Need help for PWM circuit!!

aptsys · Feb 16, 2011

Just a quick question... Do you have the part number of Q1? Also, what's the current draw on the load you want to PWM?

If Q1 is suitably rated, and you are able to hack the circuit to disconnect the cathode of the diode from the 5V rail, you can simply:

1. Attach the cathode diode to the +40V terminal of the battery
2. Connect the ground of the circuit to the negative of the battery
3. Break the connection between the negative of the battery and the load
4. Connect the switched output from the MOSFET (confusingly also labelled ground) to the negative of the load

tkhao1986 · Feb 16, 2011

I think i get what u meant. Well, that is something which first came to my mind. However, the thing is, if the wirings between the load and MOSFET gets damaged and gets connected to GND, i will be unable to control the flow of current, which will be dangerous.

Thus, i need to put some switches/transistor after the positive terminal of the battery, so that the flow of current will still be under control if any wirings were to be damaged.

I just checked on whats a optoisolator and i think it fits to what i want here. Is there any such opto isolator which has a MOSFET and can handle 40V? For your information, the current may reach around 25A.

aptsys · Feb 16, 2011

tkhao1986 said:
I think i get what u meant. Well, that is something which first came to my mind. However, the thing is, if the wirings between the load and MOSFET gets damaged and gets connected to GND, i will be unable to control the flow of current, which will be dangerous.

A short circuit from your battery will be dangerous under any situation, hence why fusing is important.

If the current is reaching 25A, you will almost certainly need a MOSFET driver as you will not be able to saturate the MOSFET fully at 5V unless there's a logic level MOSFET with a low Rds(on) and current handling capability greater than 25A. I'll have a search...