Not so much magic smoke, more an explosion...

Google Groups
1

Someone who shall remain nameless managed to kill our dehumidifier by
shutting a door across the mains cable, which took out the ring main as
well. When I took it to bits the internal 3A fuse on the PCB had
shattered. I replaced the cable and the fuse, turned it on and nothing
happened. When I took the cover off again the internal fuse had gone
again, so I disconnected the 240V output to the compressor in case that
was the problem, replaced the fuse and tried again. This time the
internal fuse exploded in a most gratifying manner, showering the room
with glass shards. I’m guessing there’s a dead short somewhere on the
AC side. There is a rectangular (polyester?) capacitor across L & N and
something I don’t recognise, a FNR 10K471, which google tells me is a
Varistor, which google goes on to tell me is there to protect from
voltage spikes. It also appears that when these fail, they fail
short-circuit. Does anyone know if it’s possible that’s what’s
happened? There’s no signs of damage to any of the components on the PCB.

Thanks,

Alan Burlison

2

Alan,

It sounds as if they who shall remain nameless has inadvertently
damaged some components on the mains control section of the
dehumidifier.

If you have changed the wire (I assume you have used suitable 3 core
mains which is rated for 3Amp current.) then it cannot be this

As you have already discovered there is a short circuit somewhere on
the mains side of the dehumidifier. Here is what I would do.

Remove the compressor output - as you already have done.
Remove the fuse from the PCB
Remove the 230V input cable that you have replaced.

Set your multimeter to resistance measurement - lowest sensitivity (0

  • 200 ohms normally) and place the probes across the live and neutral
    terminals. There should not be a reading on the meter (it should not
    read 0.06 ohms or similar) it might read >20M Ohms or similar…it
    depends on your meter. Mine bleeps when there is a dead short. If
    there is a dead short then you will need to remove components or look
    at the PCB until you find where the short circuit is.

The circuit is normally made up in the following way on a dehumidifier -

  1. Mains input (Live, Neutral and Earth go to PCB)
  2. Fuse and Surge protection (Varistors and 3A fuse)
  3. Mains filter (made up of capacitors and inductors - sometimes in a
    metal can)
  4. Conversion to low voltage and control section for measurement and
    relay output
  5. Output to compressor via relay or triac…this bit varies it
    really depends on how the item was designed.

The areas to look at are the tracks coming from the mains input to the
varistors and the mains filter. The likely culprit is a vaporised
track or electrolytic or polypropelene capacitor going short circuit
due to over voltage and current.

If you post pictures I may be able to help further. Please be careful
with 230V mains…particularly if you charge up capacitors. They
will hurt when you discharge them yourself by accident. To be avoided
I assure you.

Regards

AlexOn 11 May 2012 00:21, Alan Burlison alan.burlison@gmail.com wrote:

Someone who shall remain nameless managed to kill our dehumidifier by
shutting a door across the mains cable, which took out the ring main as
well. When I took it to bits the internal 3A fuse on the PCB had shattered.
I replaced the cable and the fuse, turned it on and nothing happened. When
I took the cover off again the internal fuse had gone again, so I
disconnected the 240V output to the compressor in case that was the problem,
replaced the fuse and tried again. This time the internal fuse exploded in
a most gratifying manner, showering the room with glass shards. I’m
guessing there’s a dead short somewhere on the AC side. There is a
rectangular (polyester?) capacitor across L & N and something I don’t
recognise, a FNR 10K471, which google tells me is a Varistor, which google
goes on to tell me is there to protect from voltage spikes. It also appears
that when these fail, they fail short-circuit. Does anyone know if it’s
possible that’s what’s happened? There’s no signs of damage to any of the
components on the PCB.

Thanks,

–
Alan Burlison

–
You received this message because you are subscribed to the Google Groups
"HAC:Manchester" group.
To post to this group, send an email to hacman@googlegroups.com.
To unsubscribe from this group, send email to
hacman+unsubscribe@googlegroups.com.
For more options, visit this group at
http://groups.google.com/group/hacman?hl=en-GB.

3

If you have changed the wire (I assume you have used suitable 3 core
mains which is rated for 3Amp current.) then it cannot be this

I replaced it with some 16A-rated flex, so no it’s not that.

As you have already discovered there is a short circuit somewhere on
the mains side of the dehumidifier. Here is what I would do.

Remove the compressor output - as you already have done.
Remove the fuse from the PCB
Remove the 230V input cable that you have replaced.

I’ve actually removed the PCB entirely.

Set your multimeter to resistance measurement - lowest sensitivity (0

  • 200 ohms normally) and place the probes across the live and neutral
    terminals. There should not be a reading on the meter (it should not
    read 0.06 ohms or similar) it might read>20M Ohms or similar…it
    depends on your meter. Mine bleeps when there is a dead short. If
    there is a dead short then you will need to remove components or look
    at the PCB until you find where the short circuit is.

It reads 0.2 ohms, so something’s a dead short.

The circuit is normally made up in the following way on a dehumidifier -

  1. Mains input (Live, Neutral and Earth go to PCB)
  2. Fuse and Surge protection (Varistors and 3A fuse)
  3. Mains filter (made up of capacitors and inductors - sometimes in a
    metal can)
  4. Conversion to low voltage and control section for measurement and
    relay output
  5. Output to compressor via relay or triac…this bit varies it
    really depends on how the item was designed.

I can identify bits that correspond to all those parts -on the PCB there
are 2 transformers, one looks like a power supply transformer, the other
looks like it might be some sort of isolation transformer. The
compressor is switched via a relay.

The areas to look at are the tracks coming from the mains input to the
varistors and the mains filter. The likely culprit is a vaporised
track or electrolytic or polypropelene capacitor going short circuit
due to over voltage and current.

Apart from some scorch marks caused by the last time the fuse let go
there are absolutely no signs of damage on the board or components -
before I popped the fuse it I’d checked it carefully and I’ve rechecked
it since so I’m pretty sure sure nothing else has gone bang.

On the HV side of the transformer the only components are the Varistor,
a poly cap and some diodes, so I think it’s either the poly cap or the
varistor that have gone bang. I think I’ll try removing the varistor
and the cap and checking them out - although Farnell don’t have the
exact same varistor, one with a very similar spec only costs 19p so that
versus �100+ to replace the dehumidifier seems like a reasonable exchange!

If you post pictures I may be able to help further. Please be careful
with 230V mains…particularly if you charge up capacitors. They
will hurt when you discharge them yourself by accident. To be avoided
I assure you.

Indeed :slight_smile: If you are going, I’ll being the PCB to Hacman on Wednesday.

Thanks very much for your advice Alex, much appreciated.

Alan Burlison

4

I think I’ll try removing the varistor and the cap and checking them out

Removed both, they both seem to check out yet there’s still a short,
even with them removed - now I’m v. confused…

Alan Burlison