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I'm using the following circuit with a P-channel mosfet to switch the positive line to a transceiver. I want to keep the transceiver grounded and switch it off to stop current draw from the battery.

The circuit gives the desired results and switches the transceiver off, and I get near 0 current draw when the digital pin is high.

My question is in relation with the capacitors on the transceiver. After power is cut from the transceiver if I place a voltmeter across the load (Drain and ground), I get a voltage of ~0.5V which decreases. I'm assuming this is the capacitors discharging when the voltmeter is applied. There is no current drain if I connect an ammeter to the batteries negative side, as you would expect. I'm not sure of the specs for the capacitors but I'm assuming they are typical for filtering noise etc.

Do I need to be at all concerned with this discharge voltage from the capacitors, and can it negatively impact the mosfet?

P-Channel Mosfet Switch

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  • \$\begingroup\$ A simpler hack is a suitable resistor (maybe 10K) across the transceiver's supply (i.e. drain to GND). It draws some power when the transceiver is on, and maintains a load to drain the caps better than the transceiver, which will go high impedance i.e. draw very little current when the caps fall to a volt or so. Some judgment needed to find the best compromise between wasted power and discharge speed. \$\endgroup\$
    – user16324
    Feb 18, 2020 at 11:32
  • \$\begingroup\$ Thanks I tested a 10K resistor in parallel with the transceiver and it discharges the capacitor very quickly and has very little draw current. \$\endgroup\$
    – quinn123
    Feb 21, 2020 at 2:59
  • \$\begingroup\$ That only leaves the inrush current to be concerned about. I'm not sure how to calculate this, but I'm going to assume the resistor on the gate will reduce the inrush sufficiently enough. Hopefully someone more knowledgeable than me can comment. \$\endgroup\$
    – quinn123
    Feb 21, 2020 at 3:07

1 Answer 1

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There are a couple of potential harms:

  1. The inrush to the capacitor when the PMOSFET is turned ON. Some calculation is needed to be sure that the peak current is not too high. I guess that's no problem, but it's better to be sure.

  2. Does the power-on reset circuit work if there's some voltage left? A resistor could discharge it surely. I do not know if any reset circuit exists.

  3. In the past connecting the power on later in a part of a system could cause malfunctions and even smoke because some parts didn't stand input voltages during the power-up. But this is probably no problem with properly designed modern parts. Again: better to be sure!

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  • \$\begingroup\$ Some good points I will have to have a think about. The radio has a sleep mode but it's difficult to program. So this was meant to be a simpler option. \$\endgroup\$
    – quinn123
    Feb 18, 2020 at 11:06
  • \$\begingroup\$ The transceiver is a cc2530. I will find a schematic tomorrow. It only really supports sleep with spi and I'm using uart, so hence I need a method to turn off. \$\endgroup\$
    – quinn123
    Feb 18, 2020 at 11:31

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