Hi Guys,
So I have a circuit that is being built up to run motors with an atmega. The power source is a 2 - 4s LiPO pack, and this device will run the motors through a half bridge.
To date, this has worked just fine. However I'd like to integrate a power switch to turn off the logic side of my circuit. But I won't always have a switch connected - if it's not connected, it should default to always-on, while if connected, the switch will turn on or off the logic.
So my approach is to use a P-fet in between the LiPO pack and the logic voltage regulator. The gate will be pulled down to ground (for a default on-position) and a connected switch will short the gate to battery voltage (vBatt) to turn it off.
The catch is the P-fet is rated for 30v, however that gate is +/-20v. I would expect that there will be spikes of up to 25v on vBatt (despite ~16.8v being the normal max voltage) and have catered for it in the half bridge and regulators, etc. The P-fet should handle it, but I suspect the gate will not.
I've had a bit of a scratch around and found that a Zener will be the likely option. I've seen a few examples (including on here) and think that I have it set up right. I've attached my schematic and would really appreciate a sense check first... Also, the SBR diode is on the input side of a 5v LDO, and has been used to prevent the logic from backfeeding the motors and browning out in my old design. I also suspect this is now redundant with the introduction of the P-fet.
- The zener is 15v.
- J1 is the power switch connector.
