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

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.

--
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 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.

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

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.

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.

Oh, I've definitely bought a cheapo power supply

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

Pete

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.

> 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

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.

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