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'Driving 24V relay with 5V'
2004\10\06@033824 by Russell McMahon

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> Anyhoo, attached gif shows an energising circuit for a 24V relay
> so it can pull in with a 12V supply, which will hold it. Still tinkering
> at this stage, and one or two components (eg BC557) might get
> changed.

Looks pretty good to me.
If you are worried about the -0.6v across the capacitor you could put a
diode in the BC548 collector to raise the capacitor end by a diode drop.

I'd use BC327, BC337 as of right because they are about as capable as
anything in that class at the price, and they are often cheaper than the
wimpier alternatives (probably because everyone buys them). They are even
reasonably cheap from RS/Farnell if you get the right version.

You could add a custom sized resistor in series with the coil to further
reduce hold-in current.



       RM




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