I'm following Birk Peterson's published plans (Machinist's Workshop
April/May 2003) to build a magnetizer/demagnetizer using the
coil from a surplus solenoid. On first pass it worked real well but after
two or three 2 second cycles it burned up the coil. The coil came from a
big Dormeyer intermittent duty relay. If I use a coil from a continuous duty
relay will it survive longer?

Any advice?

It's a 115 Vdc circuit with a bridge rectifier to energize dc to mag and A/C
to demag.

Thanks in advance.

Rick

Rick Legge wrote:
I'm following Birk Peterson's published plans (Machinist's Workshop
April/May 2003) to build a magnetizer/demagnetizer using the
coil from a surplus solenoid. On first pass it worked real well but after
two or three 2 second cycles it burned up the coil. The coil came from a
big Dormeyer intermittent duty relay. If I use a coil from a continuous duty
relay will it survive longer?

Any advice?

It still may burn out. The coil winding is
sized for use in a closed magnetic circuit.
Open the circuit to put your work object in
and the power dissipated in the coil goes
way up.

Short of a rigorous engineering study, give
it a try. Keep the duty cycle low and use
your nose. I've heard of people giving the
coil a shot of freeze-mist to cool it down.

On Tue, 06 Jan 2004 23:25:07 GMT, "Rick Legge"
wrote:

On first pass it worked real well but after
two or three 2 second cycles it burned up the coil.

Magnetising ?

The coil came from a
It's a 115 Vdc circuit with a bridge rectifier to energize dc to mag

Doh ! No wonder it failed. Try to find a design from someone who has
something resembling a clue.


You don't need a constant field to make a magnetiser, you need a
unidirectional field. So long as it retains the same polarity, a
pulsed field is OK.

A solenoid winding (relay, transformer or motor) is a reactive load,
which means that it has a higher impedance at AC than at DC. For AC,
even at 50Hz, this impedance limits the current. Run it on DC though
and it's the same as sticking a fairly short length of copper wire
straight across the supply - no wonder it burned out !

Most decent magnetisers are pulsed, using a capacitor bank. Real ones
also monitor the winding temperature, or else limit the duty cycle.

--
Smert' spamionam

I built a demagnetizer by making a coil of some (16ga?) magnet wire that
I had laying around. Its resistance is way too low to connect to 110
directly, so I connect it in series with a high-wattage device. E.g., a
blow dryer. Works fine. Bob

Bob Engelhardt wrote:
I built a demagnetizer by making a coil of some (16ga?) magnet wire that
I had laying around. Its resistance is way too low to connect to 110
directly, so I connect it in series with a high-wattage device. E.g., a
blow dryer. Works fine. Bob

To demagnetize I use an Weller Electrical soldering gun:
Insert screw driver or whatever in the loop and pull the
trigger. Withdraw slowly and move it away to arms length.
Let go of trigger. Done. HTH.
--
SATOR AREPO TENET OPERA ROTAS
Have 5 nice days! John
******************************
--- ILN 000.000.001 ---

John wrote:

Bob Engelhardt wrote:

I built a demagnetizer by making a coil of some (16ga?) magnet wire that
I had laying around. Its resistance is way too low to connect to 110
directly, so I connect it in series with a high-wattage device. E.g., a
blow dryer. Works fine. Bob


To demagnetize I use an Weller Electrical soldering gun:
Insert screw driver or whatever in the loop and pull the
trigger. Withdraw slowly and move it away to arms length.
Let go of trigger. Done. HTH.
IIRC, the loop current is around 100 amps. So it is a 100A-turn winding when
the tip is good.

I have wound custom windings this way on old scrap units.

Martin

--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder

"Martin H. Eastburn" wrote in message
m...
John wrote:

Bob Engelhardt wrote:

I built a demagnetizer by making a coil of some (16ga?) magnet wire that
I had laying around. Its resistance is way too low to connect to 110
directly, so I connect it in series with a high-wattage device. E.g., a
blow dryer. Works fine. Bob


To demagnetize I use an Weller Electrical soldering gun:
Insert screw driver or whatever in the loop and pull the
trigger. Withdraw slowly and move it away to arms length.
Let go of trigger. Done. HTH.
IIRC, the loop current is around 100 amps. So it is a 100A-turn winding
when
the tip is good.

I have wound custom windings this way on old scrap units.

Martin

--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder
----------------------
I build a simple demagnetizer by using a discarded 1/3 hp motor field coil
using the 220 connection on 120 volts AC and a heavy duty push button Simply
hold the tool in the field, push the button and withdraw the tool.
Don Warner
----------------

A common power supply transformer of about 75-100 VA with a 120V primary and
an EI core is easy to convert to a demag. Simply hacksaw(don't cut the
windings) off one side of the core to make it a E core. Clear the ends of
the secondary winding(s) and tape them up.

Mount in a open top box with the new core poles up and cover them flush with
a thin plastic or other non-conductive cover. Add a line cord and a
momentary PB to feed the primary and there you have it. Just don't hold
down the PB more than about 15 or 20 sec. I find that 2 or 3 sec is all I
need.

Randy

"Bob Engelhardt" wrote in message
...
I built a demagnetizer by making a coil of some (16ga?) magnet wire that
I had laying around. Its resistance is way too low to connect to 110
directly, so I connect it in series with a high-wattage device. E.g., a
blow dryer. Works fine. Bob

On Wed, 07 Jan 2004 18:53:19 GMT, "Randal O'Brian"
wrote:

A common power supply transformer of about 75-100 VA with a 120V primary and
an EI core is easy to convert to a demag. Simply hacksaw(don't cut the
windings) off one side of the core to make it a E core. Clear the ends of
the secondary winding(s) and tape them up.

Mount in a open top box with the new core poles up and cover them flush with
a thin plastic or other non-conductive cover. Add a line cord and a
momentary PB to feed the primary and there you have it. Just don't hold
down the PB more than about 15 or 20 sec. I find that 2 or 3 sec is all I
need.

Randy




This works fine and one of the nice things about it is that the
basic design of power transformers is such that almost any transformer
large or small, with one ironside removed, generates enough flux for
effective demagnetisation. Smaller transformers just demagnetise a
smaller area and so may need more than one pass. Given a choice choose
a transformer with a centre iron core size wider than your largest
workpiece.

For full demagnetisation it is not sufficient to just put the
workpiece on the pole area and flip the power on and off. This still
leaves residual magnetism - the amount is dependent on the precise
time in the powercycle that power was removed.

The right technique is to first put the workpiece on the pole
area. Then switch the power on and slowly (about 1 second) pull the
workpiece along until entirely clear of the pole area. Once clear,
switch the power off - transformers modified this way overheat
rapidly.


The above system is pretty good for roughly flat workpieces but
less convenient for round items - milling cutters or large drills. The
trick here is to recycle a junk colour television. These have two
large demagnetising coils wrapped around the large diameter flare of
the cathode ray tube. Every time a colour TV set is switched on, these
coils are energised with a current that decays over a few seconds to
demagnetise the colour tube.

Remove these coils and do the bandsaw flip trick on each of
them so that the large single loop formed by each coil becomes a
triple loop. Tape the two triple loops together to form a single
sextuple loop about 3" bore. Connect the two windings in series -
finish of winding 1 to start of winding 3.

With this device, apply AC power, pass the workpiece through
the coil (about 1 second), remove power.

With care this can also be used a magnetiser. Feed the coil
from a 10a bridge rectifier and flip it "on" for not more than 5
seconds (a tenth of a second is enough!) by a switch ON THE AC SIDE of
the rectifier.

For simple jobs like a screwdriver, one flip with the
business end well inside the coil is all that is needed.

To magnetise proper permanent magnet materials a closed iron
circuit is needed. This means that the magnet plus any assorted chunks
of mild steel must be arranged to form a continuous path in iron to
allow the flux to flow in a roughly (can be VERY roughly) circular
path encircling one leg of the magnetising coil.

This is a mickey mouse magnetiser - it will do a
reasonable job on small magnets but is not a substitute for the large
capacitor discharge professional machines or jury rigged fuse blowers.
The magnetising force can be roughly doubled by connecting the two
coils in parallel but you'd then need to be pretty nifty with the
power switch!

Jim