Turning off external sensors with Arduino: MOSFET vs I/O pin

Using Arduino Motors, Mechanics, Power and CNC
#1

Hi everyone,

For a project, I made an environmental sensor system, consisting of an Arduino Pro Mini 5V, a data logger module and three sensors.

To save power between data logs I used several techniques I read about online, including turning off the power to the sensors using a MOSFET (I used the IRLB8721PbF).

Recently, I started reading a bit more about MOSFET's and turning off external devices with Arduino's, and I realized that since my sensors only use about 4mA, I could have used the a I/O pin (set to output) on the Arduino instead.

As I was curious to see if there were any difference in current draw between the two methods, I tested the current draw when using the MOSFET vs when using the I/O pin to turn the sensors on. When using the MOSFET, the current draw increased with 4 mA when the sensors are turned on, corresponding to the sensor draw. However (!) when using the I/O pin the current draw first shortly increased with 6 mA, after which it settled at a 4 mA increase. These differences would suggest that it is more power efficient to use a MOSFET, even with low current draw apllications.

Now my question is (1) why do I see this short spike to 6 mA increase in current draw when using the I/O pin to power on the sensors? (2) And is it more preferable to use a MOSFET even with low current draw applications and why?

Another thing related to this: I recently read that a resistor in series with the gate is necessary when using a MOSFET, however I have been using the MOSFET for my application without a resistor and I have had no problems whatsoever for more than 6 months and counting. (3) Could someone explain why it has been working fine for my application? Does it have something to do with the fact I use the MOSFET to switch a low current draw load? Or does it have something to do with the type of MOSFET I am using (the IRLB8721PbF).

Lot's of questions, I'm kinda new to electronics...

Thanks in advance for taking the time to answer!

Difster

#2

A N channel mosfet can only switch the low side, or common, of a device. This works for two lead devices such as solenoids, relays, lamps and led’s but it does not always work for devices that have more than one current path.

Powering a low current device with an output pin can work but since you don’t mention actual devices or provide a schematic of how you connected things, it’s difficult to comment in detail.

#3

Hi,
What are your sensors?
If any are I2C or SPI you may need to restart the sensor each time you re-apply power.

Tom.... :slight_smile:

#4

Difster88:
Recently, I started reading a bit more about MOSFET's and turning off external devices with Arduino's, and I realized that since my sensors only use about 4mA, I could have used the a I/O pin (set to output) on the Arduino instead.

No, probably not a good idea.

Two big problems doing that:

  1. Output pins have a fairly high impedance, 40 ohms or so, so instead of providing a solid 5V rail to the
    sensor you have one that will sag as the sensor's current consumption changes, imposing that variation
    onto the sensor's output voltage if its analog 4mA x 40 ohms = upto 0.16 V variation.

  2. You forget about decoupling. The sensor's supply needs decoupling, and a decoupling capacitor is too
    harsh a load to drive from an Arduino pin - logic signals can be expected to drive a few 100pF capacitive
    load, but 100nF or more? Not a good idea as the charging spike for the cap might be 100mA or more.

One or other of these issues will apply, 1) for analog sensor, 2) for digital.

#5