shumifan50
- Jan 16, 2014
- 579
- Joined
- Jan 16, 2014
- Messages
- 579
The schematic is a proposed circuit for using a N-channel FET to switch on/off the circuit as well as a load on the battery(J2). To start the circuit you have to press SW1 and hold it until the pic raises GP5 at which point, if the circuit works as desired, the FET will ground the Com of the LM7805 as well as pin 1 of J2, completing the circuits(for the load on J2 and the power to the PIC).
Note: This is to replace the relay on my LiPo protection project on another thread.
A few things bother me:
1. Is the circuit the right way around to use a N-channel FET switching ground rather than the positive voltage?
2. What will the LM7805 do when it has 12V on its input but no ground? Will it still draw current because of the load connected to its output? And if so, what will the output voltage be?
3. Do I need a resistor between the FET and the PIC? I think not as the gate current should be very low, but I don't know which figure to look at in the data sheet or how to calculate it.
This is my first attempt at using a FET so feel free to shoot me down. Any suggested improvements will be greatly appreciated.
I looked at using a P-channel FET so I could switch the positive side, but that would require a FET driver, which this does not as the FET used can be switched with the 5V from the PIC even though the Rds(on) will be quite high (0.045R) which will result in quite high heat when switching on 30A or so. I might need 2 FETs in parallel but I will cross that bridge when I get to it - first the basics.
The intended operation.
1. When the battery is connected, nothing happens as the LM7805 has no ground and the J2 negative terminal is not connected to ground, so the load, connected to J2, remains switched off.
2. Push the start switch(SW1) and that connects the ground of the 7805 and the negative terminal of J2. The PIC starts running and raises GP5.
3. Let go the push button and the FET takes over and keeps the LM7805 and the negative terminal of J2 connected to ground.
4. It is not possible to switch the circuit off as I am out of inputs on the PIC. I could add another switch(normally closed), but can easily do without.
@Steve:
I am not hard headed about using a LM7805, it is the regulator I know(and have lots in my bitsbox) and it is easy to use but I will change it to suggested improvements.
Note: This is to replace the relay on my LiPo protection project on another thread.
A few things bother me:
1. Is the circuit the right way around to use a N-channel FET switching ground rather than the positive voltage?
2. What will the LM7805 do when it has 12V on its input but no ground? Will it still draw current because of the load connected to its output? And if so, what will the output voltage be?
3. Do I need a resistor between the FET and the PIC? I think not as the gate current should be very low, but I don't know which figure to look at in the data sheet or how to calculate it.
This is my first attempt at using a FET so feel free to shoot me down. Any suggested improvements will be greatly appreciated.
I looked at using a P-channel FET so I could switch the positive side, but that would require a FET driver, which this does not as the FET used can be switched with the 5V from the PIC even though the Rds(on) will be quite high (0.045R) which will result in quite high heat when switching on 30A or so. I might need 2 FETs in parallel but I will cross that bridge when I get to it - first the basics.
The intended operation.
1. When the battery is connected, nothing happens as the LM7805 has no ground and the J2 negative terminal is not connected to ground, so the load, connected to J2, remains switched off.
2. Push the start switch(SW1) and that connects the ground of the 7805 and the negative terminal of J2. The PIC starts running and raises GP5.
3. Let go the push button and the FET takes over and keeps the LM7805 and the negative terminal of J2 connected to ground.
4. It is not possible to switch the circuit off as I am out of inputs on the PIC. I could add another switch(normally closed), but can easily do without.
@Steve:
I am not hard headed about using a LM7805, it is the regulator I know(and have lots in my bitsbox) and it is easy to use but I will change it to suggested improvements.