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

[Harry Bissell]

If the zener is much higher than the diode drops, the error will be small. If not you really
need to match the diodes. (the percentage change of the diode drops is more significant at
lower voltages)

Old designs for precision clippers used the CA3019 monolithic diode array... kind of
unobtanium these days. Using the CA3046 as diodes could work as well, if you cared that
much.

In most cases the diode bridge is a good idea for symmetrical clamping.

If I really care, I use active (opamp) based clamp circuits and a precision reference. It can be
troublesome to make this work at high frequencies...

H^) harry


----- Original Message -----
From: David G. Dixon <dixon at interchange.ubc.ca>
To: 'Harry Bissell' <harrybissell at wowway.com>, 'Mark Rivera' <marr at lumin.us>
Cc: 'SDIY List' <synth-diy at dropmix.xs4all.nl>
Sent: Fri, 28 Jan 2011 13:59:48 -0500 (EST)
Subject: RE: [sdiy] Zener clipping Was : ...that 1K resistor on the output

> 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.


-- 
Harry Bissell & Nora Abdullah 4eva