Dan Michaels wrote:
>
> Tony Nixon wrote:
>
> >
> >You're right, I only catered for about 470uF at present, but the PCB is
> >still 'open' for awhile yet. I figured what I learnt at electronics
> >school - 1000uF / amp (ball park).

Terrible advice me thinks.

> I/C = dv/dt --> 1A/1000uF = 1000v/sec
>       --> 1v/msec --> 20v/20msec, where 20msec = period of 50hz AC
>
> [Check my math - it's too early in the morning]. It says, if you suck
> 1A out of a 1000uF cap over the period of your AC input, then Vcap
> will drop 20v. Yes?

My heads a bit heavy from last night, but here goes..

12VAC 1A wall wart unloaded gives around 13.8VAC

The PicPocket draws 50mA quiescent and 100mA while programming.

Wall wart output with a 130mA load is around 13.6VAC

13.6VAC = 19.2VACPeak

1.2V drop through bridge = 18VAC peak

I need 13V + 2V min for 78xx regulation.

18 - 15 = 3V

Take a reasonable(?) margin and say 1V ripple allowed.

C = Iload / 2 X F X DV

C = .1 / 2 X 50 X 1  (60Hz for US even better)

C = 1000uF

Take a narrow margin and allow 2V ripple and I can use the 470uF.

Probably better with the 1000uF though, in case the ICSP circuit draws a
bit of current.

For about $2 more, could use LM2930-8 instead of 7808 for better ripple
margin.

--
Best regards

Tony

mICro's
http://www.picnpoke.com
mailto:sales@picnpoke.com

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