I've this circuit:


simulate this circuit – Schematic created using CircuitLab

I have a doubt, when I press the SW1 the LAMP1 turns on, when I press the SW2 the light turns on but the P channel MOSFET (IPB120P04L-03) is reverse polarized. What damage can it cause? And can it be resolved? Thanks in advance

  • \$\begingroup\$ Assuming V2 and V3 are the same voltage, why do you believe that the resultant voltage can damage the device? \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 11 '17 at 20:33
  • \$\begingroup\$ @IgnacioVazquez-Abrams thanks for answers. I don’t know if device can be damaged. I don’t know the causes because I can’t simulate the circuit \$\endgroup\$ – Giovm Jun 11 '17 at 21:17
  • \$\begingroup\$ @KingDuken this is the datasheet of PMOS mouser.com/ds/2/196/… \$\endgroup\$ – Giovm Jun 11 '17 at 22:19
  • \$\begingroup\$ As drawn, the diode would have 12V on either side and would not be reverse biased. If the supply attached to SW2 ends up several tenths of a volt higher than supply V2 then it would forward bias the diode and could potentially damage that, if V2 and V3 were able to supply enough current. One fix would be to put a series diode between R2 and the FET, with the cathode on the FET's source terminal. That would prevent any current from flowing through the FET body diode. \$\endgroup\$ – 65Roadster Jun 14 at 4:08

That is not a good circuit.

In real life voltage sources vary from their nominal values. For example in a car the 12v battery can be anywhere between 9v and 16v.

If V3 is more than about 0.7v more than V1 and SW2 is activated you could get damaging current flowing through the body diode of the MOSFET.

Similarly is V1 is higher than V3 and both SW1 and SW2 are closed you could get damaging current flow through the MOSFET into V3

One way to modify the circuit if you want the lamp to operate if SW1 or SW2 is closed is to add two diodes feeding the lamp: one from the MOSFET and one from SW2.

That way whichever supply is the higher will supply the power to the lamp.

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  • \$\begingroup\$ Thank you for your answer. If I use one voltage source it's needed a diode? i.stack.imgur.com/m40bb.png \$\endgroup\$ – Giovm Jun 12 '17 at 9:48
  • \$\begingroup\$ If V2 and V3 are the same voltage source then you don't need the diodes. In that case SW2 is directly across the FET. \$\endgroup\$ – Kevin White Jun 12 '17 at 13:57

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