[sdiy] Zener clipping Was : ...that 1K resistor on the output

[David G. Dixon]

> Back to back zeners shunt the voltage through themselves and the series
> resistance... so no current can flow into the power supply.
> 
> Downside is, the range of voltages is limited by available zeners, and
> their tolerance.  If you were trying to clamp to (lets say)
> +/- 5V, you'd need a 4.3V zenrer (plue the .7 volts that the other zener
> contributes). With 5% or 10% tolerances, you may not get
> accurate enough results. If they are too high, you pass an overvoltage,
> too low and you conduct some current in normal cases.


I learned a little-known trick the other day which basically solves the
problem of mismatched back-to-back zeners: simply put a single zener across
a diode bridge rectifier and that one zener will act as both zeners in a
back-to-back pair, but with perfect matching.  Of course, there may be
issues with matching of the two diode pairs, but in my experience these are
usually very close.