Re: trying to make an electromagnet

From: andy (news4_at_earthsong.free-online.co.uk)
Date: 07/19/04 

Date: Tue, 20 Jul 2004 00:40:57 +0100

On Tue, 13 Jul 2004 14:25:56 -0500, John Fields wrote:

> On Sun, 11 Jul 2004 20:41:52 +0100, andy
> <news4@earthsong.free-online.co.uk> wrote:

> The fusing current of #26 is about 20 amps and it has a resistance of
> about 41 ohms per thousand feet at 20°C, so assuming that it can
> survive a short 10 amp pulse out of your 12V supply every once in a
> long while means that the shortest piece of wire you can wind on the
> core will be about 30 feet.

I have built a circuit which sends a short (1 sec) pulse through the
magnet whenever an LDR goes from light to dark (i.e. to water at the end
of every day). At the moment, the current is around 800 mA, which is
working OK with a weak speaker magnet, but not with the strong neodymium
magnet i want to use for the finished system.

What i am planning to do is to split the coil i have into 4 parts of about
2 ohms each, and put these straight across 12V to get 6A through each part
of the coil. The two things i'm not sure about here are:

- can you draw this much current (24A total) from a low capacity (4 Ah)
lead acid lamp battery without hurting it?

- how to actually build this circuit - would it work with standard
breadboard plus some extra solder heaped along the tracks carrying the
high current, or would i be better using wires between the transistors and
the coil? 

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