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Topic: Sanity check; switching ICs off with MOSFET (Read 861 times) previous topic - next topic

Cosford

Apr 01, 2015, 12:23 pm Last Edit: Apr 01, 2015, 12:25 pm by Cosford
Hi,

Just wanted a quick sanity check. I've had some great help in the past (http://forum.arduino.cc/index.php?topic=290215.0) with regard to MOSFETs and switching loads on or off, which worked perfectly.

I'm now looking to switch a whole module on/off (in particular, an MFRC522 RFID reader). Am I free to use low side switching on the ground line of that? Or would current flow back through the logic lines? What would be the best method of switching the entire module on or off at will?

Datasheet: http://www.nxp.com/documents/data_sheet/MFRC522.pdf
Module pinout: http://4.bp.blogspot.com/-CypWJHefOgY/U9AnzSpeEyI/AAAAAAAABao/fZKUC4YPNRk/s1600/RFID-RC522-pinout.png

Thanks in advance,

Cosford.

Paul__B

#1
Apr 01, 2015, 12:28 pm Last Edit: Apr 01, 2015, 12:28 pm by Paul__B
Am I free to use low side switching on the ground line of that?
Never.  Well, almost never.

Or would current flow back through the logic lines?
That would be the reason.  And any external connections.  Mind you, even with high-side switching you must disable the I/O lines.

What would be the best method of switching the entire module on or off at will?
Generally, high-side switching with a logic level (P-)FET.  If the voltage is higher than the Arduino 5V, you need a NPN transistor (or N-FET) to control it.


Cosford

Great, thanks for that. Intuition that it wouldn't be as simple as that was right. Aha.

So, use a p-type FET to switch power to it. You mention disabling the I/O lines even with high side switching, I assume that's because it would attempt to source current from the I/O lines? How exactly would you go about 'disabling' them?

Just had a proper read through the datasheet rather than a skim, and it seems that I can power down the module near entirely, by holding reset line low, so this shouldn't actually be necessary, but for future reference this will be useful.

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