[sdiy] really inaccurate zeners

Guy McCusker guy.mccusker at gmail.com
Tue Jun 30 23:13:27 CEST 2020


I took a quick look at the datasheet for the Vishay 1N4732 and if i read it
right it seems the dynamic resistance around 1mA current is 500 Ohm so
quite a bit of voltage variation with current is expected when current is
low.

The nominal voltage is achieved with current of 53mA.

So I don’t think the devices David has are out of spec - it’s just that the
spec isn’t what he wants it to be.

On Tue, 30 Jun 2020 at 21:59, <mskala at ansuz.sooke.bc.ca> wrote:

> 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/
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