SMPS blown up - any ideas why?

[Tim Woodall]

I'm in the UK on nominal 240V mains - actually about 245V

I've got some computers run from 12V switch mode power supplies (look
like laptop power supplies but slightly higher current rating than my
laptop at least). Today I was doing some fiddling which involved
shutting one of the computers down and completely unplugging the supply.
A few hours later I plugged the supply in again (not connected to the
computer although I don't think this should matter) and there was some
alarming crackling, my ups started beeping and the overload light came
on. I quickly unplugged again and I found the 5A fuse on the power lead
to the UPS had blown. I changed that fuse.

Being brave (or a fool I thought I'd try the power supply again,
although this time not via the UPS - but another 5A fuse blew. Third
attempt with the power supply plugged into the computer but nothing at
all - I assume the 3.15A fuse in the power supply has now blown as well.
(I can't see the fuse and I can't see how to open the power supply,
thats just what's printed on the supply)

I have more than one of these supplies and I've now got the machine
running from another one.

Checking the input power to the power supply, (because it's easy for me
to do), it peaks at about 0.30A on power up and sits around 0.2A after
that. 55VA, 30W according to the power meter (I'm surprised the power
factor is as bad as that). The supply says 12V, 6.6A. So this doesn't
look overloaded to me.

Does anyone have any ideas what might be the problem? Have I just been
unlucky or is there something else I should be checking for?

I could put an ammeter on the output but it's a little tricky to do, so
unless it's going to give me anything else useful I can't be bothered to
try it. Presumably the output current is going to be no more than 20x
the input current.

I need to order a new power supply. Should I get the same 80W supply or
would it be advisable to get the 110W supply (8.5A)? I would prefer to
go with the 80W one because the whole point of this setup is that it's
fanless and the 110W PSU has a fan but the machine in question could go
into the loft (attic) if needs be.

Tim.

--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

http://www.woodall.me.uk/ http://www.locofungus.btinternet.co.uk/

 

[Phil Allison]

"Tim Woodall"

I'm in the UK on nominal 240V mains - actually about 245V

I've got some computers run from 12V switch mode power supplies

** SMPS blow up - just like dogs bark and cats go meow.

Off line SMPS are basically accidents looking for somewhere to happen.

Just finished repairing a particularly horrible example in a Yamaha
4-channel power amplifier - an off line, self oscillating square wave
inverter using BJTs !!!!

Seems the thing self destructed after two small electros in the base drive
ccts went high ESR.

With no soft start system and no load current limiting at all - it was bound
to blow up sooner rather than later.



...... Phil

 

[Baron]

Tim Woodall wrote:

I'm in the UK on nominal 240V mains - actually about 245V

I've got some computers run from 12V switch mode power supplies (look
like laptop power supplies but slightly higher current rating than my
laptop at least). Today I was doing some fiddling which involved
shutting one of the computers down and completely unplugging the
supply. A few hours later I plugged the supply in again (not connected
to the computer although I don't think this should matter) and there
was some alarming crackling, my ups started beeping and the overload
light came on. I quickly unplugged again and I found the 5A fuse on
the power lead to the UPS had blown. I changed that fuse.

Being brave (or a fool I thought I'd try the power supply again,
although this time not via the UPS - but another 5A fuse blew. Third
attempt with the power supply plugged into the computer but nothing at
all - I assume the 3.15A fuse in the power supply has now blown as
well. (I can't see the fuse and I can't see how to open the power
supply, thats just what's printed on the supply)

I have more than one of these supplies and I've now got the machine
running from another one.

Checking the input power to the power supply, (because it's easy for
me to do), it peaks at about 0.30A on power up and sits around 0.2A
after that. 55VA, 30W according to the power meter (I'm surprised the
power factor is as bad as that). The supply says 12V, 6.6A. So this
doesn't look overloaded to me.

Does anyone have any ideas what might be the problem? Have I just been
unlucky or is there something else I should be checking for?

I could put an ammeter on the output but it's a little tricky to do,
so unless it's going to give me anything else useful I can't be
bothered to try it. Presumably the output current is going to be no
more than 20x the input current.

I need to order a new power supply. Should I get the same 80W supply
or
would it be advisable to get the 110W supply (8.5A)? I would prefer
to go with the 80W one because the whole point of this setup is that
it's fanless and the 110W PSU has a fan but the machine in question
could go into the loft (attic) if needs be.

Tim.

Its quite possible that one of the capacitors had gone bad ! Whilst not
affecting an already running PSU, could cause the failure at switch on.
It happens sometimes with bad caps on computer mainboards, often with
bad results.

--
Best Regards:
Baron.

 

[Jasen Betts]

On 2008-07-12, Tim Woodall <devnull@woodall.me.uk> wrote:

I'm in the UK on nominal 240V mains - actually about 245V

Does anyone have any ideas what might be the problem? Have I just been
unlucky or is there something else I should be checking for?

prolly bad luck or bad parts. a while back Morex used a bunch of dodgy
capacitors in their powersupplies (well the ones I saw were in the
DC-DC circuit not the powersupply)

I could put an ammeter on the output but it's a little tricky to do, so
unless it's going to give me anything else useful I can't be bothered to
try it. Presumably the output current is going to be no more than 20x
the input current.

Doing that would tell you how much power your computer is using.
another thing to look for is heat, are the supplies running hot?

I need to order a new power supply. Should I get the same 80W supply or
would it be advisable to get the 110W supply (8.5A)?

why not measure it? you have DC plug cable from the dead supply, that may
make getting an ammeter in-line easier.

Bye.
Jasen

 

[Ross Herbert]

On Sat, 12 Jul 2008 18:59:03 +0000 (UTC), Tim Woodall <devnull@woodall.me.uk>
wrote:

:I'm in the UK on nominal 240V mains - actually about 245V
:
:I've got some computers run from 12V switch mode power supplies (look
:like laptop power supplies but slightly higher current rating than my
:laptop at least). Today I was doing some fiddling which involved
:shutting one of the computers down and completely unplugging the supply.
:A few hours later I plugged the supply in again (not connected to the
:computer although I don't think this should matter) and there was some
:alarming crackling, my ups started beeping and the overload light came
n. I quickly unplugged again and I found the 5A fuse on the power lead
:to the UPS had blown. I changed that fuse.
:
:Being brave (or a fool I thought I'd try the power supply again,
:although this time not via the UPS - but another 5A fuse blew. Third
:attempt with the power supply plugged into the computer but nothing at
:all - I assume the 3.15A fuse in the power supply has now blown as well.
I can't see the fuse and I can't see how to open the power supply,
:thats just what's printed on the supply)
:
:I have more than one of these supplies and I've now got the machine
:running from another one.
:
:Checking the input power to the power supply, (because it's easy for me
:to do), it peaks at about 0.30A on power up and sits around 0.2A after
:that. 55VA, 30W according to the power meter (I'm surprised the power
:factor is as bad as that). The supply says 12V, 6.6A. So this doesn't
:look overloaded to me.
:
oes anyone have any ideas what might be the problem? Have I just been
:unlucky or is there something else I should be checking for?
:
:I could put an ammeter on the output but it's a little tricky to do, so
:unless it's going to give me anything else useful I can't be bothered to
:try it. Presumably the output current is going to be no more than 20x
:the input current.
:
:I need to order a new power supply. Should I get the same 80W supply or
:would it be advisable to get the 110W supply (8.5A)? I would prefer to
:go with the 80W one because the whole point of this setup is that it's
:fanless and the 110W PSU has a fan but the machine in question could go
:into the loft (attic) if needs be.
:
:Tim.


Did you check the specs for these SMPS? Are they specified to run without a
load? Some SMPS I have encountered definitely DO NOT like running withoput a
load and promptly exhibit the same symptoms as yours when asked to do so.