I am having an issue with a control signal.

I have a CEP50P03 P channel trench MOSFET that is supposed to switch a 12v power supply.

I am having an issue I can not figure out. When I power the board with 5V everything works as it should.

However when I add the GND connection to the 12v side to use the 7805DT voltage regulator the Mosfet will come on and latch on and will not turn off.

I then tried removing the voltage regulator and power the circuit with only 5v and the fet does not switch at all. I have not been able to figure out this weird behavior.

I attached the shematic below. This circuit is used in an automotive rev limiter module where tach signal is a 0-12v square wave used to read rpm that is then converted to a 0-5v square wave the ATMEGA328P reads and if the frequency is above X ammount the Atmega sends a signal to turn the MOSFET off for X time and the mosfet should turn back on.

However like mentioned above when I have 12V coming in the MOSFET latches on and stays on.


  • \$\begingroup\$ Why are there diodes (D1, D3 and D4) used with the 7805? Moreover, could you please encircle the CEP50P03? \$\endgroup\$ – Huisman Apr 28 '19 at 19:55
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    \$\begingroup\$ Ah... please correct your schematic symbol for the CEP50P03. You drew a PNP transistor :-) \$\endgroup\$ – Huisman Apr 28 '19 at 19:58
  • \$\begingroup\$ Sorry that's what the Eagle library I used had for it. \$\endgroup\$ – Hunter Inman Apr 28 '19 at 20:05
  • \$\begingroup\$ @Huisman I placed the diodes there to see if I was some how having reverse flow through the regulator. No change with them there or not. \$\endgroup\$ – Hunter Inman Apr 28 '19 at 20:08
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    \$\begingroup\$ Sorry but your schematic diagram is really hard to decipher. Doing good schematics would help a lot avoiding errors. However R11 shall go to 12V instead of 5V. Moreover the SIGNAL input conditioning looks wrong too. \$\endgroup\$ – carloc Apr 28 '19 at 20:29
  1. The CEP50P03 is a PMOS which requires a negative gate source voltage.
  2. If the threshold voltage is (typically) -2V, you should not drive it with -2V, but with a bigger negative voltage.

A fast and easy way to get a rough indication what \$V_{GS}\$ to use: check the conditions that are given for \$R_{DS(on)}\$. In this case: \$V_{GS} = -4.5V\$ and \$V_{GS} = -10V\$.


Solution to the problem was too connect R11 to 12v instead of 5v. Too obtain a lower Vgs


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