Pulldown resistor on switched supply wire


I have a sensor module, which is switched with a P-channel FET as a high side switch. When the module is switched off after operation, I measured an unexpected 1,3V on the switched line (OUTPUT), which discarges in approx. 15sec. Now I'm sure it is "coming back" from the sensor, because if the sensor is not connected, I measure 0V instantly after FET switch off.

Would it do any harm if I connect e.g. a 100k resistor between the OUTPUT line and GND?
(I think it would discharge faster any voltage "left" in the sensor module.)

Thanks in advance.

Resistor would dissipate the floating voltage just fine once Q1 is turned off and no load is connected.

It's probably just the bypass caps on the module, discharging. Why do you care about the voltage? Surely you're not sampling the output after you turn it off?

@CrossRoads: thanks for the confirmation.

@aarg: it's a quite complex module with it's own voltage regulator, MCU and stuff. There could be a situation, when I need to reset the module. I thought it would be the best for a hard reset, if the input voltage for the sensor module really drops around 0V first, then switch back again. The 15 sec discharge time seems problematic for me.

Oh. Then please tell us what the module is. If 15 seconds is a guess, so is 10, so is 5, so is 2, etc. etc.

It is a millimeter wave radar sensor. 15 sec is not a guess, I measured it.

I only have a screenshot of a shorter FET switch OFF-ON. After 800ms it is still 1,3V.

FET switch OFF-ON without the sensor module:

I meant guessing about the effect... not just the decay. What testing have you done to determine the actual power down/ power up reset behaviour?

Also still vague.. "a millimeter wave radar sensor"... is it top secret?

It looks to me like the voltage drops almost instantly to about 1.5V as soon as power is removed.

Why not write a test sketch to turn it off for a variable few milliseconds and see if it's reset or not? It's extremely unlikely that a tiny residual voltage like that would influence the reset at all...

R4 serves no purpose at all. :roll_eyes:

Have you considered the sensor may misbehave as its supply falls?
If its OK in those conditions why not provide a switched discharge path for the module's bypass caps?
Alternatively - does it matter if that voltage is there for 15 seconds?

Such brown-out condition is always dangerous in a complex system. The designers of the module probably expected the brown-out may happen and implemented proper countermeasures. But unless you have very detailed documentation you cannot be sure they did and even if they tried "something" if it covers every possibility.
In other words draining the decoupling caps to 0 V before applying the power again is a very good idea.

Your 100k was just a guess, right? Do you know how much current you have to draw, to drain the capacitance in some target interval, like 100ms?

If it's too high, your drain resistor will waste a lot of power when the device is on.

RESET from the Arduino is an output as well as an input - you could connect it to some active circuit that would ground the device power input until the processor starts up.