The circuit can be separated into 2 parts:
- One side is used to power the Pi. I go from the battery (12V, 6A) to a buck converter that I use to drop the voltage to 5V. This seems to work fine. I was able to set current limitation and it would be effective.
- The second part is used to control the LED. Here I go from the battery to a first MOSFET controller ( MOSFET Tube Driver Module For Arduino SCM ARM Raspberry Pi with a IRF520 ) plugged to the ground on the Pi and to GPIO16. The circuit continues then to a buck converter that limits the voltage to 3V and that should normally limit the current to 750mA. It goes then to a second MOSFET controller that is used to control the LED status. This MOSFET board is connected to the ground on the Pi and to GPIO21.
Now my problem is with the 2nd system. If instead of a LED I attach a multimeter and try to read the current I get the following:
- MOSFET 1 and 2 are off --> current 0mA
- MOSFET 1 on, 2 off --> current 0mA
- MOSFET 2 on, 1 off --> current 0mA
- MOSFET 1 and 2 on --> the current starts low (0.25A) and continuously rises. After 1 min it is around 0.5A and keeps going. I am new to electronics so I didn't know if it could damage the Pi. Therefore I did not leave it on to see where is stops climbing.
If I switch off the 2 MOSFETS for a while and then switch on again, the current values has dropped a bit and rises again.
My questions are:
- What could potentially limit my current in the 2nd system at 250mA when I start up?
- This current increase looks like something is charging somewhere. Could some part of the current go into the Pi? Or is my reading wrong by closing the system with my multimeter?
- How can I prevent this unwanted behavior and have the wished 750mA right away when I switch on the 2 mosfets
Thank you in advance