electromagnet wiring

[ratcateme]

I am trying to make a electromagnet.
i want to run it off a separate battery and i don't need to be able to control the strength of it all i want to do is turn it off and on.
but how do i do that with it running off a different battery than my Micro Controller?
and how do i wire it so it wont drain my batter really fast.
it will properly need to be on for about 20-30 seconds then it will be off for a while.
how should i connect this and what do i need to make it go.
the battery i am thinking for using is 1400mah and 7.2V.

Scott.

[airman00]

put a resistor load connected to the electromagnet.

[ratcateme]

that's what i was thinking but ho many ohms should it be?
(this is my first ever robot and i am really noob at electronics)

Scott.

[ArcMan]

I am trying to make a electromagnet.
i want to run it off a separate battery and i don't need to be able to control the strength of it all i want to do is turn it off and on.
but how do i do that with it running off a different battery than my Micro Controller?

Scott.

I would suggest using a MOSFET to drive the electromagnet with a separate battery source.  Google something like "MOSFET switching circuit" to see how.

[airman00]

that's what i was thinking but ho many ohms should it be?

how much ohms is the electromagnet coil?

[ratcateme]

i don't know yet i haven't built it but it is going to be a budget homemade one with some wire wrapped around a piece of iron

Scott.

[airman00]

i don't know yet i haven't built it but it is going to be a budget homemade one with some wire wrapped around a piece of iron

Scott.

take a multimeter and measure the resistance

[AndrewM]

How big (physical size) of an electromagnet are you planning on making?  And how much force do you need it to have?

[ratcateme]

i just need a really small one i want to pick up 4g coins.(New Zealand currency is magnetic)
i am still looking around for a piece of iron
when i was reading something (i can no longer find) i think i need to make sure it is pure iron otherwise it will get magnetized when i turn off the power so it wont drop the coin.

Scott.

[AndrewM]

Actually you need to make sure it is NOT pure iron.   Carbon steel (like 1006, 1008, 1010) will usually do the trick.

Considering you are planning on having the magnet on for 20-30 seconds, you will need to pay major attention to the current you pass through it, inductive coil wire (ie magnetic wire) usually has two ratings, the operating current and the fusing current.  Operating current is what level of heat it can dissipate continuously on its own, fusing current is the current that immediately starts melting the coating and wire together.  For 20-30 seconds you are looking at something around 10-20% of the fusing current.  All of this is dependent on the type of sealant used (nylon, acrylic, etc), how tightly wrapped the coil is (so heat can dissipate), etc.

Here's some general numbers for the more common wire sizes (available at radio shack):

22awg - op cur: 0.65a  fuse: 41a
26awg - op cur: 0.25a  fuse: 20a
30awg - op cur: 0.1a    fuse: 10a

[paulstreats]

You could control it with a standard h bridge driver ic. It will allow you to use an external power source as long as the grounds are common.

 This would give you more control such as controlling the strength which means that you can calibrate it to only work at the strength required and conserve power

[Admin]

You can use a simple motor driver to control it, or just a single MOSFET with the gait hooked up to a micro.

and how do i wire it so it wont drain my batter really fast.

You can add a resistor in series, but that will make the electromagnet weaker.

Electromagnetic strength is proportional to both voltage and current. Raise either, and you get a better magnet.

I recommend using wire taken out of an old motor, as its optimal for electromagnetics.

You can also expect your electromagnet to heat up fast. I find electromagnets to be fairly weak, too.