[sdiy] really inaccurate zeners

David G Dixon dixon at mail.ubc.ca
Wed Jul 1 03:30:53 CEST 2020

Sorry, I mistyped.  I meant 1N4732 (4.7V), not 1N4372 (which is a 3.0V
zeners -- what genius picked those numbers, anyway?  It's as if people
intentionally do things with electronic parts to increase confusion, like
putting the power pins on the wrong sides of quad opamps so they need to be
oriented backwards relative to every other IC).  The zeners I measured today
are 1N4730A (3.9V), 1N4731A (4.3V), and 1N4732A (4.7V).

So, who ever runs a zener diode with 20mA of current?  You could heat your
home by using your synth as a fireplace.

Also, there was a 0.5V difference between zeners from the same denomination.
The datasheet suggests +/-5% tolerance for the A denominations, and I'm
seeing much looser tolerances than that.

I'm going to go out and measure the voltages vs current by using a bunch of
different resistors, draw a curve, and report back with my findings.  (I'm
predicting that the voltage won't change that much as long as the current is
higher than about 1 mA.)


-----Original Message-----
From: Neil Johnson [mailto:neil.johnson71 at gmail.com] 
Sent: Tuesday, June 30, 2020 2:23 PM
To: David G Dixon
Cc: Synth-diy at synth-diy.org
Subject: Re: [sdiy] really inaccurate zeners

Hi David,

> The amplitude control is achieved in each core with a single zener diode
inside a "zener bridge" consisting of four 1N4148s.  Since the 4148s drop
approximately 0.5V, a 3.9V zener (1N4370) should give a total drop of
roughly 5V, which should guarantee waveforms which are 10Vpp.

Umm, I thought the 1N4370 was a 2.4V device?  All the datasheets I can find
confirm this.

> One was about right, but the other was way low.  I replaced the zener 
> of the low one, and it got worse.  So then I decided to actually 
> measure some zeners.  After much grumbling, the bottom line is that I 
> had to delve into my bag of 1N4372s (4.7V) before I found two zeners 
> that read almost exactly 3.9V.  They were all very low relative to 
> their nominal values.  (I tested them by putting a 3.3k resistor 
> between +15V and the cathode, and connecting the anode to ground.  I 
> use 3.3k resistors on the comparator in my VCO core.)

Umm, and 1N4372 are supposed to be 3.0V.

You may want to recheck the part numbers?

> So now I have decided that I have to measure each and every zener diode,
and separate them into plastic bags based on their actual voltages rather
than their denominations.  In this way, after a certain amount of tedium, I
will have a collection of precision zener diodes (within +/- 0.05V).
However, I shouldn't have to do this.  I would expect zeners to be off by
0.2 (or, at most, 0.3V) from their nominal values, but to cross over one or
two (or even three) denominations?  That is unacceptable.  What are these
zener diode manufacturers doing wrong?

Well, zeners - like all electronic components - have tolerances.  In your
case, assuming you have 1N4372 diodes, without any suffix they have a 10%
tolerance.  You can get tighter tolerance parts but you pay more for them
(they sort them on the production line).

Also, as Tom pointed out, the advertised voltage is at a stated current (Izt
- Zener test current). For the 1N437x series this is 20mA.  At lower test
currents you will measure a lower reverse voltage.  For example, if you dig
out the Motorola datasheet on these


on page 6 you can see how much the zener voltage varies with current
(answer: quite a lot!)


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