# High side switching of a current source with 3.3V logic

I have built a Voltage Controlled Current Source that has no ON/OFF switch. This VCCS simply receives a voltage and outputs current to a DUT. The DUT is basically a high-power variable resistor (10 to 20 ohm).

So for example, if I give 100mV to the VCCS's input, it will try to supply 100mA to the DUT (regardless of its current resistance). And if I give 2V to the VCCS, I get 2A at the DUT (again, regardless of its resistance). So the VCCS is all fine and dandy.

Now, I need to implement a high-side switching between the VCCS and the DUT, that can be controlled by a 3.3V logic from an MCU. I tried to come up with some solution, and here is where I am currently stuck at:

And here is the result:

Can you please guide me to wire this circuit correctly? Also, I will appreciate it if you can name a NMOS and PMOS that can be used in this configuration. I have chosen the mos-es in the LTSpice arbitrarily.

• I think this is an XY problem... Your issue will be keeping the p-channel on when the current setting is low enough so the output is below the Vgs threshold. What does your current source circuit look like. Commented Nov 16, 2017 at 17:26
• I'm not sure what you are trying to accomplish with M1, for a simple switch it can be removed. You also need to watch the absolute maximum for Vgs on the P-FET with a 40V source you are probably right at the edge of damaging the device. A 10V zener in parallel with R2 can be used to limit Vgs regardless of the input voltage (watch the zener power rating when the switch is on, current governed by R3) Commented Nov 16, 2017 at 17:26
• @Trevor any ideas as to why I get 3.6A through the load? Commented Nov 16, 2017 at 22:30
• Not sure how much source resistance? Commented Nov 17, 2017 at 0:54
• @trevor you are right i had 0 ohm on sources Commented Nov 17, 2017 at 8:11