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need a simple electromagnet 100mA to pick up small things.

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Patrick

Sat Sept 01 2012, 05:22AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Hey i need some oppinons form others on this matter...

The need: i need to pick up 20gram or less objects, steel sheet is what there made of. with low current like 50-100mA.

my idea: im thinking of those big block auto and indutrial relays, for mains switching... if i take out a 120 or 12 volt coil i think everything is right there? right? and what would be better the 12VDC or the 120VAC coil?

any others care to advise me?

Forty

Sat Sept 01 2012, 05:32AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

A relay would be a good start. could also try a transformer with one side of the iron removed, or a solenoid with its external iron removed. For 50-100mA of current, winding your own would probably take forever.

Patrick

Sat Sept 01 2012, 06:16AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok Forty, look at this...

Are there any poisonous metals in relays? like cadmium?

a 120 volt and 12 volt coil based relay.

at 12 volts i barely picks up a screw... i may need to add a metal concentrater to the gap...

Sulaiman

Sat Sept 01 2012, 07:48AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

One thing to consider is releasing the object,
the residual magnetism in the electromagnet core (remanence)

must be low enough to allow the weight of the object to pull it off the electromagnet,
one technique that reduces remanence is to have a non-magnetic space between the electromagnet core and the object, air, plastic, brass, aluminium etc.
Also, try to use an electromagnet as small as possible to reduce the remanent magnetic field.

The small relay you showed above;
- use sandpaper or a file to flatten-off the two raised parts of the core,
allowing the sheet of metal to come into contact with a greater area of the core
- 20 gr. of steel sheet will be easier to pick up than a screw due to the area in contact with the magnetic core.

Dis-assemble a small mains transformer,
stack all of the 'E' laminations and replace the transformer bobbin/windings.
A 120 Vac primary winding should work well on 12 Vdc or less.
or
use a 'hacked' dc relay below its rated voltage (e.g. 24 Vdc relay using 5 Vdc)
for lower current as 20 gr. is a very light load..

Patrick

Sat Sept 01 2012, 08:15AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

it seems to letgo easy enough, its just too weak the reliably pick or hold things...

Dr. Slack

Sat Sept 01 2012, 11:16AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

... its just too weak the reliably pick or hold things...

At the risk of adding complexity, you could pulse it initially with a high current to make a strong field for pickup, then allow it to reduce once the object is closer.

For instance a boost charger run off 5v, charging an output 'lytic to 30v. When you switch the coil to it, it gets a 30v belt for the few 100mS that it takes to drain the cap, then it bottoms out running from the power supply and the boost diode. Just a thought.

Conundrum

Sat Sept 01 2012, 01:18PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

A fix for sticky magnet syndrom is to "degauss" the core with a descending AC pulse ie start of at 100% 10kHz and ramp down to 0% in about 5 seconds.

Also I've used bundles of ferrite cores from defunct UPS's before, works fine with 500 turns or so wound around it.
Select the appropriate wire gauge to get a sensible current at the chosen voltage and if possible make the coil in two sections which are then connected in parallel to provide some degree of redundancy.

-A

Forty

Sat Sept 01 2012, 03:39PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

you could even use a permanent magnet, moved towards or away from a non-magntic spacer by a small motor or solenoid. kinda like how the hand lifters for sheet metal work.

Patrick

Sat Sept 01 2012, 06:36PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Forty wrote ...

you could even use a permanent magnet, moved towards or away from a non-magntic spacer by a small motor or solenoid. kinda like how the hand lifters for sheet metal work.

im thinking of an RC micro servo, that moves a tiny super magnet away, which would let go.

radiotech

Tue Sept 04 2012, 06:25PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

If you are using AC, i.e. an AC relay coil, it will just buzz on contact with steel unless
it has a shading pole piece on the end. This appears as a heavy copper slug on one part of the
of the pole.

If you try DC on a 'slugged' relay, it will delay the release time.

That is how dial telephones knew how to let the number pulses through, and set themselves up
for the next digit, when the dial was released.

You said 0.05- 0.1 Amps. The magnetic force wil depend on amp-turns of the coil. For a fixed
voltage, this makes the distance between the electromagnet and the workpiece to be attracted,
dependent on the conductivity of the coil winding.

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