I have part of a circuit that I wish to disable power to depending on two input signals (digital, active low). How do I realise this, preferably using transistors?

The two signals comes from a LiPo protection circuit representing over-charge and over-discharge. When either is low the rest of the circuit must be disabled to prevent battery damage and potential danger.

This is what I tried, using two N-channel enhancement mode MOSFETs. It works for one of the signals, but when input 2 is low, it does not fully turn of (LED is still lit). What the schematic software didn't allow me to do is that there is a diode between source and drain of the transistors, as in the second picture. An explanation to that would also be nice, as I have a feeling it affects the setup.

Example circuit (Vdd = 2.7 – 4.3 V)
Example circuit

Actual symbol of MOSFETs used
MOSFET symbol
Source: http://simreal.com/mediawiki/images/2/26/Fig06-02.jpg


1 Answer 1


Q1 is upside down and the internal body diode is going to conduct.

Also note that you do not need a P channel MOSFET.

  • \$\begingroup\$ Thanks for your reply! Would this hold true even if the diode is replaced with a general load, e.g. a resistor? The P channel MOSFET was included in the picture I stole... \$\endgroup\$ Apr 27, 2015 at 13:22
  • \$\begingroup\$ Yes this circuit works with any type of DC load. If the load resistance is quite low you might need to choose MOSFETs with low on resistance so that when the load is activated by both inputs being high the full Vdd is switched to the load with only a small volt drop across both FETs. Where did the picture come from btw? \$\endgroup\$
    – Andy aka
    Apr 27, 2015 at 13:36
  • \$\begingroup\$ Great, I'll try that once I have access to my board again. Added source to the image in the post. \$\endgroup\$ Apr 27, 2015 at 13:57
  • \$\begingroup\$ Which website (or maybe book) did the incorrect circuit diagram come from) \$\endgroup\$
    – Andy aka
    Apr 27, 2015 at 14:34
  • \$\begingroup\$ I drew it myself (using Digikey SchemeIt), but the general idea came from protection circuit for a LiPo. (powerstream.com/p/GMB_PCM308EB.2.5V(SMS8205).pdf) \$\endgroup\$ Apr 27, 2015 at 14:41

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