Jim Tellier wrote:

>David Duffy wrote:
>
>
>>About the only thing I can think of right now is that the layout
>>of the PSU (0V line in particular) is causing the problem. If
>>you're drawing the solenoid current via the same 0V wire as
>>the LM317T is getting its reference from you'll have problems.
>>Does the -ve of the solenoid go back to the 4700u capacitor
>>negative directly? Also, 2500u on the 5V line looks excessive.
>>
>>
>
>Well, electrically, the -ve of the solenoid must connect to GND, but you're
>suspecting that the 'path' to ground is perhaps too resistive? Thus creating
>a voltage drop?  I'll check that out.
>

Yes, the path of the 0V line is what I was getting at. If the return path
for the solenoid goes via the same wire to the filter cap as the LM317
you'll have problems. Take the 0V line of the 5V section and the 0V
line of the solenoid back to the main filter cap seperately. View the
following ASCII art with a monospaced font: 0 = main filter cap -ve,
S = solenoid -ve, R = 5V regulator 0V line.


Bad wiring:

0             S            R
----------'---------'

Also bad:

0             R            S
----------'---------'


Good wiring:

0             S            R
|---------'              |
'-------------------'

>Re: the 2500uF at 5V node.. I've always wondered about "correct" techniques
>for choosing the size of filter caps in situations like this. It seems like
>more 'seat of the pants' methods are often used, rather than calculations,
>as far as I can tell.  Anyone aware of a deterministic, engineering approach
>to this?   I understand the applicability of the RC time constant to the
>goal of smoothing instantaneous voltage fluctuations due to current spikes,
>but what about relatively stable loads?
>

About 1000uF - 2000uF per amp of current is in the ballpark for
filter caps. Putting a huge cap on the output of some regs can make
them unstable I've heard. If you're only powering a low current PIC
circuit with the 5V, a few hundred uF and a 100n should be right.
David...

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David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: www.audiovisualdevices.com.au
___________________________________________

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