[sdiy] really inaccurate zeners

[mskala at ansuz.sooke.bc.ca]

On Tue, 30 Jun 2020, Tom Wiltshire wrote:
> I saw a conversation recently on DIYStompboxes about the zener regulation
> inside an old phaser pedal. It was similarly producing a voltage much lower
> than expected. It was pointed out that the circuit was running it at around
> 0.5mA, and the zener was spec’d at 10mA.
> Could this affect your circuit too? By my reckoning, your test with 15V /
> 3K3 is only running a bit over 3mA through the zener. How much current does
> it need to meet the spec?

That's be my thought as well.  Common Zener diodes havea soft knee in
their voltage/current curves; they're rated for a voltage at some
specified current which is often pretty high, and in effect they have a
lower Zener voltage if you run them at lower current.  I had to do a fair
bit of experimentation to choose the right diodes for my Fixed Sine Bank
design in particular because it's using Zeners "off-label" as it were,
inside op amp feedback loops at a much lower current level than the data
sheet nominal value.

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before tribes.
https://ansuz.sooke.bc.ca/