What obvious thing am i missing?

breadboardguy

New Member

• Aug 7, 2009

• #1

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.

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

 

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Last edited: Aug 7, 2009

Hero999

Banned

• Aug 7, 2009

• #2

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.

 

P

Pommie

Well-Known Member

Most Helpful Member

• Aug 7, 2009

• #3

Swap the motor and fet positions. Also connect the negatives of the batteries together.

Mike.

 

be80be

Well-Known Member

• Aug 7, 2009

• #4

One that's no h-bridge If it was would look like this **broken link removed**
Read this H-bridge secrets part 1

 

be80be

Well-Known Member

• Aug 7, 2009

• #5

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

• Aug 8, 2009

• #6

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?)

 

Last edited: Aug 8, 2009

Hero999

Banned

• Aug 8, 2009

• #7

be80be said:

One that's no h-bridge If it was would look like this **broken link removed**
Read this H-bridge secrets part 1

Click to expand...

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

• Aug 8, 2009

• #8

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

 

Last edited: Aug 8, 2009

breadboardguy

New Member

• Aug 8, 2009

• #9

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.

 

blueroomelectronics

Well-Known Member

• Aug 8, 2009

• #10

RC servos work fine with 5V