Magic Smoke

Hi

The magic smoke has escaped from my CNC PC. It looks like it's the 400 volt input capacitor that holds the circa 300 volts DC before it gets chopped up into the 5 and 12 volts.

What are the chances it is just the cap has gone? The PC will be around the 10 year old mark. The HDD is on a parallel bus and not SATA. I would like to keep it because it runs Mach3 and don't want issues trying to run it on later OS's.

TIA

Cheers Michael.

You can easily get replacement boards with IDE and ATA buses from a well known auction site

Search for "LGA 1156" for example

If it's a desktop with a separate power supply then new psus are fairly cheap and you wouldn't need to replace the motherboard. That's hoping that the board you have wasn't damaged.

This is the cap that's gone. I have some old psu's but they seem to use 2 caps in series at 200v each. I have found one on ebay so will order one up. I will check the bridge rectifier to make sure that it hasn't gone short because that would have put AC across the cap which I don't think it would like. Thanks for your input guys.

Cheers Michael,

Good luck with changing the cap. If you can wind it up slowly on a variac in case of nasty surprises.

If you need to replace the whole psu (imho the easiest solution) the modern ones have mostly SATA power connectors and not many of the old molex connectors. Luckily the wire colours are (normally) the same so a simple way out suggests itself.

Hi Mike,

if you can't see any other damage it's worth trying the replacement. you're unlike to cause any further damage that hasn't already been done, but make sure it's cased up when you switch on and for goodness sake make sure the cap is the right way round!"*

Neil

* not meaning to be patronising, but I've heard the bang from a tiny little one going pop!

Make sure it's fully discharged before trying to remove it. They can give a nasty belt. Just chuck a heavy screwdriver across the terminals. (not literally).

I have removed the cap and when I put a meter across the holes it was low ohms and a diode looks like it's the same. To buy a cap it's about £5 so I have just bought a second hand psu for £7!

Neil.I know what you mean about electrolitics. I put a low voltage one in a higher voltage circuit. It nearly took my finger off. Lots of confetti though.

Cheers Michael

Hi Michael

If you replace the capacitor ( approx. 470 uF 200V )

change the identical one next to it

the two capacitors are wired in series to make a 235 uF 400 V capacitor

**LINK**

you may find it cheaper to replace the AT power supply with a new ATX supply

an ATX power supply can be used with an adaptor or by connecting the two 6 way molex connectors off

your old power supply to the new ATX supply

see

**LINK** or http://uk.rs-online.com/web/p/computer-power-cable-assemblies/7961110/

for details of the required connections between an ATX supply and a AT motherboard

NOTE - the orange AT "power good " wire needs connecting to the grey ATX wire

the orange ATX wires are not used (+3.3V only used on ATX motherboard))

to switch on a ATX power supply the green " power on" wire needs connecting to a black ground wire

John

Edited By john swift 1 on 22/11/2014 18:18:18

One reason for using 2 x 200V caps in series was to be able to use the input rectifier in voltage doubling mode for 120V input, giving a 340Vdc bus. Those PSUs would typically have a 120/240 voltage selector switch on the side.

Modern PSUs tend to have a power factor corrector (PFC) which boosts the input voltage up to around 380-400Vdc regardless of the mains voltage. That's not a problem for electrolytics in itself.

Generally, when a bus cap goes, there will be several other key components taken out with it, like the inrush current limiter and the diode bridge - and the internal fuse. Your second hand PSU is definitely the way to go. Even a new one would cost very little and they seem to be very backwards compatible.

With time, the electrolytics dry out and this is often the cause for the PSU failing. As they dry out, their resistance goes up so they get hotter still and the ripple voltage increases. The cans are designed to vent in the case of an internal short.

If you replace the failed caps, you may find it fails again fairly soon due to the other caps having dried out. They were probably all designed(?) with a similar lifetime. Yes, I've been designing switched mode PSUs for some years now....

Murray

Hi Michael,
I agree with all of the comments about replacing the PSU with a new or second hand one. When something fails in a switch mode PSU it generally takes out a number of other components. Also if you are not totally competent with electronics and are well equipped particularly with an isolating transformer do not attempt to fix switch mode power supplies.

Les.

Best one I,v seen go off was with a new( Philips) amplifier that I look after at out church. Just after we got it, during a service there was a good bang(the amp was in a back room), the sound system died, so I went and investigated, the room was full of white smoke, and the amp had a bump on top. It was a 2500uf electro, and only just at the working voltage, I replaced it with one 50% higher voltage, and it has been OK for the last twenty years.

Ian S C

Before caps were fitted with vents, they could be lethal if they detonated. I once saw a 38mm diameter can punch a clean hole through the suspended ceiling above it. This was during an abnormal (overvoltage) test, so wasn't completely unexpected but it could have put someone in hospital (or a box) in different circumstances. Now they tend to vent in a more controlled (but still alarming) fashion!

Murray

A friend of mine was working on a CO2 wire feed unit when a cap blew up, apart from temporary deafness from the bang he looked very good in the foil wig he was now wearing. The breaker feeding an inverter drive tripped so I reset it, the bang was loud to say the least and I may well have the record for the longest jump from a standing position, big cloud of dust and smoke followed, I always get someone else to reset CBs on drives now. Tantalum bead caps seem to make one hell of a bang for their size.

Mike

Edited By Michael Poole on 24/11/2014 14:40:55

Tantalums are crap. They don't like any sort of ripple current and they fail short circuit if they get upset or see a tiny overvoltage, so they are utterly pointless - or worse. Any competent engineer will religiously avoid using them. Those who don't will struggle to give any compelling reason for their use. They are the sign of someone who needs to be in another job!

Murray

Oh bother, and I've just used a tant on my latest PCB, in the buck converter for the GPRS module. New job here I come; probably be demoted to a shelf stacker.

Andrew

Often a power supply will blow because of a dry joint. This is because a power supply is flow soldered with many different sizes of components. Try examining the board for dry joints and re soldering them after fitting new components.

I know your game Andrew. You're just trying to look human and fallible by putting in a token tantalum. It'll still work perfectly I'm sure!

Murray

I had the same issue with my CNC PC, I just bought a 2nd hand OEM powersuppy for about £10 delivered out of small form factor PC, it wasn't the right size or shape but I made it fit and just ensured it wouldn't slide about or damage anything.

Up until a year or 2 ago I kept a bunch (5 or more) of odd size and shape AT/ATX PSU's in case I needed a sound 3.3 /5 or 12 volt supply - but the other half forced me to get rid of the "junk" - I could really do with one now for some makeshift 12v lighting and suds pumps etc.