Let's Make Robots!

Reversed battery protection

Hi guys, i'll start by giving you this circuit:


The circuit breaker (the PTC at the top named "CB1") and the Zener diode ("D1") have been added for overcurrent and overvoltage protection. But now my concern now is about protection from reversed batteries. What i wanted to ask you is: is it better to add the p-channel mosfet or shall i take it out?

As for as i understand the circuit, there would be reversed battery protection anyways, since the current would pass through D1 thus short circuiting the robot and activating the PTC, but i am worried that this way the PTC would be stressed to much.  


PS: oops i think i connected the Q1 the other way round. (switched source and drain)


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The P-FET and the PTC are generally for use with higher current usage (100s of mA up), the zener would be for lower current needs (tens of mAs). If higher current, use a linear regulator, or even a switcher. If going low current, a simple diode can replace the P-FET for reverse protection.
I don't really get it. The switching regulator i am using doesn't have reversed battery protection and that's why i wanted to add it to my circuit. The diode would be ok but i don't like its voltage drop.
couldn't you put the diode prior to the switching regulator? the voltage drop wouldn't affect the output at least... a 9.6volt battery would then give you 9 volts....not really that bad.....

i could, but i see no reason to :)

i mean, i could, yes, but if i find a better way to do that its better! (for my understanding and for the circuit) 

Use a linear or switching regulator in addition to the P-FET and PTC, as all would be capable of running higher current.

A zener diode is only good for low current regulation, if any at all. It would be better paired with a simple diode, as both are low current devices.

the zener diode is not there for current regulation. This is how i thought it would work.

suppose the vreg is not in there (actually it is not in there) and you insert a 20V battery instead of the normal 10V you should have inserted. Since i don't want the 20V to ruin my circuits i short it through the diode (which only passes voltage higher than 15V) and activates the PTC. (this is what the diode is doing in case no vreg is there or if the unregulated current accidentally reaches circuits which should be regulated)


Now, if i insert the battery the other way round, the current would likely pass through (i guess) the zener diode (shortest way to GND) and get in a short circuit and this would activate the PTC. 


 The current will be blocked since the gate voltage won't let the current pass through the mosfet.

what i wanted to ask you is which of these two is better 

If the circuit that is being powered draws say 200 mA at 10 volts, the zener diode has to shunt the remaining 10 volts (20 -10) times the 200 mA through itself to perform it's reverse breakdown "regulation". The zener will thermally fail in this, probably in a very short time, and the circuit will then be getting the full 20 volts that it was trying to prevent in the first place. That is why a regulator should be used in higher current applications rather than a zener.
But i AM using a regulator, and the zener is only a last resort. Say a screw which falls on the board accidentally touches the input and output pin of the v reg. If this happens the zener will burn out but at least save the circuit. Are you sure that when the zener burns out the circuit will receive its 20 volts? I have heard somewhere that if it gets destroyed it will still act as a conductor. 
Yes, I am sure. Zeners fail as an open circuit, not a conductor.
The blocking P-FET will be much better than hoping a zener will protect in reversing the battery.