Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: Robotboy86 on July 18, 2007, 01:42:24 AM

Title: I've adapated someones H-Bridge and I would like comments on it
Post by: Robotboy86 on July 18, 2007, 01:42:24 AM

I saw an h-bridge schematic on http://www.mcmanis.com/chuck/Robotics/tutorial/h-bridge/bjt-circuit.html while I was reading about h-bridges and I adapated his schematic to work without those opto-isolator LEDs.  I then went searching for N-Channel MOSFET's and P-Channel MOSFET's.  I ran into a problem though..

I could only find N-Channel PWM optimized ones..  and to be hoenst, there aren't a lot of decent logic level P-Channel H-Bridges.  As such, how do you convert an h-bridge that uses both P and N bridge transisitors, into that only uses N-Channels??  I put my half way done schematic on the bottom, as you can tell I still have the old p-channel's on there. 


This is my first attempt at a real circiut kind of thing..  besides a simple one on my breadboard that hit an LED on :) So if there are things that are completly out of whack, let me know. 

Thank you again,

Dan


(http://img440.imageshack.us/img440/7995/hbridge2sv4.gif) (http://imageshack.us)

Title: Re: I've adapated someones H-Bridge and I would like comments on it
Post by: Ro-Bot-X on July 18, 2007, 04:49:29 AM

Actualy the schematic as you draw it it will not work. The controll signal for the upper transistors needs to be inverted.

To use only N-channel MOSFETs all you have to do is swap in your schematic the emiter with the collector for the upper transistors (and tie the 10k resistor from the base to the ground). Then it will work as espected. In other words, a 1 on the FWD line and 0 on the REV line will make it work like this: from battery positive through the collector and emiter to the motor (on the left side) and through it will go to (on the right side) the collector and emiter to the ground. The other transistors are off.

I just wanted to add that this design lacks the ability of braking. When both FWD and REV lines are 0, all transistors are off. If the motor was spinning, it will turn into a generator and all the power generated will go to the batteries (and if it is large enough will charge them). Some H-bridge ICs will short the motor leads when both lines are on 1 or 0, this creating the "electronically brake effect" because the power generated will go back to the motor generating a reversing motion thus making the motor to quickly come to a stop.

Title: Re: I've adapated someones H-Bridge and I would like comments on it
Post by: Robotboy86 on July 18, 2007, 11:34:44 AM

Thanks man, thats just what I wanted!

And ya I know about the braking part..  I gonna add that in later today on the image thing.  If you don't mind I am going to post a revised schematic in a while..  when I do i'd really appreciate some feedback.  I subscribe to the idea of the more planning/annoying folks with questions you do.. the less frustarted you are when you build

Title: Re: I've adapated someones H-Bridge and I would like comments on it
Post by: Robotboy86 on July 18, 2007, 01:31:14 PM

Been reading more on it still, it would seem some people say that an all n-channel mosfet h-bridge is very difficult to create?  I am confused about your "The controll signal for the upper transistors needs to be inverted." comment.  The control signal for both lower and upper transisitors are the same.. why would I need to invert it?  Because the signal is going "backwards" on it?  If so, then do I need to change it from 5v to a -5v signal?  I have changed it so that it grounds to the negative on the upper ones.. but after that I am somewhat stuck.

Also, you stated that I need to swap the emitter and collector for the upper ones... how would I represent that in the schematic?  Would I simply just change the arrow's position to match the bottom?  I understand that when I built this, I would replacite the design onto a breadboard(for testing) and simply making sure all the n-channel's emitters are pointing towards the "bottom".  IE Upper emitter to the motor, lower to the ground.

My other question is this, why would I connect the 10k to the ground?  I assume this is because I am inverting the signal from positive to negative for the control??

Thanks a lot.

Title: Re: I've adapated someones H-Bridge and I would like comments on it
Post by: Admin on July 22, 2007, 12:02:37 PM

I personally discourage making your own drivers, and recommend just buying them. They have gotten really cheap in the last year.

This is my schematic for back when I used to make them:
(http://www.societyofrobots.com/images/schematics_h-bridge.jpg)

Title: Re: I've adapated someones H-Bridge and I would like comments on it
Post by: Robotboy86 on July 22, 2007, 03:00:50 PM

Oddly enough, I read the dirver comment in about 6 diffirent places.

Then you said it and now I am ok with using drivers lol

Title: Re: I've adapated someones H-Bridge and I would like comments on it
Post by: Brandon121233 on July 22, 2007, 07:40:49 PM

You can get complete H-Bridge chips for free- like the L298, its a whole lot easier than trying to build one yourself.