I was reading on the internet about reading the remaining voltage level of a battery operated device using an ADC. This circuit is designed so that the ADC is only used when it is needed to be used, rather than it constantly draining the battery.

However, I do not understand what R3 and C1 are used for. I'm guessing it's some kind of MOSFET driver, but I am not sure.

Could someone help me to understand what they are for? Thanks

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    \$\begingroup\$ Can you link to the source of that schematic so we can see it in context? \$\endgroup\$ – Phil Frost Jan 28 '14 at 3:03
  • \$\begingroup\$ With R3 = 0Ω, wouldn't turning on Q2 have undesirable consequences for Ic at Q2 and for power consumption should Q2 survive? \$\endgroup\$ – RedGrittyBrick Jan 28 '14 at 9:25

R3 is to pull the gate up close to source potential when Q2 is turned off (which turns Q1 off).

Q2 pulls the gate down to ground potential to turn Q1 on (the gate will be at -Vcc with respect to the source).

C1 uses up PCB real estate, costs money, and does nothing of any value. It just slows down the turn-off of Q1 a bit, and the turn-on hardly so you would notice it.

  • \$\begingroup\$ That's helpful thank you. So I do not need to use the capacitor? \$\endgroup\$ – user9993 Jan 28 '14 at 2:06
  • \$\begingroup\$ I see no value in it. The MOSFET has about 70pF of gate capacitance and the BJT has 3pF of collector capacitance. Adding 100pF does not make much difference to anything. \$\endgroup\$ – Spehro Pefhany Jan 28 '14 at 2:10
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    \$\begingroup\$ Caps attached to mosfets in this way are useful when the mosfet is driving a motor, in order to prevent the motor from accidentally moving each time the power is turned on -- to overcome possible stray capacitance to GND from the wire connected to the gate. I agree with Spehro that in this battery-voltage-level measurement circuit, leaving the cap out it would work just as well. \$\endgroup\$ – davidcary Jan 28 '14 at 15:43

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