I was hoping you guys could help me check over my circuit. I'm building a circuit to supply a fan (12 V and 1 AMP) 12 Volts when an atx power supply starts and then switches to 5 Volts when the MCU signals so. The 12V is connected to the fan through the PNP transistor and the MCU controls the NPN transistor which connects 12V to the base of PNP. The 12V and 5V are each connected with a schottky diode and then join to the same wire, so when the NPN turns on the PNP the 12V turns off and 5V takes over. MCU is +3.3V and seems to work fine with the NPN. The gain on the transistors is a minimum of 1000.
Initially, I want the PNP and NPN bases connected to ground, so I hooked up some pull-down resistors. Is there a way to calculate what resistor value to use? I've just seen the recommended values 10k+. 10k Seems to cause some leakage for me, so I used 100k which works. Thanks.
Initially, I want the PNP and NPN bases connected to ground, so I hooked up some pull-down resistors. Is there a way to calculate what resistor value to use? I've just seen the recommended values 10k+. 10k Seems to cause some leakage for me, so I used 100k which works. Thanks.
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