Drive 2N7000 direct from 4071 CMOS OR

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dannix

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Hi,

I am wondering if the attached will work before I get the parts in. Would appreciate it if one of you fine experts cast an eye of the attached schematic.

I am wanting to turn on 4 fans if one logic input is high, thus the DP switch or turn any fan on seperatly via the associated switches using the other OR input, the output drives a 2N7000 mosfet to switch the fans on.


Thanks in advance!
 

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Hi Dannix,

You forgot to add pull-down resistors on the inputs of the 4071's,
use 5 x 100k or so. You also forgot to connect diodes across the
motor terminals. The presence of R1 is not clear to me, it's not
needed.

on1aag.
 
Hi, R1 was me playing with pull up/down resistors in a simulation. I did not have any diodes as the fans are brushless with internal protection. But I may add them just in case they get changed in the future.

So I need 100K pull down on the inputs? what is the purpose of this?

Forgive me I am still learning. I read that I should add a ferrite bead before the 2N7000 gate, I dont want to do rappid switching, so I guess these are not required.
 
I wonder, even with brushless motors whether we need the Back emf protection diodes. After all the mosfet is switching OFF the current thro the motor. even then back emf is generated and the mosfet
 
How much current do the fans draw?

The 2N7000 is only good for 200mA.

You're better off with BJTs for small loads like this, a 2N2222A would do the job.
 
Hi, I have added the back emf diodes anyway. The fans are 160mA each, I have 2N7000's already so unless there is a reason I should not use them I will stick with them.

I would have done this on bread brd 1st, but I only have one OR gate IC left and really wanted to be sure there was no obvious mistake.

Can anyone explain the need for the pulldown resistors on the inputs?

Thanks everyone, most appreciated!
 
hold on, is it so the input is not floating when the switch is off? to prevent the IC going nuts from some outside infulance?
 
As you said you don't need back EMF protection on brushless motors with built in drivers.

The pull down resistors prevent the gate inputs floating and picking up noise which would give unexpected results.
 
Ok, thanks for you help guys! I am going to build this on breadbrd tonight (time permitting) and will let you know how it goes tomorrow.

Thanks again
 
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