"Fizzing" power supply.

P

Pete Verdon

Guest
Hi,

I'm building a piece of demonstration equipment for schoolchildren, that
includes an electromagnet (representing the write head of a hard drive).
The electromagnet is home-made, a few hundred turns of varnished wire on
an old bolt. To power it, I've just obtained a 2-amp bench power supply
like this:

http://www.radiotronics.co.uk/item.php?partcode=P004C

The magnet works well, with the supply set to 3V. I've put a multimeter
in series with it, and it appears to be drawing about half an amp. It
didn't get noticeably hot.

However, when the magnet is connected, the supply makes an audible
"fizzing" or "hissing" noise. Is this normal? What is it likely to be,
and does it indicate a fault with the supply or that the supply and the
magnet are not compatible in some way?

Thanks,

Pete
 
Pete Verdon wrote:

Hi,

I'm building a piece of demonstration equipment for schoolchildren,
that includes an electromagnet (representing the write head of a hard
drive). The electromagnet is home-made, a few hundred turns of
varnished wire on an old bolt. To power it, I've just obtained a 2-amp
bench power supply like this:

http://www.radiotronics.co.uk/item.php?partcode=P004C

The magnet works well, with the supply set to 3V. I've put a
multimeter in series with it, and it appears to be drawing about half
an amp. It didn't get noticeably hot.

However, when the magnet is connected, the supply makes an audible
"fizzing" or "hissing" noise. Is this normal? What is it likely to be,
and does it indicate a fault with the supply or that the supply and
the magnet are not compatible in some way?

Thanks,

Pete
That sounds really weird. Pun unintended. I assume that there is no
Click to expand...
burning smell, nothing getting hot. The supply shouldn't have anything
in there that could make a noise. Unless the transformer has loose
laminations. They would make a buzzing sound! I would have thought
that it would be over current protected anyway, so its not due to
overload.
--
Best Regards:
Baron.
 
On Sat, 09 Feb 2008 16:22:10 +0000, Pete Verdon wrote:

Hi,

I'm building a piece of demonstration equipment for schoolchildren, that
includes an electromagnet (representing the write head of a hard drive).
The electromagnet is home-made, a few hundred turns of varnished wire on
an old bolt. To power it, I've just obtained a 2-amp bench power supply
like this:

http://www.radiotronics.co.uk/item.php?partcode=P004C

The magnet works well, with the supply set to 3V. I've put a multimeter
in series with it, and it appears to be drawing about half an amp. It
didn't get noticeably hot.

However, when the magnet is connected, the supply makes an audible
"fizzing" or "hissing" noise. Is this normal? What is it likely to be,
and does it indicate a fault with the supply or that the supply and the
magnet are not compatible in some way?

Thanks,

Pete
Click to expand...
Try a six ohm (or so), 2 watt resistor in parallel with the coil. If
that stops the 'fizzing' then the supply is oscillating because of the
inductive load. If it works, then you can put a big capacitor (I don't
know what to recommend, but I'd start with 1000uF, observing polarity) in
series with the resistor, which can now be reduced to a 1/4 watt device.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
 
Tim Wescott wrote:

On Sat, 09 Feb 2008 16:22:10 +0000, Pete Verdon wrote:

Hi,

I'm building a piece of demonstration equipment for schoolchildren,
that includes an electromagnet (representing the write head of a hard
drive). The electromagnet is home-made, a few hundred turns of
varnished wire on an old bolt. To power it, I've just obtained a
2-amp bench power supply like this:

http://www.radiotronics.co.uk/item.php?partcode=P004C

The magnet works well, with the supply set to 3V. I've put a
multimeter in series with it, and it appears to be drawing about half
an amp. It didn't get noticeably hot.

However, when the magnet is connected, the supply makes an audible
"fizzing" or "hissing" noise. Is this normal? What is it likely to
be, and does it indicate a fault with the supply or that the supply
and the magnet are not compatible in some way?

Thanks,

Pete

Try a six ohm (or so), 2 watt resistor in parallel with the coil. If
that stops the 'fizzing' then the supply is oscillating because of the
inductive load. If it works, then you can put a big capacitor (I
don't know what to recommend, but I'd start with 1000uF, observing
polarity) in series with the resistor, which can now be reduced to a
1/4 watt device.
Click to expand...
I think Tim could be right! It didn't occur to me since lab power
supplies are normally very stable and it is something that could easily
happen if the coil causes something to become resonant.

--
Best Regards:
Baron.
 
On Sat, 09 Feb 2008 11:51:32 -0600, Tim Wescott wrote:

On Sat, 09 Feb 2008 16:22:10 +0000, Pete Verdon wrote:

Hi,

I'm building a piece of demonstration equipment for schoolchildren,
that includes an electromagnet (representing the write head of a hard
drive). The electromagnet is home-made, a few hundred turns of
varnished wire on an old bolt. To power it, I've just obtained a 2-amp
bench power supply like this:

http://www.radiotronics.co.uk/item.php?partcode=P004C

The magnet works well, with the supply set to 3V. I've put a multimeter
in series with it, and it appears to be drawing about half an amp. It
didn't get noticeably hot.

However, when the magnet is connected, the supply makes an audible
"fizzing" or "hissing" noise. Is this normal? What is it likely to be,
and does it indicate a fault with the supply or that the supply and the
magnet are not compatible in some way?

Thanks,

Pete

Try a six ohm (or so), 2 watt resistor in parallel with the coil. If
that stops the 'fizzing' then the supply is oscillating because of the
inductive load. If it works, then you can put a big capacitor (I don't
know what to recommend, but I'd start with 1000uF, observing polarity)
in series with the resistor, which can now be reduced to a 1/4 watt
device.
Click to expand...
I forgot to mention -- try it with just resistors, enough to draw the
same current at the same voltage (6 ohms at two watts). If it _still_
fizzes then you've bought a cheapo power supply.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
 
On Sat, 09 Feb 2008 16:22:10 +0000, Pete Verdon
<usenet@verdonet.organisation.unitedkingdom.invalid> wrote:

Hi,

I'm building a piece of demonstration equipment for schoolchildren, that
includes an electromagnet (representing the write head of a hard drive).
The electromagnet is home-made, a few hundred turns of varnished wire on
an old bolt. To power it, I've just obtained a 2-amp bench power supply
like this:

http://www.radiotronics.co.uk/item.php?partcode=P004C

The magnet works well, with the supply set to 3V. I've put a multimeter
in series with it, and it appears to be drawing about half an amp. It
didn't get noticeably hot.

However, when the magnet is connected, the supply makes an audible
"fizzing" or "hissing" noise. Is this normal? What is it likely to be,
and does it indicate a fault with the supply or that the supply and the
magnet are not compatible in some way?

Thanks,

Pete
Click to expand...
It's probably current-limiting. What's the coil resistance? Many power
supplies are unstable in current limit, probably worse with an
inductive load. Do you have access to an oscilloscope? The voltage
vaveform would be interesting.

John
 
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John Larkin wrote:
It's probably current-limiting. What's the coil resistance? Many power
supplies are unstable in current limit, probably worse with an
inductive load. Do you have access to an oscilloscope? The voltage
vaveform would be interesting.
Click to expand...
Reminds me of my physics lesson last Thursday. The teacher was
demonstrating what an oscilloscope does and hooked it up to one of the
cheap 'educational' power supplies abundant at our school. The waveform
out of the 'AC' connections was nearer a square wave than a sine.

- --
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
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Tim Wescott wrote:

I forgot to mention -- try it with just resistors, enough to draw the
same current at the same voltage (6 ohms at two watts). If it _still_
fizzes then you've bought a cheapo power supply.
Click to expand...
Oh, I've definitely bought a cheapo power supply :)

The fizzing isn't a problem provided it doesn't indicate imminent
self-destruction.

Pete
 
Pete Verdon wrote:

Tim Wescott wrote:

I forgot to mention -- try it with just resistors, enough to draw the
same current at the same voltage (6 ohms at two watts). If it
_still_ fizzes then you've bought a cheapo power supply.

Oh, I've definitely bought a cheapo power supply :)

The fizzing isn't a problem provided it doesn't indicate imminent
self-destruction.

Pete
I disagreee! If the fizzing sound is caused by the PSU self oscillating
Click to expand...
due to the inductive load, it could very well cause damage or self
destruct. Not to mention any potential damage that could be caused if
you connected voltage sensitive equipment while its doing that.
--
Best Regards:
Baron.
 
Baron wrote:
Pete Verdon wrote:

The fizzing isn't a problem provided it doesn't indicate imminent
self-destruction.

I disagreee! If the fizzing sound is caused by the PSU self oscillating
due to the inductive load, it could very well cause damage or self
destruct.
Click to expand...
Um. I don't mean to be rude, but that was kind of my point. I said that
it's a problem *if* it indicates damage or destruction.

Not to mention any potential damage that could be caused if
you connected voltage sensitive equipment while its doing that.
Click to expand...
I'll bear that in mind, but I don't think I own any voltage-sensitive
equipment :). My limited knowledge is very much of the "electrical"
rather than "electronic".

Pete
 
Pete Verdon wrote:

Baron wrote:
Pete Verdon wrote:

The fizzing isn't a problem provided it doesn't indicate imminent
self-destruction.

I disagreee! If the fizzing sound is caused by the PSU self
oscillating due to the inductive load, it could very well cause
damage or self destruct.

Um. I don't mean to be rude, but that was kind of my point. I said
that it's a problem *if* it indicates damage or destruction.
Click to expand...
Yes so you did.

Not to mention any potential damage that could be caused if
you connected voltage sensitive equipment while its doing that.

I'll bear that in mind, but I don't think I own any voltage-sensitive
equipment :). My limited knowledge is very much of the "electrical"
rather than "electronic".

Pete
Click to expand...
--
Best Regards:
Baron.
 
Tim Wescott wrote:

Try a six ohm (or so), 2 watt resistor in parallel with the coil. If
that stops the 'fizzing' then the supply is oscillating because of the
inductive load. If it works, then you can put a big capacitor (I don't
know what to recommend, but I'd start with 1000uF, observing polarity)
in series with the resistor, which can now be reduced to a 1/4 watt
device.
I forgot to mention -- try it with just resistors, enough to draw the
same current at the same voltage (6 ohms at two watts). If it _still_
fizzes then you've bought a cheapo power supply.
Click to expand...
Unfortunately I don't have anything like those values in my parts box.
However, I tried shorting it out briefly (a bench supply ought to cope
with the occasional misconnection, right?) with both plain wire and then
with a 100kO (I think) resistor. Neither fizzed, which I assume means
that the noise wasn't due to overload (as someone else suggested).

Hopefully given the name of the newsgroup my near-total lack of
electronic knowledge is not going to be too much of an embarassment :)

Pete
 
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