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```What is the purpose of using a power MOSFET in the main voltage input of a
circuit? Why not just directly connect the main voltage input to a power
supply? Why go through the MOSFET?

I am trying to understand a circuit that goes directly from a 12V battery
to Drain of a P-channel power MOSFET. The Gate is connected to ground
through a 1MOhm resistor and connected to Source through a Zener diode. The
circuit is connected to the Source pin.
```
 0
Reply jmeyer8879 (30) 7/20/2009 4:49:15 PM

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```On Mon, 20 Jul 2009 11:49:15 -0500
"eeboarder" <jmeyer@emittechnologies.com> wrote:

> What is the purpose of using a power MOSFET in the main voltage input
> of a circuit? Why not just directly connect the main voltage input to
> a power supply? Why go through the MOSFET?
>
> I am trying to understand a circuit that goes directly from a 12V
> battery to Drain of a P-channel power MOSFET. The Gate is connected
> to ground through a 1MOhm resistor and connected to Source through a
> Zener diode. The circuit is connected to the Source pin.

Accidentally reverse the battery.  Repeat your circuit analysis.
Illumination should follow.

--
Email address is currently out of order
```
 0

```
eeboarder wrote:

> I am trying to understand a circuit that goes directly from a 12V battery
> to Drain of a P-channel power MOSFET. The Gate is connected to ground
> through a 1MOhm resistor and connected to Source through a Zener diode. The
> circuit is connected to the Source pin.

That is a protection against the battery reversal. Mosfet has less of
the voltage drop compared to a diode.

DSP and Mixed Signal Design Consultant
http://www.abvolt.com

```
 0
Reply nospam (2805) 7/20/2009 5:29:37 PM

```"Vladimir Vassilevsky" ...
> eeboarder wrote:
>> I am trying to understand a circuit that goes directly from a 12V battery
>> to Drain of a P-channel power MOSFET. The Gate is connected to ground
>> through a 1MOhm resistor and connected to Source through a Zener diode.
>> The
>> circuit is connected to the Source pin.
>
> That is a protection against the battery reversal. Mosfet has less of the
> voltage drop compared to a diode.

And when analyzing, remember the 'parasitic' drain-source diode inside the
MOSFET - it is an essential part of the trick...

Arie

```
 0
Reply nospam21 (19049) 7/20/2009 5:41:53 PM

```>
>> eeboarder wrote:
>>> I am trying to understand a circuit that goes directly from a 12V
battery
>>> to Drain of a P-channel power MOSFET. The Gate is connected to ground
>>> through a 1MOhm resistor and connected to Source through a Zener
diode.
>>> The
>>> circuit is connected to the Source pin.
>>
>> That is a protection against the battery reversal. Mosfet has less of
the
>> voltage drop compared to a diode.
>
>And when analyzing, remember the 'parasitic' drain-source diode inside
the
>MOSFET - it is an essential part of the trick...
>
>Arie
>
>

Thanks again for the help!
```
 0
Reply jmeyer8879 (30) 7/20/2009 7:21:39 PM

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