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I completed an IR emitter circuit with two selectable power settings. It's working on the breadboard and I'm asking if I need the gate-source resistors for stability? As the circuit is battery powered I want to prevent wasting power. 38kHz is created by a 328P VCCmax = 8.4V V_38kHz=3.3V

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  • \$\begingroup\$ IF the drive sources are always high or low and never floating then R2 R3 R4 are not needed. If any of these are sometimes open circuit / high impedance then IF you care that the FETs may turn partially on and stay that way then the pull downs are needed. Really only R2 is enough as Q8 Q9 do nothing if Q6 is off. || As current per resistor at 5v drive is i = V/R = 5/100k = 50 uA then odds are this will not make a lot of difference (20 hours per mAh) UNLESS the low or high drive is left on when the 38 kHz is off/low. \$\endgroup\$ – Russell McMahon Feb 22 at 2:34
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IF the drive sources are always high or low and never floating then R2 R3 R4 are not needed.

If any of these are sometimes open circuit / high impedance then IF you care that the FETs may turn partially on and stay that way then the pull downs are needed.

Really only R2 is enough as Q8 Q9 do nothing if Q6 is off. As current per resistor at 5v drive is i = V/R = 5/100k = 50 uA then odds are this will not make a lot of difference (20 hours per mAh) UNLESS the low or high drive is left on when the 38 kHz is off/low.

As others have noted - you can save an I/O pin, a FET and a pulldown by having low power setting set by a resistor where Q9 is and add to it with Q8 as required.

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  • \$\begingroup\$ ah now I get it what you mean with the resistor replacing Q9. That's a smart way. I will do it like that and leave only R2. I'm not sure if there's floating. If there is no failure the MCU 328P outpur should be high or low \$\endgroup\$ – user3435167 Feb 22 at 3:14
  • \$\begingroup\$ @user3435167 The processor may float the I/O line very briefly at startup before the port is initialised. This will be for a very short time and is unlikely to matter in this application. \$\endgroup\$ – Russell McMahon Feb 22 at 5:23
  • \$\begingroup\$ @user3435167 Note that others have commented on using a resistor in place of Q9 in rsponses to your previous questions. It pays to read all comments and answers carefully and to understand the points being made. You can learn much that way. \$\endgroup\$ – Russell McMahon Feb 22 at 5:25
  • \$\begingroup\$ to be honest I didn't understand the reply by the other user and got no more answer to it \$\endgroup\$ – user3435167 Feb 22 at 11:49

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