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Can two N-Channel MOSFETs be connected in series to perform a "AND" function?

e44-72

Member
Hello,

I'm designing a circuit to ring a bell using a code set by the user. When powered the circuit steps through the code which is set via DIP switches to decide where a "ding" or a space should be within the code. Each "on" position within the code actuates a modified car relay which hits the bell. The code is 8 positions long and once finished it remains in an end state where nothing more happens so long as power is still applied to the circuit. Upon the next power on the code is reset and it starts the process again.

The circuit uses a 555 to produce the pulses (at about 1.5Hz - 4Hz) which actuate the relay arm, and a 4017 counter with DIP switches on the outputs to set the code. The output of the 555 and the DIP switches would ideally go through an "AND" gate and the result would activate a MOSFET to switch the relay on. Instead though I connected two MOSFETs in series so both have to be on at the same time to actuate the relay, the reason being that I already have a lot of them and I didn't want to buy a pack of AND gate IC's to only use one AND gate per circuit. One problem I can see with this is that the the "source" to "drain" connection between the two MOSFETs would be left floating, so I pulled it ground via a 39k resistor.

Other than that though is there anything wrong with doing it this way? I've tested the circuit on breadboard and it works well, but I didn't know if there might be any potential problems doing it like this. I have plenty of 2N3904, 2n3906 and BC337-25 transistors so if there is a better way to perform the AND logic with discrete parts please let me know.

The circuit runs off of a 12V 1.25A DC Power adapter, and the car relay is rated at 12V 200mA.
I've attached the full schematic below:

Thanks in advance for your help.
 

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e44-72

Member
Thanks for the replies.

I can't see anything wrong with it and it seems to work ok. It's just that I've seen some forum posts that imply you shouldn't connect two MOSFETs like that to perform an AND function without specifically saying why. So I thought I'd ask for some conformation on it, as I'm only an amateur enthusiast when it comes to electronics.

Thanks again for the help.
 

crutschow

Well-Known Member
Most Helpful Member
That should work fine.
You don't need R13 , since it still won't turn on until both gate voltages are high either way, but it won't hurt anything.

Below is the LTspice simulation to the 2-MOSFET AND-gate function:

1615146599295.png
 

e44-72

Member
Thanks Crutschow,

The reason I added R13 was because I was worried that the floating connection between Q1 and Q2 could potentially damage transistor Q1. Thinking about it now it can't do, because even if it floated to 12V the maximum Vgs rating for IRF510 is +/- 20V anyway. I think I will omit R13 due to the current path it forms when Q1 is on which illuminates the LED slightly when the bell isn't connected which I don't like, or change it to a higher value.

Thanks for your help.
 

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