What resistor value?

[NB]

I have a switch mode PSU that went up in smoke. Input voltage is 115/230v
50/60 hz. Straight across the mains input there was a resistor & capacitor
in series (filter). The capacitor went short & the resistor produced the
smoke & cracked. Problem is 'what was the resistor value'. The capacitor is
a 0.01mF disc ceramic & the resistor was about 1/2W. The supply works ok
without the filter but I should really replace the resistor.

Could anyone suggest what the value should be?

Thanks, Nigel

 

[Laurence Taylor]

NB wrote:

I have a switch mode PSU that went up in smoke. Input voltage is 115/230v
50/60 hz. Straight across the mains input there was a resistor & capacitor
in series (filter). The capacitor went short & the resistor produced the
smoke & cracked. Problem is 'what was the resistor value'. The capacitor is
a 0.01mF disc ceramic & the resistor was about 1/2W. The supply works ok
without the filter but I should really replace the resistor.

Could anyone suggest what the value should be?

Thanks, Nigel

Not critical, but around 100k should do the trick.

It's to reduce surge current when the capacitor charges up. (It also
helps you to stop you getting a belt if you accidently touch the mains
plug after you've unplugged it).

--

rgds
LAurence

.... Love isn't love until you give it to someone
begin the search for better software

 

[Dave D]

"Laurence Taylor" <see-headers@nospam.plus.com> wrote in message
news:3F2107F8.B3878EA7@nospam.plus.com...

NB wrote:

I have a switch mode PSU that went up in smoke. Input voltage is
115/230v
50/60 hz. Straight across the mains input there was a resistor &
capacitor
in series (filter). The capacitor went short & the resistor produced the
smoke & cracked. Problem is 'what was the resistor value'. The capacitor
is
a 0.01mF disc ceramic & the resistor was about 1/2W. The supply works ok
without the filter but I should really replace the resistor.

Could anyone suggest what the value should be?

Thanks, Nigel

Not critical, but around 100k should do the trick.

It's to reduce surge current when the capacitor charges up. (It also
helps you to stop you getting a belt if you accidently touch the mains
plug after you've unplugged it).

No offence, but you need to learn a little more about switch mode PSUs and
filter theory before you make assertions like these, which are not true.

Dave

 

[Dave D]

"NB" <n@awards...plus.co.uk> wrote in message
news:Xns93C358F65C426nawardspluscouk@195.92.193.157...

I have a switch mode PSU that went up in smoke. Input voltage is 115/230v
50/60 hz. Straight across the mains input there was a resistor & capacitor
in series (filter). The capacitor went short & the resistor produced the
smoke & cracked. Problem is 'what was the resistor value'. The capacitor
is
a 0.01mF disc ceramic & the resistor was about 1/2W. The supply works ok
without the filter but I should really replace the resistor.

Could anyone suggest what the value should be?

Thanks, Nigel

Nigel, the easiest way out of this may be to obtain an X-class capacitor and
solder it directly across the mains. Different manufacturers use different
values, but 0.1 uF or 0.33 uF values are common. (some use a 0.01uF before
and after the filter transformer) I don't much care for soldering mains
filter caps straight across the mains socket pins like many manufacturers
do, I prefer to see them after the PSU's mains fuse, but that may not be too
practical in your case.

Dave

 

[Laurence Taylor]

Dave D wrote:

It's to reduce surge current when the capacitor charges up. (It also
helps you to stop you getting a belt if you accidently touch the mains
plug after you've unplugged it).


No offence, but you need to learn a little more about switch mode PSUs and
filter theory before you make assertions like these, which are not true.

If you would be so kind as to correct me, I shall stand corrected.

The series capacitor-resistor device sounds like a standard snubber
network, often found across switch contacts, thyristor dimmers, and
the like, to reduce interference. Usually they are 0.1uF and 100k.

I don't think I've seen one on an SMPS, in my experience they use
capacitors straight across the mains, often with a couple of chokes
inline as well.

--

rgds
LAurence

.... Love isn't love until you give it to someone
begin the search for better software

 

[Dave D]

"Laurence Taylor" <see-headers@nospam.plus.com> wrote in message
news:3F21869F.75E0AF8A@nospam.plus.com...

Dave D wrote:

It's to reduce surge current when the capacitor charges up. (It also
helps you to stop you getting a belt if you accidently touch the mains
plug after you've unplugged it).


No offence, but you need to learn a little more about switch mode PSUs
and
filter theory before you make assertions like these, which are not true.

If you would be so kind as to correct me, I shall stand corrected.

Sorry if I seemed abrupt, Laurence ;-)

The series capacitor-resistor device sounds like a standard snubber
network, often found across switch contacts, thyristor dimmers, and
the like, to reduce interference. Usually they are 0.1uF and 100k.

I suspect that in this case the cap would be in series with a low
resistance, maybe it was cheaper than using an X-Class capacitor. The
resistor is not in series with the cap to protect the user from a shock from
the plug, it will be either to form a tuned filter, or to 'fail safe' in the
event the (probably a cheap non X-class device) cap shorts, maybe both.

I don't think I've seen one on an SMPS, in my experience they use
capacitors straight across the mains, often with a couple of chokes
inline as well.

You are correct, An X-class across the mains, a (preferably) common mode
choke, and another x-class on it's output would be a nice filter
arrangement. Even nicer would be the addition of a couple of Y class caps to
ground from L and N. I suspect this might not be a PC power supply!

Dave

 

[Laurence Taylor]

Dave D wrote:

The series capacitor-resistor device sounds like a standard snubber
network, often found across switch contacts, thyristor dimmers, and
the like, to reduce interference. Usually they are 0.1uF and 100k.


I suspect that in this case the cap would be in series with a low
resistance, maybe it was cheaper than using an X-Class capacitor.

I think I should have said 100 ohms, not 100k. Apologies.

The
resistor is not in series with the cap to protect the user from a shock from
the plug, it will be either to form a tuned filter, or to 'fail safe' in the
event the (probably a cheap non X-class device) cap shorts, maybe both.

No, judging by the number of times I've known that to happen, I don't
think the manufacturers bother with such niceties!

I see why it could be used as a failsafe device - cap goes phut,
resistor opens.

I don't think I've seen one on an SMPS, in my experience they use
capacitors straight across the mains, often with a couple of chokes
inline as well.


You are correct, An X-class across the mains, a (preferably) common mode
choke, and another x-class on it's output would be a nice filter
arrangement. Even nicer would be the addition of a couple of Y class caps to
ground from L and N. I suspect this might not be a PC power supply!

Indeed. You used to be able to buy delta units in a nice plastic case,
but I haven't seen them for ages.


rgds
LAurence

.... Love isn't love until you give it to someone
begin the search for better software

 

[NB]

"Dave D" <someone@somewhere.com> wrote in
news:bfrjbq$nbf$1@hercules.btinternet.com:

Nigel, the easiest way out of this may be to obtain an X-class
capacitor and solder it directly across the mains. Different
manufacturers use different values, but 0.1 uF or 0.33 uF values are
common. (some use a 0.01uF before and after the filter transformer) I
don't much care for soldering mains filter caps straight across the
mains socket pins like many manufacturers do, I prefer to see them
after the PSU's mains fuse, but that may not be too practical in your
case.

Dave

Thanks for the replies. This series R/C was after the PSU's mains fuse &
after a filter transformer. I think the resistor may have been about 10k
(measuring both remaining halves). An X-class capacitor may be an easy
solution.

N