Pulse Width Modulation

[Harald Kapp]

Just what this figure actually is depends on how hard the mosfets are turned on.

The max. Threshold voltage is Vgsmax = -4 V. The 555 operated from 12 V can easily generate a lower gate-source voltage (note that Vgs is negative since source is connected to 12 V). The MOSFET should be turned on hard.
At Rds = 0.018 Ω the observed voltage drop is equivalent to a current of Ids = 166 A -> unlikely. The voltage drop is most certainly due to other effects.

• What is the gate-source voltage at the mosfet when on?
• What is the drain current of the mosfet when on?
• Is the mosfet properly heatsinked? (at 30 A it will dissipate 16 W! See the note at the bottom of the schematic in post #19)

 

[Alec_t]

the voltage supplied to the motor is only 8.5 to 9

Most DMMs won't give accurate readings of a pulsed voltage (PWM).