On Wed, 25 Jan 2006 12:57:30 -0500, andrew kelley wrote:

> Hello all,
> 
> I have a solid state relay controlled by a 'c508 which is powered from
> a simple power supply (transformer, rectifier, regulator with 470uF
> cap on input and 10uF on output).  We have a microwave which when you
> open or close the door generates +/- 0-8v spikes on the power *to the
> micro*..  We think that these power glitches are causing RAM data to
> change and the PC to change and alter the execution of further code. 
> Any suggestions on filtering these spikes?

Glitches like that will certainly cause random malfunctions.  Are those the *only* capacitors involved?  At 
that size they are good as reservoirs, but no good at spike filtering or decoupling.  For a start, put a 0.1uF 
ceramic on each side of the regulator, and if there's more than a short distance from the regulator to the 
PIC,  another across the power connections close to the PIC itself.  

And as an aside, with significant capacitance on the output side, it's a good idea to put a reversed diode 
from the output to the input of the regulator - 78-series regulators (actually you don't mention what you're 
using) don't like to have a higher voltage on their output than input, and the diode will stop this from 
happening after you power-down.  This won't affect the spike problem you have but it might improve the life 
expectancy of the regulator!  :-)

Cheers,


Howard Winter
St.Albans, England


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