Temperature controller explosion.

This is perhaps distantly related to workshop electronics, but I have (or had) a laboratory waterbath in my dungeon. It blew up in a somewhat spectacular fashion a few days ago. A loud bang, toxic smelling smoke, and the workshop MCB tripped. I had a look and it seems a capacitor has blown. Crappy pic below - I can't seem to sort out how to get stuff from my phone to Win10 . Anyhow, the cap marked is marked 470N X2 and seems to be wired between live and neutral on the choc block which brings the mains in, as far as I can tell without further dissection. The questions I have for the electronics wizards amongst you are (a) is it reasonable that there should be a (presumably 470nF) capacitor between live and neutral on the input, (b) if so why, and (c) is it likely that the electronics downstream have survived the explosion?

Rob.

(a) Yes

(b) To reduce conducted emissions

(c) Quite possibly

Correct that the capacitor value is 470nF. As the X2 indicates it is an X capacitor, specifically designed to go across live and neutral. If you replace it you must use another X rated capacitor. They're pretty common in the catalogues, so finding one shouldn't be a problem.

Andrew

[a] Yes.

[b] Suppression of radio frequency noise either incoming or outgoing.

[c] Probably. I've known them to fail for no other reason than they felt like it with no problems after replacement.

Thee may be 2 other caps. ( probably smaller ) from neutral/ground and phase/ground in a 'Delta Suppressor' arrangement. Not always fitted.

D.

PS Andrew has faster fingers it seems ...

Edited By Dave Daniels on 11/05/2017 23:05:40

Edited By Dave Daniels on 11/05/2017 23:07:12

Hi Robin,
Itis reasonable to have such a capacitor across the mains. (The X signifies that they are reated for this type of application.) They are used in that position to help filter out electrical noise. It could be to reduce external electrical noise getting into the equipment or to reduce electrical noise generated by the equipment from getting back into the mains. (A common source of this noise is from swtch mode power supplies. There is a reasonable chance that the rest of the equipment has not been damaged. The equipment should function with this capacitor removed. It sould be replaced with an X rated capacitor for long term use.

Les.

The filter capacitor on our dishwasher gave up, after about 12 years, recently. Took the mcb trip out once, then blew the fuse and tripped the mcb. It was blackened, but only tripped the mains occasionally (took a couple of weeks to determine the cause of the trips) and was not too expensive to replace. One went bang in a washing machine several years ago - that one was obvious!

I doubt it would affect anything down-line, unless flying debris. If in doubt, reconnect without the filter to test, if you were not going to repair the controller if duff.

89p at your local Maplin store, cat. no. N68CN.

I remember back about 1956, the suppressor capacitor on mums Singer sewing machine blew up. I was in bed at the time, and I crept out to the living room, to find mum sitting in a cloud of smoke, and quite speachless. Dad came home an hour or so later, removed the suppressor, gave the machine a test run. It got sold about twenty years ago in almost as new condition. After removal of the suppressor I stopped getting interference on my crystal set.

Ian S C

That Maplin part is only 47nF, not 470nF. However, the simplest solution is just to snip out the remains and move on. If you find you get interference on the radio or your nearby equipment starts to misbehave then you can think about replacing it but the specific installation has a lot more to do with the EMC performance than one single capacitor.

These X caps are supposed to be self healing ie the fault should clear itself. However they make the most godawful stink and if it fails in one place internally it seems likely to fail somewhere else later.

When I powered up my 30 year old CNC controller, 2 of the X caps popped within the first few hours, so I removed the rest of them. The whole thing is in the skip now but it worked fine without them.

Murray

Note the reference to "Incoming". Where I work, there was a vacuum cleaner that put so much noise onto L-N that the class X capacitor in nearby equipment passed sufficient current to blow the internal 20mA fuse. If that fuse had not been there, what would have failed instead? 230V x >20mA (no idea how much greater) is a lot of power to put into a small capacitor.

It might be prudent to check for any nearby sources of major electrical noise. Unlikely, but it can happen.

The power dissipated in the capacitor is nowhere near 230V x 20mA. It depends on the ESR times the square of the current and as the ESR will be less than an ohm will be very small, less than a microwatt at 20 mA.

Russell

True, but what happens to the ESR when a voltage spike punctures the dialectric?

Dave

They are, to some extent, self-healing.

If the environment is prone to nasty voltage spikes the device needs a MOV. That's what they were designed to deal with.

Incidentally, MOVs degrade over time. We had to change them every year on some sensitive kit.

D.

Thanks to all for your replies. I now understand what the capacitor is doing there, and it sounds like there's a good chance that the rest of the circuitry will have survived. I'll have to strip it down to clean, as the blown capacitor has sprayed foul smelling brown gunk (the dielectric presumably?) all over the innards. I guess the thing to do take the capacitor out, replace fuses, and if it runs replace the cap.

I'll be pleased to get this thing going again - paid 15 quid at a salvage yard (don't think they had any idea what it was), got home and googled - around £1k from Cole-Parmer! It's a PID controller, spec says it will hold temperature to +/- 0.01 C, which is good - amongst other things it can cook a perfect soft-boiled egg. Yolk at about 63.27 C is my preference.

Thanks again, Robin.

Just be careful as there is the possibllity that there are other capacitors inside that are still charged to mains voltages My first operation would be to check any electrolytic capacitors and discharge them .