Re: [N8VEM-S100:2417] 6502 CPU Board V1 components question

[Fabio Battaglia <hkzl...@gmail.com>]

On 17/02/14 19:31, David Riley wrote:
> On Feb 16, 2014, at 11:56 AM, Fabio Battaglia <hkzl...@gmail.com> wrote:
>
> 5% should be fine; 5% was actually the "expensive" grade of resistor
> when these parts were new. 1K, more or less, is generally just a
> nominal value unless it's for a termination network (e.g. for SCSI),
> in which case you usually want the given value with a 5% tolerance to
> match the characteristic impedance of the transfer medium. The
> recommended value in the datasheet should probably be fine; you don't
> want an output cap that's too big on a 78xx part, because when you cut
> off the power supply, you could end up reverse-biasing the regulator
> if there's a lot of stored energy in the cap, and my recollection is
> that the 78xx regulators don't have much in the way of protection. You
> can always drop a diode from output to input to protect it, though
> that's obviously not ideal if the PCB doesn't already allow for that.
> - Dave 
Yep, I've seen 7805 being destroyed by reverse voltage caused by a big
cap discharging. It shouldn't be an issue on this case though: I don't
have the datasheet handy, but I remember that the reccomended value
wasn't that much distant from 1.0uF (probably less). 1.2uF just struck
me as a weird value though.

While the diode is probably not needed, It might not be a bad idea
either if no one advices against: It's not hard to solder on the back of
the board and I have plenty of good-enough silicon diodes...