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What obvious thing am i missing?

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breadboardguy

New Member
I need to get my h-bridge up and running for my robot, but i cant even make a simple n-channel fet to work!

Can someone tell me what is the obvious thing that i am missing in the circuit below, its what afrotechmod said to do on youtoube.

I have heard that low voltage on fet can cause issues, but i tryed with 6volts on the drain and ... nothing.

Need help.Quick please.

http://www.youtube.com/watch?v=Te5YYVZiOKs
 

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Hero999

Banned
You're using it as a source follower so the gate to source voltage will only be 3.5V which isn't enough to turn it on.
 

Pommie

Well-Known Member
Most Helpful Member
Swap the motor and fet positions. Also connect the negatives of the batteries together.

Mike.
 

be80be

Well-Known Member
If you want to switch a motor with a fet it's like this the 5 k and 20k may not be needed if you get the right fet
 

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breadboardguy

New Member
I did'nt knew that, must be my newbieness.

So i will drive the two n-channel on the high drive side with two 2sa684 at 9 volts, i tested it and it works. Its just more complicated.

If i use a pic mcu at ttl levels for the gate input, should i connect the two batteries(of the dc motors and of the pic mcu) ground together?

You really helped me, i was going crazy.

(is it normal that the irf511 floats high if i dont ground the gate?)
 
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Hero999

Banned
One that's no h-bridge If it was would look like this

Read this H-bridge secrets part 1
The high side MOSFETs on that circuit either need to be bootstrapped, be P channel or receive the gate drive from a higher voltage than the motor.
 

be80be

Well-Known Member
Hero the OP said H-bridge he posted a fet being used to switch a motor. I didn't draw that I just posted it to show what one looks like. If you or OP wants to know how to make one then please read here H-bridge secrets part 1
 
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breadboardguy

New Member
Completly random question: Can i source an rc servo motor with a 5 volt regulator? (7805)

My circuit (with the2SA684 and the irf511) works very well, whats is only left now is to solder 8 irf511 fet, 4 2SA684, 2 7805, 3 ic socket, 10 screw connectors, 2 big caps and a lot of diodes and resistors on a 2 inches by 3,5 inches veroboard... It should take me at least two days for a such small thing.

Thanks for the help.
 
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