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Again with a power supply input protection

Started by George Herold May 27, 2016
On Tue, 31 May 2016 17:07:18 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>On Tue, 31 May 2016 16:42:12 -0700 (PDT), Phil Hobbs ><pcdhobbs@gmail.com> wrote: > >>+1 for Polyzens--they're magic. A 16V PolyZen with a 10V zener in series with the ground lead will do a great job. A series Schottky from there to the load will help protect againdt supply reversal and input shorts. >> >>Cheers >> >>Phil Hobbs > >We use polyfuses with gigantic (SMB package) unipolar transzorb >zeners. Seems to work.
SMB isn't gigantic. I have a DO218 (or equivalent) TVS diode on almost every board (not on daughterboards that get power from the motherboard).
> >The surface-mount polyfuses are pretty bad, so we generally use the >radials.
Surface mount fuses aren't much better, though I use them for prototypes.
On Tuesday, May 31, 2016 at 7:42:16 PM UTC-4, Phil Hobbs wrote:
> +1 for Polyzens--they're magic. A 16V PolyZen with a 10V zener in series with the ground lead will do a great job. A series Schottky from there to the load will help protect againdt supply reversal and input shorts. > > Cheers > > Phil Hobbs
Huh, OK split load. I'll have to think more... Testing the last poly fuse thing, I found that somewhere in V-I space, the poly/zener has to operate as a (crapy) voltage regulator... And happily waste whatever power you ask. It'd be nice if poly fuses had a sharper knee... Oh.. polyzens... I haven't tried those. Do they make 24- 30 Volt ones? George H.
>Testing the last poly fuse thing, I found that
somewhere in V-I space, the poly/zener has to
>operate as a (crapy) voltage regulator...
And happily waste whatever power you ask.
>It'd be nice if poly fuses had a sharper knee... >Oh.. polyzens... I haven't tried those.
Do they make 24- 30 Volt ones? They've been widening the range, but last time I checked they only went up to 16V or so (hence the auxiliary zener). Still, way over half of the zener dissipation is in the Polyzen, so it really helps sharpen up the knee. Cheers Phil Hobbs &nbsp;