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Author Topic: Sheeva PSU preventive maintenance  (Read 1736 times)
cjm
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« on: May 31, 2010, 06:26:33 AM »

Hi all,

my Sheevaplug is constantly powered on for a couple of months now (240VAC, although limited to around 230VAC by UPS) and the repeated stories about failing power supplies make me expect mine to fail pretty soon. Instead of hooking up an external PSU, I'm planning to replace the capacitors with better ones before they dry out completely.

However, I use this box as my central home server (email, DNS, Internet, ...) and I'm afraid that the capacitors have aready degraded to the point where it won't turn on after I opened it up to find out which capacitors I actually need...

Does anyone of you have a list of capacitors (capacity, voltage and approximate size) in the Sheevaplug PSU?

While waiting for a reply, I guess I'll try and see whether I can get a backup of the plug's root disk working on my desktop under qemu-system-arm -- I always wanted to see whether this is a viable solution for temporarily replacing the plug with a desktop while getting the plug itself fixed...

Thanks,
--Christian
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superpat
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« Reply #1 on: May 31, 2010, 07:06:07 AM »

Hi Christian.

The problem with the capacitors is not that they dry out, but that the liquid electrolyte in the capacitors heats up due to the  electrolyte conducting  ac ripple current and heating, eventually blowing  the electrolyte out through the overpressure vent, (the radial frangible lines on the end of the capacitor).

The "goodness" or ESR (equivalent series resistance), of the capacitor can vary with age and ambient temperature, normally upwards.  The heating effect is Watts = (Iripple) squared x ESR , or the heating effect is directly proportional to the value of the ESR. 

READ :- http://wiki.xtronics.com/index.php/Capacitors_and_ESR

This is a mains switching power supply. It can give you a nasty shock.  MAINS IS CONNECTED DIRECTLY TO THE CAPACITOR WITH NO SAFETY ISOLATION.  Please make sure that the PS is disconnected from the mains and the caps are discharged BEFORE you work on the supply.

The capacitors in the Sheevaplug  V 1.3 ( second generation) 's power supply:-

The problem capacitor is the mains full wave rectifier smoothing cap. Fitted as standard  is a general purpose 400V 6.8 ufd 105 deg C aluminium electrolytic.   I would recommend fitting a special purpose power supply filter capacitor with defined LOW ESR, 400V 6.8ufd  125 deg C aluminium electrolytic.

Also worth changing is the "snubber" capacitor, a 400v 22 ufd 105 deg C capacitor. Again I would replace this with a special purpose power supply filter capacitor 400v 22 ufd 125 Deg C LOW ESR  part.

The 125 deg C parts will also give you a bit more margin.

You will have to find a supplier of good quality power supply filter capacitors. Places like Maplins will sell you general purpose capacitors, that will be no better than the ones already fitted.


Once again I must reiterate, if you are not trained to  work on hazardous  mains power supplies, then get someone who is competent to do it for you!



regards

Patrick


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cjm
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« Reply #2 on: May 31, 2010, 09:23:28 AM »

Thanks, Patrick,

I can only second your warnings for everybody reading this. I once ruined a screwdriver discharging a high-voltage capacitor in an old valve radio. I thought the capacitor had already discharged and just wanted to play it safe but it must have had the full charge for such an effect and this would mean around 311V (220VAC in Germany back then). These days we have 240VAC/340V peak...

Back to the subject: so you would say only those two capacitors need replacing? I was wondering whether I should replace all of them while I'm at it... Also, do you remember the size of those two (diameter and height)?

Thanks,
--Christian
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