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    Use an OR wired with diodes to control the gate in a PMOSFET

    I have two outputs from an open collector IC connected to diodes to achieve an OR wired. The circuit is used to control the turn ON and turn OFF of a PMOSFET, shall I use a pull up resitor in the source input of the PMOSFET? Does the design need a pull down in the PMOSFET gate?

    The MOSFET is always turn ON, even when the gate is 5V, how is possible?

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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    The MOSFET is always turn ON, even when the gate is 5V, how is possible?
    That's by design, need to pull the gate to source potential (+12V) to turn the MOSFET off.

    Respectively, the circuit can't work as sketched. Please clarify the intended function. Why do you connect fixed voltage of +5V to one input. OR function related to MOSFET on state would require to pull one or the other input low, respectively reversed diode polarity and gate-source resistor.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    I have placed the pull up between the source and the gate and the reverse diode but the circuit is still not working.
    I have connected the fixed voltage to simulate the circuit, both inputs are connected to a NPN circuit and to the schottky diode and the gate.
    I have simulated the new circuit but doesn't work. What else should I change?



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    "Not working" means? Diodes still have to be reversed.

    Diodes aren't necessarily required, open collector circuit does already provide wired or.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Not working means the gate is always 12V. It doesn't change.
    I have placed the diodes looking at the gate because I want an OR gate so when one input is "1" the gate is turn off.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    You have some problems.

    The diodes prevent the NPN's from ever turning on the gate.

    Second you need a wired nand it seems:
    If all inputs are 0 -> good -> pull down gate -> turn on pfet
    If any input is 1 -> bad -> float gate -> turn off pfet

    Using transistor logic add a single NPN that pulls down the PFET gate. Now 1=on.

    Now use more NPN's in a wired OR configuration to control THAT NPN's gate.

    Might be nice to look at an integrated high side load switch like FPF2700 which will have a logic level input.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Since your schematic had its parts too far apart and some were sideways, I re-drew it.
    Notice that the Mosfet has nothing to turn it on or turn it off.

    An OR gate output is low when both inputs are low. Its output is high when either or both inputs are high.
    Your circuit don't doo dat.Click image for larger version. 

Name:	OR gate wrong.png 
Views:	4 
Size:	23.1 KB 
ID:	154577



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Quote Originally Posted by Audioguru123 View Post
    Since your schematic had its parts too far apart and some were sideways, I re-drew it.
    Good job Audioguru. OPs thoughts are also equally scattered. Maybe this'll help him understand the circuit shortcomings.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Post #7 and #8 are behind the discussion, OP partly corrected the original circuit in post #3.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Quote Originally Posted by FvM View Post
    Post #7 and #8 are behind the discussion, OP partly corrected the original circuit in post #3.
    Since I recently got a new e-mail address I am also a NOOB here with a new name and my posts are delayed since a moderator must approve them.
    In the partially corrected circuit then the Mosfet is always turned off.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Replying the post #6, I don't need a wired NAND, where if one input is 1 and the other input is 0 then the wired output is 0 and the PMOSFET is on. I need an OR logic, if one input is 1 then the output of the wired is 1 and the PMOSFET is turn off.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    The circuit is again updated but still doesn't work totally.Click image for larger version. 

Name:	Circuit_2.png 
Views:	4 
Size:	9.3 KB 
ID:	154581



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Hi,

    Your most recent schematic shows an AND-style configuration, not OR.

    AND (and NAND) are in series: A + B = C (or NOT C)
    OR would be in parallel: A or B = C

    Maybe start by putting the PMOS gate NPN drivers in parallel - not in series - with the PMOS gate; then consider adding pull-up resistors to make true discrete inverters to each pair of NPNs you have so as to make two buffers whereby any HIGH input = PMOS OFF.

    If I can, I'll attach a schematic later.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    d123 thanks, if you can send me a drawing, it will be really helpfull.

    - - - Updated - - -

    d123 I have placed the NPN in parallel with the gate and added some pull ups but it doesn't have any impact in the circuit.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    I need an OR logic, if one input is 1 then the output of the wired is 1 and the PMOSFET is turn off.
    Thanks for clarification. My understanding of OR is in fact different: If one input 1 than MOSFET turned ON.

    One more inverter needed for the intended function:

    Click image for larger version. 

Name:	or.PNG 
Views:	1 
Size:	15.3 KB 
ID:	154582



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Hi,

    Just to add to what FvM has already provided, the schematic below is a small variation of the same premise:

    Click image for larger version. 

Name:	OR NPN PMOS driver schematic.JPG 
Views:	7 
Size:	144.4 KB 
ID:	154586

    As this is only a simulation, although most work with occasional fine tuning of component values, skepticism never hurts. Simulators solve maths problems with idealised components.

    Notice lovely ground bounce here and there, that's why I added the two Schottky diodes. One of life's necessary evils.

    Hopefully, you should be able to use either schematic in the real world and get on with fine tuning pertinent details to your application now.

    Good luck!



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Quote Originally Posted by FvM View Post
    One more inverter needed for the intended function:
    Click image for larger version. 

Name:	or.PNG 
Views:	1 
Size:	15.3 KB 
ID:	154582
    So what exactly is the intended function?
    I'm confused about that.
    Zapper
    Curmudgeon Elektroniker



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Hi,

    Yesterdays version didn't fulfil the actual requirement of A or B = NOT Y, my apologies, this one does:

    Click image for larger version. 

Name:	or gate discrete pmos driver 2.JPG 
Views:	6 
Size:	130.1 KB 
ID:	154595



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    Hi,

    A simple wired OR can be made with two diodes... and to ensure valid levels one needs a pulldown resistor.
    Then one needs an extra circuit to drive the output Mosfet. A single BJT and two resistors should work.

    So my whole circuit is:
    * 2 input diodes
    * 2 (voltage divider) resistors
    * 1 bjt
    * 2 (voltage divider) resistors
    * 1 output Mosfet

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: Use an OR wired with diodes to control the gate in a PMOSFET

    So what exactly is the intended function?
    I'm confused about that.
    Review post #11, I think it's clear
    I need an OR logic, if one input is 1 then the output of the wired is 1 and the PMOSFET is turn off.



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