Tuesday, November 1, 2011

A note about H-bridges

If you are a n00b like I was back in freshman year, or just forgetful (like I am now) you might want to take note before you burn out a bunch of transistors.

At some point you may realize you only need 2 inputs to control your h-bridge. You will tie the left hand inputs (base of Q1,Q3) together, and the right hand inputs (base of Q2, Q4) together.

Likely, burning transistors will at some point result, because the inputs will be left floating. For me, I accidentally pulled out the input wires.

"The important thing is you cannot leave any of the terminals AC and BD float. Let’s look at the AC terminal in more detail. Since A and C are tied together, and if it is left float then there will be a path for current through the collector of Q1, through its base, through the base of Q3, through its emitter to the ground. So, Q1 and Q3 are on resulting a short circuit. So, you must be careful if you are thinking about reducing the control pins by tying A with C and B with D. You just can’t leave the tied terminals open or float. They should be either grounded or pulled high."

Read more here: DC Motor Interfacing to PIC Micro. Scroll down to the "Important Note."

Convergence Plots

Argh, as usual, I never really finish any personal projects thanks to schoolwork...but it isn't all bad, here's a school-related project that is really cool:

You can solve the equation shown by posing it as a problem mapping from R2->R (rather than the complex numbers C->R). You can do this by expanding it out if you substitute the term a+bi for z.

Then I used three different methods of finding minima: Newton's method, fixed step-size gradient method, and the conjugate gradient method. The gradient methods were sloooooow especially at the center where the gradient is small.

Yellow means it converged to the point [a,b] = [0,1]. Green: [1,0]. Blue: [0,-1]. Red: [-1,0]. Orange converges to the origin [0,0]. White means it didn't converge to any of those but didn't go wildly careening off in some direction (diverge). Black means it did diverge, though I actually came up with that late in the game so some of these plots might not be totally accurate according to that legend.

In other news, some progress on the laser cutter but that's about it.