A High(er) Power H-Bridge for Servo control - Prototype layout

by Ivan Hamilton 9/3/2011 9:43:00 PM

Ok So I laid out a board. This is layout for the prototype board. I'm going to make a single-sided PCB on the mill, so bottom side tracks only and jumper minimisation was the goal.

I certainly haven't perfected PCB isolation routing, so a single track between 0.1" DIP pads was the smallest width I allowed. I'll admit the component placement was largely manual (into relevant sections), but the routing was 95% auto-router. A couple of adjustments were made after the first prototype - this is pretty close to the layout.

Basic H-Bridge Layout in Eagle

I printed the layout and stuck it on the board before routing & drilling, it is a handy guide when inserting components. I certainly wouldn't advise a paper-overlay (absorbing moisture, catching fire, etc) but it's fine for a simple prototype.

Bottom of completed board

It's pretty ugly. I had numerous problems with the isolation routing (breaking tips, very wide). I've been using a Dremel 300 Series. The standard collet setup was just awful (massive run-out), I'd heard about using the Dremel 3-jaw chuck since this was more accurate. I put one in, and measured its run-out... yes, it is much better than the collet arrangement, so that's what I used on this board. After finishing the board (and breaking 2 tips), I rather accidentally was watching the tip of the spindle side-on and discovered that, when spinning, the bit was prone to break into periods of vibrating wildy (visually estimated at ~1mm). This explained the broken bits and wide cuts. Whilst invaluable as a hand tool, a Dremel is simply not a precision machine tool.

A Proxxon Professional drill/grinder IB/E has been ordered. With hardened steel collets and a manufacturer stated run-out of 0.03mm (1 thou), these seem to be preferred by the PCB routing hobbyists.

Top of completed board

So, besides a forgotten pullup resistor, decoupling capacitor and 2 x 1K resistors in series to make 2K, it's pretty close to the original design.

For an initial prototype - it'll do. When the design is a little more stable & tested, some cheap Chinese factory PCBs (ITead Studio or Seeed Studio) may be in order.

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CNC | Electronics

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"My inner nerd can beat up your inner nerd."

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