Hi-side switch

Thread Starter


Joined Sep 25, 2011
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.


Last edited:


Joined Jul 17, 2007
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.