Hi all,

i know that you have probably seen this issue and questions for this a million times, but I have a circuit that doesn't work and I have no idea why?

I am trying to switch battery voltage out to a device. The battery voltage can range from 10V down to 6.5V. Current consumption ranges from 500mA to 750 or so mA.

The switch is being controlled from a 5V PIC micro. The I/O drives the base of a MMBT3904 through a 5K resistor. The emiter is tied to ground and the collector is tied to the gate of a SI2301 FET and a 40K resistor that is connected to the FET's source and the battery voltage. The FET's Drain is tied to the device.

The I/O is changing, but there doesn't appear to be any switching going on on either the 3904 or the FET.

Any help would be GREATLY appreciated.

Thanks

Tony

 

The MOSFET may be shorted or open.

Try disconnecting the MMBT3904 from the gate, and use a 1k resistor from the collector to the battery. If you can then see the voltage on the collector changing by changing the I/O pin, the MOSFET is bad. If the collector voltage doesn't change, the 5k resistor or the MMBT3904 is bad.