# Again with a power supply input protection

Started by May 27, 2016
```A circuit will have 24V @ 1.5A (max) input from a wall wart.
I'd like to design in reverse polarity and maybe over voltage
I don't like the Zener/ polyfuse idea, cause there is going to
be a lot of power in the Zener... I'm afraid it will smoke before the
polyfuse lets go.  So some FET circuit.
Here's Jim T's offering. (from a past thread.)
http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf

The gate source voltage on most of the P-fets I've looked at is
+/- 20V.  So the 24V is a bit of a concern.  I can just put in some resistors
to make a voltage divider.. so not a big deal.

But do I need two P-fets? (Oh snap, never mind.. I had to draw the circuit.)

Well I'll make it a question anyway... are then any P-fets that can handle more
G-S voltage... (well more in absolute value.)

TIA
George H.
```
```Den fredag den 27. maj 2016 kl. 18.16.55 UTC+2 skrev George Herold:
> A circuit will have 24V @ 1.5A (max) input from a wall wart.
> I'd like to design in reverse polarity and maybe over voltage
> I don't like the Zener/ polyfuse idea, cause there is going to
> be a lot of power in the Zener... I'm afraid it will smoke before the
> polyfuse lets go.  So some FET circuit.
> Here's Jim T's offering. (from a past thread.)
> http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf
>
> The gate source voltage on most of the P-fets I've looked at is
> +/- 20V.  So the 24V is a bit of a concern.  I can just put in some resistors
> to make a voltage divider.. so not a big deal.
>
> But do I need two P-fets? (Oh snap, never mind.. I had to draw the circuit.)
>
> Well I'll make it a question anyway... are then any P-fets that can handle more
> G-S voltage... (well more in absolute value.)
>
> TIA
> George H.

just add a ~12V zener to gate

-Lasse
```
```On 27/05/2016 17:16, George Herold wrote:
>
> But do I need two P-fets? (Oh snap, never mind.. I had to draw the circuit.)
>

Yes, as you said. Two needed, one for reverse polarity and the other for
over-volts but they need not be p-channel. If your input supply is
floating using n-fets will save money per ohm rdson.

piglet

```
```On Fri, 27 May 2016 09:16:50 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>A circuit will have 24V @ 1.5A (max) input from a wall wart.
>I'd like to design in reverse polarity and maybe over voltage
>I don't like the Zener/ polyfuse idea, cause there is going to
>be a lot of power in the Zener... I'm afraid it will smoke before the
>polyfuse lets go.  So some FET circuit.
>Here's Jim T's offering. (from a past thread.)
>http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf
>
>The gate source voltage on most of the P-fets I've looked at is
>+/- 20V.  So the 24V is a bit of a concern.  I can just put in some resistors
>to make a voltage divider.. so not a big deal.
>
>But do I need two P-fets? (Oh snap, never mind.. I had to draw the circuit.)
>
>Well I'll make it a question anyway... are then any P-fets that can handle more
>G-S voltage... (well more in absolute value.)
>
>TIA
>George H.

Polyfuse+transzorb+cap works fine. A bunch of semiconductors and
passives (9 in Jim's case) will have hazards of their own. Like ESD
damage, for example.

Most fets blow out around 75 volts Vgs. There are lots of fets that
have internal g-s ESD zeners, typically +-40 volts turf, and they can
be protected with just a series current-limit resistor.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On 28/05/2016 02:16, George Herold wrote:
> A circuit will have 24V @ 1.5A (max) input from a wall wart.
> I'd like to design in reverse polarity and maybe over voltage
> I don't like the Zener/ polyfuse idea, cause there is going to
> be a lot of power in the Zener... I'm afraid it will smoke before the
> polyfuse lets go.

It might be worth considering the Polyzen and any equivalents. It is a
series polyfuse followed by a shunt Zener, the trick being that the
Zener is thermally coupled to the polyfuse so that when the Zener starts
cooking, it heats the polyfuse which makes it go high-impedance even if
it was not conducting anywhere near its trip current. In that way the
Zener is thermally protected. Availability might be a problem.

```
```On Fri, 27 May 2016 09:16:50 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>A circuit will have 24V @ 1.5A (max) input from a wall wart.
>I'd like to design in reverse polarity and maybe over voltage
>I don't like the Zener/ polyfuse idea, cause there is going to
>be a lot of power in the Zener... I'm afraid it will smoke before the
>polyfuse lets go.  So some FET circuit.
>Here's Jim T's offering. (from a past thread.)
>http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf

24 V is quite high voltage so a series diode 0.6 V voltage drop
doesn't too much harm. The diode solves the reverse polarity problem.
Select a diode with a large reverse voltage rating, say 1 kV.

If the source is floating, use a bridge rectifier, so the device works
regardless of polarity and even works on AC as a bonus.

For large positive peaks, use some series inductance to limit the peak
current, helping any parallel protection component to do their job.

However, if you expect constant over voltages, then some fusing
component in series would be required.

```
```On Saturday, 28 May 2016 06:32:41 UTC+1, upsid...@downunder.com  wrote:
> On Fri, 27 May 2016 09:16:50 -0700 (PDT), George Herold
> <gherold@teachspin.com> wrote:
>
> >A circuit will have 24V @ 1.5A (max) input from a wall wart.
> >I'd like to design in reverse polarity and maybe over voltage

> If the source is floating, use a bridge rectifier, so the device works
> regardless of polarity and even works on AC as a bonus.

you can often use that when things aren't floating too, If they're both grounded the ground connections just short out the 0v line diodes.

NT
```
```On Friday, May 27, 2016 at 10:15:07 PM UTC-4, John Larkin wrote:
> On Fri, 27 May 2016 09:16:50 -0700 (PDT), George Herold
> <gherold@teachspin.com> wrote:
>
> >A circuit will have 24V @ 1.5A (max) input from a wall wart.
> >I'd like to design in reverse polarity and maybe over voltage
> >I don't like the Zener/ polyfuse idea, cause there is going to
> >be a lot of power in the Zener... I'm afraid it will smoke before the
> >polyfuse lets go.  So some FET circuit.
> >Here's Jim T's offering. (from a past thread.)
> >http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf
> >
> >The gate source voltage on most of the P-fets I've looked at is
> >+/- 20V.  So the 24V is a bit of a concern.  I can just put in some resistors
> >to make a voltage divider.. so not a big deal.
> >
> >But do I need two P-fets? (Oh snap, never mind.. I had to draw the circuit.)
> >
> >Well I'll make it a question anyway... are then any P-fets that can handle more
> >G-S voltage... (well more in absolute value.)
> >
> >TIA
> >George H.
>
> Polyfuse+transzorb+cap works fine. A bunch of semiconductors and
> passives (9 in Jim's case) will have hazards of their own. Like ESD
> damage, for example.

Thanks,  I worry that there won't be enough current to blow the
poly fuse, and then the transorb/ Zener is sitting there dissipating
~10's of watts.. and won't last long.

We're just going to hard wire the power supply in place, not as pretty
but no fuss.
>
> Most fets blow out around 75 volts Vgs. There are lots of fets that
> have internal g-s ESD zeners, typically +-40 volts turf, and they can
> be protected with just a series current-limit resistor.

I don't know FET's that well, but a quick sampling of spec sheets seems
like they all list Vgs max as +/- 20V.  I was wondering if this is
mostly a "lazy" spec*.  In that few people really care, and it would
be too costly to spec it and measure.  It seems like a 200V (Vsd)
FET should have Vgs somewhere near 200 V.

George H.
Like the reverse bias voltage on LED's, all listed as 5V.
>
>
> --
>
> John Larkin         Highland Technology, Inc
>
> lunatic fringe electronics

```
```+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
```
```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.

The surface-mount polyfuses are pretty bad, so we generally use the