Re: Capacitor size



Jan Nielsen wrote:

Hi,

I have a simple circuit with a PICAXE that controls a relay board, when
the relay board is empty it works fine.
But when I put a simple 20W lamp on one of the relays (230Vac) the chip
resets very often when that relay starts/stops.

I have a 0.1uf on the power legs of the IC, how do I know what other
size is needed to prevent it from resetting.
I didnt make the relay board myself, but from what I can see there is no
caps on the board.

Relays produce EMI (electro-magnetic interference) particularly when switching
off the load.

This EMI can 'couple' into your circuitry and cause random resets and the like.
Changing the value of the supply decoupling capacitor is fairly unlikely to do
much to affect it. Long pcb traces that form large 'loops' are one of the worst
offenders for EMI pickup btw.

How to design for reasonable immunity to EMI is a huge subject in its own right.
You may however get some inprovement by lowering the impedance of the reset
circuit.

Graham

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