# help - electromagnet relay design

C

#### chris1617

Jan 1, 1970
0
I am trying to build a simple door/window alarm. I have built a rela
with an electromagnet (4 inch nail wrapped witcopper wire) and teste
the alarm system using a 12V - 7 amp battery but the electromagne
switch became too hot. Then I tried an AC DC adaptor I found in m
house with only 500 mA. This iwas not sufficient make a stron
electromagnet. How many amps do I need to make the magnet powerfu
enough without becoming too hot? I am using a car alarm siren. I wan
to use an AC DC adaptor as the power source

I

#### Ignoramus24343

Jan 1, 1970
0
12v relays are cheap as dirt. I see no reason to build one. I bought
50 4pdt relays for 60 cents apiece. try www.ebay.com.

i

J

#### John Popelish

Jan 1, 1970
0
chris1617 said:
I am trying to build a simple door/window alarm. I have built a relay
with an electromagnet (4 inch nail wrapped witcopper wire) and tested
the alarm system using a 12V - 7 amp battery but the electromagnet
switch became too hot. Then I tried an AC DC adaptor I found in my
house with only 500 mA. This iwas not sufficient make a strong
electromagnet. How many amps do I need to make the magnet powerful
enough without becoming too hot? I am using a car alarm siren. I want
to use an AC DC adaptor as the power source.
The magnet current is set by the applied voltage and the resistance of
the coil. The strength of the magnet is proportional to the total
amperes circling the nail (amperes through the wire times the number
of turns of wire). Using finer gauge wire will raise the resistance,
lowering the amperes drawn from a given voltage, but the extra turns
will somewhat compensate the force for the lower current per turn.

The proportionality factor between ampere turns and force is related
to the core structure of the relay. The force is proportional to the
rate of change of total flux per increment of armature movement. So
the more the total flux varies between released and pulled in
positions of the armature, the more force each ampere turn will apply
to this movement. In order to get a large change in total flux as the
armature moves, you have to provide a flux path that has only one air
gap in it. And that is the one between the armature and the pole that
attracts it. If there are other air gaps in series with the magnetic
flux as it loops out of one magnetic pole, through the armature and
back through the other pole, then the armature movement closing one
air gap will not have much total effect on the total reluctance
(magnetic field resistance) of the whole flux circuit, so will not
cause much of a change in the total amount of flux, so will not
produce much force. This is why commercial relays have an iron
structure that guides the flux all the way round this magnetic
circuit, through the hinge on the armature.

M

#### mc

Jan 1, 1970
0
Try a tremendously larger number of turns around the nail (and for that
matter, a nail is not an ideal core). You probably have something like 20
turns of wire. Try 2000. You can get magnet wire which is insulated with a
thin layer of enamel rather than with a plastic sheath. For that matter,

P

#### Paul Hovnanian P.E.

Jan 1, 1970
0
mc said:
Try a tremendously larger number of turns around the nail (and for that
matter, a nail is not an ideal core). You probably have something like 20
turns of wire. Try 2000. You can get magnet wire which is insulated with a
thin layer of enamel rather than with a plastic sheath. For that matter,

P

#### Pooh Bear

Jan 1, 1970
0
chris1617 said:
How many amps do I need to make the magnet powerful
enough without becoming too hot?

How long is a piece of string ?

Graham

I

#### Ian Stirling

Jan 1, 1970
0
John Popelish said:
The magnet current is set by the applied voltage and the resistance of
the coil. The strength of the magnet is proportional to the total
amperes circling the nail (amperes through the wire times the number

Also - for a given packing fraction (round wire) and a given material -
say copper, the power needed to generate the same magnetic field is
identical, whether you've got 10 turns, or 10000.

M

#### mc

Jan 1, 1970
0
The magnet current is set by the applied voltage and the resistance of
Also - for a given packing fraction (round wire) and a given material -
say copper, the power needed to generate the same magnetic field is
identical, whether you've got 10 turns, or 10000.

But the volt/amp ratio is what needs changing here... he wants to use more
volts (of the voltage he is applying) and fewer amps.

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