220 nF), part of the EMI filter, but I had a cheap PSU with no EMI filter at all, and installing just that capacitor didn't reduce TV or radio interference noticeably. I also had to install chokes in series with each AC line, after removing the jumpers that were in their place (coils are T5 in the diagram -- here two coils are wrapped around a single core) before I could hear the Reverend Frisby Miracle Healing Hour on AM radio again.

Many PSUs also have .001-.01 uF (1-10 nF) disk capacitors (C2, C3 in the diagram) between L and ground and between N and ground and rated for 250-600 VAC and certified Y1 or Y2. I think that Y ratings are more stringent than X ratings and the capacitance values are much lower because if the PSU wasn't connected to earth ground and one of those disk capacitors failed, the shock hazard could be a lot more dangerous. In the lousier PSUs, these capacitors had no X or Y certifications or AC voltage ratings.

If you add choke coils, be sure that they'll be safe for high voltage and covered with thick insulation (heatshrink is good) so they can't short against anything.

The capacitors need a bleeder resistor across L and N (R1 in the diagram) to discharge them. A 500K resistor should be rated for at least 1/2 watt, 250K for at least 1 watt.
 
Thank you for the info.

So the capacitors are part of input filter (EMI). The cheap one does not have!

This diagram also indirectly reveals how to mod Volt as well.

Wonderful!

If I am to add input filter to a cheap PSU, I need:
- C1 220nF 275V (ceramic)
- C4 220nF 275V (ceramic)
- R1 560Kohm 1/2 Watt (metal film)
- Z1 ?
- T1 ?
- 2xT5 ?

Pls recommend Z1, T1, T5
Pls also keep your webpage. It's very informative and useful.
 
Last edited:
Hi,
If you dont want to go into the complexity of creating your
own emi filter --- most electronics suppliers sell the filter
moulded into the back of the 3-pin socket. Usually in a tin box.
-- or- if you are like me - just whip the circuit off an old AT power
supply.
 
Thanls for the link to the "generic" ATX schematic. That is priceless! That's all I need to get a good start on troubleshooting my dead PP-412.

Hoot
 
Hoot..... can you mod a generic PSU so the voltage output for 3.3V lines = 3.43V, and 5V lines = 5.15V or something like that.
 
Krairach, be sure that those ceramic capacitors are rated for 275V AC, not DC!!!

My guess is that Z1 is also rated for about 275VAC RMS.

I don't know what T5 should be, but the chokes are usually about 0.5-1" outer diameter with 20-30 turns of thick solid wire wrapped on it. www.delta.com.tw shows that some of their EMI filters, the tin can type Couchpotato mentioned, are rated for approximately 1-5 mH. Another manufacturer is at www.cor.com . These can filters often have the schematic printed on them. The amp rating can vary greatly, from 3-10A. Some don't include a bleeder resistor and should have a 500K, 1/2W resistor wired across the two AC lines.

I took EMI filter parts from old PSUs, but I added extra insulation to the chokes to prevent their thinly insulated wires from touching other components, like a jumper that ran right underneath one. I put Teflon tubing over that jumper, wrapped chokes with polypropylene tape (yellow Mylar tape is usually used for this purpose), or used pieces of 1mm thick polycarbonate. The funny thing about those old PSUs is that their components seem to be just as heavy as those in the new ones, even when those old PSUs were rated for only 200W.
 
asw7576 said:
Hoot..... can you mod a generic PSU so the voltage output for 3.3V lines = 3.43V, and 5V lines = 5.15V or something like that.

Sure, all you need to do is change the feedback resistor values for those outputs. That what they do in ones that have trim-pots in them. First, you have to figure out which resistors on the board are used in the feedback circuit, then you need to replace them with trim-pots and tweak them for the voltage you desire, within a certain +/- or the original center. In other words, you can't go way out of bounds or you'll probably trigger some overvoltage protection circuit. 3.43 and 5.15 do not sound too far out from center.

Hoot
 
Hoot: Thanks... BTW if I send you a picture of my PSU, can you tell me where to change the resistor. Is it okay with you?
 
I don't have any information about PFC circuits, but you should be able to find PFC chips at www.linear.com, www.ti.com, or www.national.com.

The voltage adjustment resistors are usually a different color than the other resistors because of their higher precision, So instead of the last color band being gold (5% tolerance), it's red (2%) or brown (1%). But this is confusing if the first color band is also red or brown. Also you don't want to have the mobo hooked up when you make any adjustments because if there's no over voltage protection the voltage could go way too high and damage the chips. So use somebody else's mobo or the load. Also be really, really careful when making adjustments while the power is running because those resistors could be located right next to high voltage components or even be on the high voltage side of the PSU!!! I'd probably get a 10-turn pot rated for about twice the resistance of the original resistor and solder wires between it and the circuit board and mount the pot very securely on a piece of wood and use an all-plastic screwdriver to adjust it. Not only that, but I'd have the PSU plugged into an isolation transformer or at the very least a GFCI
 
Larry... can you ask your friend Christina where and how to mod the 5V and 3.3V lines?

Regarding the resistor (brown and red), I will look at it again. Should that precision resistor near voltage reference regulator IC ? ( the IC that looks like this "D" with cathode, anode and ref). In my PSU there are 3 of this kind.
 
Those D-shaped voltage reference ICs are a possibility, but on my Antec SmartPower, one of them is used for the fan speed control circuit. You could take some resistors that are about ten times as large as the ones in the PSU and try connecting them across the existing resistors to see if the output voltages change.
 
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