Cheap PSU: Piece of junk or semi-decent?

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

I'm trying to understand how MOV varistors work, and how to select their specs according to application ...

That 150V DC is maximum continuous operating voltage, right? So something like this would do?:

…

Read my later reply, I suggested something VE09M00131K cause 150V continuous is really slightly too low.

tom66: I disagree, big caps could withstand like maybe 20-30% higher-than-nominal voltage spikes. But it is still decreases their life. I am trying to select as low varistors as possible above operating voltage cause it means longer input caps life. It's both cheaper and simplier to replace varistor than input cap.

[tom66]

Re: Cheap PSU: Piece of junk or semi-decent?

You'll often see a 350~375V AC MOV on the input to a power supply.

It seems a lot higher than you'd expect but it's because a MOV isn't there for brief transients. If it were to attempt to absorb them, it would be quickly damaged... there are hundreds of them per day in the UK... I wouldn't know about other countries with less reliable electrical systems. A MOV is for the "really bad" transients (such as phase faults, nearby lightning, etc.) that might start a fire or severely damage the rest of the power supply.

So, it should be rated at much more than the continuous operating voltage of the PSU. I would guess that the big caps can withstand a higher operating voltage for some short amount of time, and the two primary switchers are usually 13007, or two 400V switches in series. In the majority of the PSUs I've seen with 115V/230V switches flicked (which is similar to a really bad surge), the only thing that goes bang is the fuse.

[Th3_uN1Qu3]

Re: Cheap PSU: Piece of junk or semi-decent?

So I can learn, would you mind explaining why a MOV in parallel with a 200V rated bigcap, in a 230V PSU, must be 150V DC?

It must NOT be 150V DC, else it'll blow first time you plug it in. It should be rated high enough to allow for the worst case - a 10% tolerance of mains voltage at zero load, which would be 253v. Multiplied by 1.414 (peak voltage of sine wave) this gives 178v per cap. So the MOV should be 180 volts or 200 volts.

If you use a single MOV in a half-bridge PSU, you will not get full protection. The two big caps in a half bridge work as a voltage divider, so if you only clamp the top to the bottom you could still get voltage imbalance. But given that MOVs are intended to absorb short-term transients which are over much too fast to cause any imbalance between the two big caps, you should be okay with a single one. Use a 360 or 400 volt MOV.

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

... I'm saving the picture to my hard drive (if you don't mind, of course...

Not at all, help yourself .



I'm trying to understand how MOV varistors work, and how to select their specs according to application ...

... There are two empty parking slots near them marked 'Z1' and 'Z2', could these be for MOV varistors? If so, which MOV specs (volts, joules, ns) would be adequate for this PSU?

Z1 is in parallel with one big cap, Z2 is in parallel with the other bigcap.

... If they are on the potential between the two caps, than it would be something like 115 Vrms/150 V DC ...

That 150V DC is maximum continuous operating voltage, right? So something like this would do?:

So I can learn, would you mind explaining why a MOV in parallel with a 200V rated bigcap, in a 230V PSU, must be 150V DC?


What if we add a single MOV to the PSU as per this scheme:

RV1 = MOV

Perhaps a ~271K MOV would be adequate for a 230V PSU in that position?

[momaka]

Re: Cheap PSU: Piece of junk or semi-decent?

Thanks for your very informative and enlightening advice Momaka .

You're welcome.

Either they knew something that we don't ... or they simply screwed up and put a resistor with the wrong Ω .

I think it's the latter, but since the PSU has worked for so many years up until it got the bad caps, then it's probably nothing to worry about.

BTW, forgot to mention I washed the PCB of this PSU in the sink with abundant water

I've done that with PC motherboards a few times. As long as they are left to dry properly, it's not a problem.

That is very cool . Topcat should really put this on the front page of BCN .
I'm saving the picture to my hard drive (if you don't mind, of course... I have a small picture collection of kludges and fixes from thereifixedit.com and other websites, and I think this definitely deserves to be saved ).

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

.... congrats on getting the PSU working .

Thanks for your very informative and enlightening advice Momaka .

If there ever was an award for spending the most time and effort to improve a cheap PSU, I think you would easily win .

I concur :

That's crazy! This is the first time I'm seeing this. Normally, those resistors have the same resistance value. No idea why Deer/Allied did that.

Either they knew something that we don't ... or they simply screwed up and put a resistor with the wrong Ω .



BTW, forgot to mention I washed the PCB of this PSU in the sink with abundant water while it was devoid of caps. Then 'centrifuged by hand', hair-driered, and left it 24 hours placed just over an electric heater to make sure it dried completely. And no afterwards.


Extra touches :

[c_hegge]

Re: Cheap PSU: Piece of junk or semi-decent?

Broken fingers?! What kind of fans are you using?

4500RPM 120x38 Deltas? I have two of those on my load tester, and I'm sure one of those would quite easily break your finger. I've run the load tester near openings and windows before, and one of the fans ingested a small stick with some leaves that fell off a tree, and turned it into sawdust.

[momaka]

Re: Cheap PSU: Piece of junk or semi-decent?

Seems like I missed a lot of action in this thread. Well, congrats on getting the PSU working .

Added a DIY heatsink for the rectifier bridge.
http://www3.picturepush.com/photo/a/...g/11471416.jpg

If there ever was an award for spending the most time and effort to improve a cheap PSU, I think you would easily win . That said, this PSU is actually something I would trust now for reliably powering an old computer.

+5V is right on the mark!

Yup, these older Deer/Allied/L&C PSUs always seem to have spot-on 5V rail. I have a slightly older but still similar PSU - a Deer DR-250ATX powering a 933 MHz Pentium 3 PC with 2 HDDs and a Radeon 7200 video card. The 5V rail is rock-stable in mine as well, usually between 5.00 and 5.05V depending on load. The 5VSB is also very close to spot-on at 5.01V.

R1 is in parallel with second bigcap, R2 is in parallel with first bigcap.

That's crazy! This is the first time I'm seeing this. Normally, those resistors have the same resistance value. No idea why Deer/Allied did that.

Also keep in mind the "suction" side eats fingers much more readily than the "blower" side ... trust me, I know...

+1
That's because when you touch the suction side, the blades are going towards "pulling in" your fingers. On the blower side, they push everything outwards, so when you put your finger there, the blades just try to ride over it - hence why you can't get injured.

I would be more worried about breaking your fingers You can replace fan, but finger…

Broken fingers?! What kind of fans are you using?

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Pics or didn't happen

Unfortunately I lack the means at home to photograph flow dynamics. I can however post some draws & pics to help you understand.

Just look at the front and rear fairings in the fans inside wind tunnels :

[eccerr0r]

Re: Cheap PSU: Piece of junk or semi-decent?

I've noticed some fans have "heavy" hubs...This fan I'm holding right now has a metal shaft (brand/model is Sanyo Denki Pico Ace 15) and hub that the fan blades attach to, whearas most fans have one metal shaft connected to a plastic hub that the blades attach to. This is a 60mm x 10mm fan so it doesn't store way too much inertia, but scaled up to a 120mm I wouldn't put my finger into it...

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

However the reduction in turbulence and noise are immediately noticeable .

Pics or didn't happen

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

The hubs are some ladies cosmetic shit caps, extremely light pieces of thin plastic, a couple grams in weight. The influence on propeller balancing of this minimal mass, so close to the axis of rotation, is negligible. No extra vibrations, don't sweat.

However the reduction in turbulence and noise are inmediately noticeable .

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

Can you achieve their precision? I guess not. So you will just centre-out the rotor and put more stress on bearings. Maybe even create some viration. That will bring you more noise than all the turbulence. People discovered it is sometimes better to add 100 RPM on better fan with same noise to achieve more than with all this mods. Similar with CPUs and stuff.

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Just curious what are thos bloody things on those fans?

'Though this be madness, yet there is method in't!'


Have a look at this table:

Notice the form with best (lowest) aerodynamic drag coefficient is teardrop (streamlined body), 0.04 .

But as they come, with flat center both at front and rear, PC fans behave like a short cylinder, with a higher Cd of 1.15 .


Now have a look at how they make the REAL fans:

Front hub & rear cone .

[Behemot]

Re: Cheap PSU: Piece of junk or semi-decent?

Just curious what are thos bloody things on those fans?

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

Wow, success beyond my expectations !

Plugged The Thing inside the good old PIII i440BX Tualatin-S @ 1600/150:

Posted OK, and BIOS reported 12.16V / 4.99V / 3.36V / -11.86V / -5.20V, with MMD reading 12.22V / 5.03V. Nice start .

Launched XP without a glitch, and found that what Aida64 and Speedfan now report is almost exactly the same what the DMM reads in the molex .

Then proceeded to test voltages under torture:

+5V is right on the mark! . +12V is within 2% . +3.3V is within 2% . Comp is granite-rock-stable now .

Also another unexpected benefit is a remarkable improvement in overall system cooling, CPU temps are 3~4 ºC lower now. Cool!


Thanks a lot gentlemen , this wouldn't have been possible without your help, patience and understanding. I'm learning in spades from you.

Token of gratitude:

... My finger healed surprisingly well ...

Glad to know .

... Out of curiosity: What soundcard do you have in there? ...

Sound Blaster AWE64 Gold.

[lti]

Re: Cheap PSU: Piece of junk or semi-decent?

Sorry to hear that. That was a 3000rpm fan, at those high speeds they bite harder than the usual ~1500 rpm PC fan.

I think it had unusually sharp edges on the blades. That fan is inside a power supply now. My finger healed surprisingly well.

[eccerr0r]

Re: Cheap PSU: Piece of junk or semi-decent?

Also keep in mind the "suction" side eats fingers much more readily than the "blower" side ... trust me, I know...

If you had a "suction" side exposed please put a grille on it especially if it's high RPM or has a heavy rotor/fanblades.

[Th3_uN1Qu3]

Re: Cheap PSU: Piece of junk or semi-decent?

Out of curiosity: What soundcard do you have in there? I used to have the same setup a couple years ago, including the push-pull fan arrangement on the PSU, both wired to 5v. Now the mobo i'm using for my dual Tualatins no longer has an ISA slot, but it has AGP 4x.

[TELVM]

Re: Cheap PSU: Piece of junk or semi-decent?

The Tacens Aura II 80mm fans that now dwell in my crappy PSU are 1500 rpm.

And that's one of the reasons for adding a second fan.