Protection Circuit

[Tabsoft]

Saw this online.

May be of some help.

http://www.instructables.com/id/Reve...-circuit-with/

[pedja089]

10-33K resistors should be fine(depending on zener leakage current).
D1, D3, D4 protect MOSFET's gate from over voltage. Usually Vgs is +-20V, so any zener between 5 and 15V should be ok.
Vmax should be Vd2+Gate Threshold Voltage of Q2(usually 1-3V).
MOSFETs depends on your max voltage and current.

[Charlie]

A series diode will protect from reverse polarity. A cheap 1N4000 series is simplest at the cost of half a volt or so depending on load. If your supply has room, a diode bridge will let you connect up power backwards and still operate correctly.

[Scampy]

Thanks for the suggestions guys.

I'm designing the project to use a 12v DC source to power some miniature lights. The problem is that this would be dependent on the end user's own installation, so you could have upwards of 16-25 amps @ 12v. The 12v supply is switched by 16 MOSFETS which in turn are turned on/off or dimmed via the control circuit ( which runs at 5v taken from the 12v input and currently regulated via a linear 7805 regulator with associated capacitors). So I would assume I would not only need the protection to cover the PIC and associated IC's, but also the 16 FETs and thus need to handle the high amperage ?

Any suggestion for the FET to use in the link Tabsoft provided ?

[richard]

currently regulated via a linear 7805 regulator

yuk , try a R-78E5.0-0.5 . no heat : - 92% efficient , pin compatible 7-28v input . try one ,you will never go back to nasty hot wasteful linear regs.

I would use a fuse for the lights and just protect the 5v reg and its circuits, the body diodes of the lighting mosfets will blow a fuse nicely .

better still get the pic to operate a 12v load relay when its had a look around and made sure everything is nice after power-up

[HenrikOlsson]

Plenty of P-channel MOSFETs to choose from.
The SPD50P03 is 30V, 50A, 7mOhm, TO252 and the IPP80P03 is 30V, 80A, 4.4mOhm, TO220.

The Recom switching regulator modules are very nice but if the current drawn by the low level circuit is low(ish) then a linear 7805 is just fine for going from 12V to 5V and a lot cheaper (about 1/5) than the Recom module. And IIRC the Recom module needs a minimum load for the regulation to work - which usually isn't a problem but something to look out for.

/Henrik.

[Scampy]

Thanks once again for the input guys. The 5v is purely to run the PIC, a DS1307, 2 x 16 LCD and a PCA9865, so not really a huge load, and whilst the 7805 is not efficient, for the purpose it does juts fine.

I'll check out the FETs that Henrik suggest, and look for some fuses for the lights. Thinking about it, as the 12v flows through the lights it doesn't matter which way round they are connected as the lights have no polarity being small incandescent lamps, so reverse polarity on the connections to the lights isn't required.