Calling Gordon McComb

Greetings Gordon, Regarding your H-Bridge Motor controller on page 260 of 'Robot Builders Bonanza' 2nd Edition, (figure 18.6), using 4 TIP41's.

My Scenario; I cannot, whatever I do, get the circuit up & running. I've constructed two of these, side by side, on perfboard using the chassis of my TrackBot as a heatsink. All transistors are insulated with no shorts to the heatsink nor to each other. Out of desperation, (in case I made a wiring error), I also breadboarded the circuit using new components to no avail. I'm running two Motors in parallel from each H-Bridge, which are connected as shown in your circuit diagram. My Motors are rated at 24 Volts and are being run at 36 Volts in this configuration. My test signal is a 9 Volt Battery running into a 7805 Voltage Regulator with the Earth, (-ve), side connected to the Earth side of the H-Bridge and the + ve lead as a signal to R1 - R4 etc. The forward & reverse leads have never been connected at the same time.

My Problem; When triggered with the +5 Volt signal, the transistors are not turning hard on and the motors are running about 1/10th speed. I'm using 1K Resistors as R1 - R4.

My Attempted Solutions; My initial thought was that, as the transistors are not turning hard on, that they are not being driven hard enough. So, the next step was to connect

2N2222 as drivers to the TIP41's. Nope - same result. I then replaced the TIP41's with NPN Darlingtons TIP122 - same result. I've also tried a direct signal to the bases of each pair, (in all cases above), to no avail.

Is there an error in this circuit or should I be looking elsewhere for the error? What I want to do is run a pair of H-Bridge's from 4N25 opto-isolators so I can use the Parallel Port of a Laptop to control them.

TIA

-- Regards, Murray McKenzie New Zealand. A person who works with his hands is a labourer. A person who works with his hands and his brain is a craftsman. A person who works with his hands, his brain and his heart is an artist

The book talks about that circuit for simple demonstration purposes, and later on mentions there are better methods of creating H-bridges. This point probably wasn't made clear enough, though. You either need to use a voltage pump for the high-side transistors, or complimentary-pair transistors. The errata sheet on robotoid.com discusses this a bit more.

Chuck McManis has a pretty good all-purpose H-bridge on this page:

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It's a more complex circuit (I tried to make mine too simple!), and he uses small signal transistors in opto-isolators to drive the power transistors. Precisely what you want to do. The power transistos are complimentary pairs, as I noted above. Try it out.

-- Gordon

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