Motor H-bridge

The specifications of this h-bridge were mostly dictated by the small Japan Servo 127K9720 servos I am using. They have a fairly common encoder resolution but were designed for 12V operation, which is fairly low. Maybe this will be an advantage rather than a handicap.

Since these motors are only rated 20W continuous and 100W peak, I am using components with appropriately small ratings in order to reduce cost to a minimum. I am sure most of you will appreciate that, as there seems to be very little in the way of economical open source motor control designs out there. I am aiming to keep the whole design below $20 per controller, but without sacrificing efficiency or reliability.

This is my first power electronics project, so suggestions and comments are appreciated. -fenn


N channel mosfet, 35A peak 60V, used for switching power to high and low sides of the bridge. $0.37 each in qty. 50 from digikey, part no. =IRFZ44N -- oh no! it seems the reason these mosfets were so cheap is that digikey is trying to close out the product line, so get them while they're still there! IRFZ44E would work well as a substitute.
stupid.. little.. gray pads!! grrr!
the mosfet heatsink tab is connected to the drain terminal, so i need to isolate it from the heatsink.. digikey p/n 345-1079-ND $0.17 in qty 10
freewheeling diode,
may not be necessary due to the relatively fast reverse recovery time (95ns) of the IRFZ44N. :other candidates:|
  • SB560-E3/1GI-ND 60V 5A schottky $0.54 ea qty 1;
  • 1n5400 or 1n5400-t, 3A 50V standard and fast recovery, $0.20 and $0.41 each, respectively
  • 1N4148 digikey part no.. ? $0.07 ea. - exactly which version was jmk talking about?
  • 1N4004| general purpose diode, 400V 1A, $0.069 in qty. 10 from digikey, part no. 1N4004-E3/23CT-ND -- recovery time of 2us is way too slow to work as a freewheeling diode; forget it.
clamp diode qty 2
30V 1W zener diode to keep inductive kickback and back-emf from destroying the mosfets. digikey p/n 1N4751A-TPCT-ND $0.10 each min qty 10
power capacitor
to supply power at the switches, and reduce EMI on the +24V supply lines
  • CAP,ELEC,SNAP-IN,4700UF,50V,105C,20%,25X30MM, Jameco p/n 331897PS $2.19 min qty 1 (chosen for its small size)
  • CAP,MYLAR,.22uF,100V, Jameco P/N 26972PS $0.19 each min qty 10
gate driver transistors (NPN and PNP)
to convert 5v logic from the micro/parport to appropriate levels (+32V) for driving the mosfets. MPSA06 80V 500mA transistor, $0.17 ea from digikey, $0.05 ea min qty 10 from jameco p/n 26462PS, and MPSA56, $0.26 ea from digikey and $0.06 ea min qty 10 from jameco p/n 210606PS
current sense resistor
0.1 ohm 5W wirewound (? looks like a sand resistor) from futurlec p/n R0001R5W $0.20 ea; this will give me 0.5V at 5A, so will have to diddle with adc voltage reference to pick up such small voltages.
current limiter
an op amp to amplify the voltage across the current sense resistor and cut off voltage to the mosfets if above a certain value. need to latch it so it acts like a breaker - how to do this? is this necessary with fuses? oh well a little redundancy never killed anyone. digikey p/n NJM#4558D-ND $0.36 each qty 1
power supply connector
3 pole pcb mount screw terminal for 24V 36V and GND- p/n G12942 $0.20 each min qty 5
motor connector
I had a really hard time finding connectors that fit the 127k9720, but eventually I figured out that if i chopped up an atx power connector into columns and whittled one of the plugs down to a "D"-shape, it would work!
shown in the background are the tools used: a dremel with saw blade, and hobby knife. Since there is no retaining clip, the connector tends to fall out, so I will probably use some heatshrink tubing over it.

Attach file: fileatx-hacked-connector.jpg 63 download [Information]

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