1
\$\begingroup\$

I'm using an N-channel mosfet to supply power to a servo. The problem is that even with the gate set at low, I'm still getting some amount of voltage (~0.9V in a 5V system) coming through. Do I need a pull-down in case the pin is floating?

Here's the current schematic:

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Why the 250 uF capacitor at that place? \$\endgroup\$ – Wouter van Ooijen Apr 9 '14 at 19:21
  • \$\begingroup\$ This drives a servo. The cap is there to minimize servo "chatter" \$\endgroup\$ – kolosy Apr 9 '14 at 21:49
  • 2
    \$\begingroup\$ You have answered it for yourself. By placing the elco as you did you get an unknown inrush current every time you switch the FET on (which will degrade the cap's lifetime, or forces you to get one with a high current spec), and as a bonus the servo will work up to an unspecified time after you switched it off. \$\endgroup\$ – Wouter van Ooijen Apr 11 '14 at 13:00
  • 1
    \$\begingroup\$ i appreciate the advice (and will take it), but the easter egg hunt is unnecessary and condescending. if i had the answers, i wouldn't come to a Q&A site to get them. \$\endgroup\$ – kolosy Apr 11 '14 at 13:07
  • 1
    \$\begingroup\$ In my second comment I gave the solution. When you asked for the reason I gave it in my next comment. \$\endgroup\$ – Wouter van Ooijen Apr 11 '14 at 14:32
8
\$\begingroup\$

Yes, if you just tristate after pulling it high, then the gate will stay floating high. You either need a resistor to pull it down to ground or you need the input signal to drive it low.

The resistor can/should be relatively high valued compared to your input resistor to prevent excessive voltage drop when you have it set as a high input. You only have to drain the inherent capacitance on the MOSFET gate when you're pulling it low so even at a high resistance to ground the RC time constant is usually relatively short.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.