The zener diode is a transient voltage suppressor, and the 'resistor' is
a PTC fuse for current limiting. If the input voltage exceeds 30v, the
TVS diode shunts it to ground, and the PTC fuse limits the current to
about 200 mA.
I've cranked the input up to about 60v before and something smoked, but
it kept working. The TVS failed partially short though, so after that
it drew a lot of current.
On 1/6/2012 11:07 AM, Edward wrote:
> I see that the OT2 uses a 7805 voltage regulator. I also see that a
> Zener diode and resistor are used in front of the 7805. It is my
> understanding that the Zener diode and resistor combination is often
> used to create a simple voltage regulator.
> Question: Why is a Zener diode and resistor used along with the 7805?
> What benefit does it have over using the 7805 by itself?