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Do I really need a gate driver? Here is what I have going on.

  1. 12Volt 100 watt incandescent bulb drawing 8.33 watts (Taxi light on a Cessna 172)
  2. The switch that turns it on and off reportedly wears out from current draw and needs frequent replacement (suggested every three years). Instead of that I thought a Mosfet would allow the switch a long happy life...
  3. I’m using a IRF3205 HEXFET® N Channel Power MOSFET WITHOUT a gate driver.
  4. 12Volts is below the 20v max on the gate and works fine to turn it on. Of course it just stays on if I take the gate voltage away so....
  5. I have inserted a 270K ohm resistor across the gate to the source as a drop down resistor.
  6. leave 12 Volts applied to the gate and it stays on and when I remove the 12 volts from the gate it shuts off.
  7. Is this the WRONG way to operate this (although it works just fine) I’m not sure if It is okay to leave the 12 volts on the gate?
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  • \$\begingroup\$ Here's a better idea.. replace that incandescent with an led! Inrush current is huge plus both are short lived devices. But the reason for a gate driver is higher voltage and higher current. If you don't need a level shifter and this isn't a pwm application where you worry about efficiency then you don't need a driver. However when switching to a solid state device you want to consider the isolation that a relay provides for safety. Maybe an optocoupler would work for you. Also, you meet max rating of vgs but what is the minimum vth to operate? \$\endgroup\$ Aug 21, 2016 at 21:33
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    \$\begingroup\$ I looked at the Graph on the data sheet... Vth 2-4 volts and fully on at 10volts. I agree The really PERFECT way to do it is an LED Because it's a certified aircraft...WHELEN LED LANDING LIGHT - PAR 36 MODEL 90361 SERIES From $727.00 When I go to ask for a field approval that $727 dollars may seem cheap as opposed to dealing with the Feds. \$\endgroup\$
    – DAVE
    Aug 21, 2016 at 21:45
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    \$\begingroup\$ Do you mean 8.33 amps rather than watts? \$\endgroup\$
    – psmears
    Aug 22, 2016 at 10:30
  • \$\begingroup\$ You need approval for the light, but not the switch? \$\endgroup\$ Aug 22, 2016 at 10:32
  • \$\begingroup\$ I have a feeling this question also needs to be asked on aviation.stackexchange.com/questions - you'll get a good answer here on the electronic aspects but the legal aspects of "pimp my ride" are very different in aviation. \$\endgroup\$
    – user16324
    Aug 22, 2016 at 11:52

2 Answers 2

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There is nothing wrong with driving a FET like this if there is no need to PWM.

What I would add is

  1. Add a series gate resistor ( ~100R)
  2. consider adding a 15V zener gate-source just incase there are some transients. You do not want to burn the gate region out

NOTE: you might want something bigger than just a Zener on the gate. TVS maybe. its dependent where exactly the FET sits with regards to indirect lightning. The DO160 is not very forgiving.

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  • \$\begingroup\$ That sounds great, I added the 100 ohm Series resistor and I'll put the Zener on the gate. Thanks for your help. \$\endgroup\$
    – DAVE
    Aug 21, 2016 at 21:31
  • \$\begingroup\$ Maybe make the pulldown resistor stronger (lower value, perhaps 4.7k). This will make sure the transistor doesn't spend too long in its linear region during turnoff. \$\endgroup\$ Aug 21, 2016 at 22:43
  • \$\begingroup\$ Most TVS devices are too slow to protect against the DO-160 waveforms (particularly 4 and 5A). Littelfuse devices seem to be the best. \$\endgroup\$ Aug 22, 2016 at 13:35
  • \$\begingroup\$ well... it isn't clear what he is subjected to or where the FET will reside. a 15V zener on the gate, stearing diode back to the 12V. some series R, additional C and then 17V TVS to take any high spikes. Plenty that can be done to make an interface compatible with the environment, but info is needed for that. \$\endgroup\$
    – user16222
    Aug 22, 2016 at 13:39
  • \$\begingroup\$ I referenced some data on TVS and wave forms at this site. \$\endgroup\$
    – DAVE
    Aug 23, 2016 at 15:49
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Consider to buy an automotive solid state relay instead. Airplanes aren't a playground for experimenting with electronics basics. As you may have noticed all wiring harness is crimped. You won't find a transitor with soldered wires hanging somewhere, so my advice is to take reliable ready made relay: mechanical or solid state, then borrow some good crimping tool and make correctly the wiring.

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  • \$\begingroup\$ Some inspiration as to what might happen from what seemed like a good electrical solution on the ground turned out to be in the air: m.youtube.com/watch?v=y4pbWZ6t0Ho \$\endgroup\$
    – winny
    Aug 21, 2016 at 21:23

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