Driving Mosfet w 556

Thread Starter


Joined Feb 1, 2007
I have IRF530N, the gate of which is being driven by NE556 (Dual 555 timer). I can see the Gate going up untill 5 Volts, My DC motor is connected in the source branch.

Source terminal of MOSFET - > DC motor -> Ground.

I am seeing a huge voltage drop of about 6 volts across Drain and source, and the source being at 2 volts.

Could you tell me why is this drop? as the gate is crossing the minimum threshold, the MOSFET should be in the ON state...right?

am I missing something here...?
Vcc that is being applied = 11 Votls

Here are the links for the two datasheets of
1) rf530n
2) NE556



Ron H

Joined Apr 14, 2005
You need to understand that, in order to be fully on, the gate must be more positive than the source, generally by several volts (depends on the MOSFET and the drain current). If your source were near Vcc (11 volts), the gate-to source voltage would be -6V (5V-11V). It would be OFF, so this won't work. Even if the source were at 5 volts, it would be OFF.
You are running the MOSFET as a source follower. The source will only rise to the voltage where it reaches equilibrium. To turn it ON, you need to either drive the gate to about 20V, or use a P-channel MOSFET, with the source at Vcc and the drain going to the motor. The gate would need to swing from Vcc (OFF) to around 0V to 2V (ON).