H-Bridge Driving

Cool. Now tell us something about it's construction and how you have things wired up.

It's possible that the PIC can't drive it, but I can't believe that the TIP120 won't do it. Are you using two separate power supplies (one for the PIC and one for the bridge/motors)? If so, are the grounds connected? Measure how much current the H-Bridge draws from the 5V supply that "works" and report that back to us.

There is a pic of it and a crude (sorry) diagram.

I'm sure the Tip120 can do it, but again, The pic is not activating the Tip120.

I'm Not using 2 power supplies yet, but will.

Thanks for the Help,

Rick-

As it turnsout the PIC will directly activate the H-Bridge. I had it programmed incorrectly and didn't realize it.

Rick-

NPN bridges are okay to learn from, but I wouldn't depend on the design for anything long-term or serious. The issue here is that the "high side" transistors will never fully turn on. So, there will be drop of several more volts than what you'd expect just through the junctions of the transistors, and there will be some extra heat dissipated. For small motors and larger transistors this should not be a problem.

H-bridge design is very much both science and art. Once you master the basic approach, you can try such things as voltage doublers on the high-side transistors (c.r.m. contributor Steve Vorres has a nifty design-around that uses a 9-volt battery and no pump--don't know if he's documented it anywhere), complementary-pair PNP/NPN bridges, MOSFET bridges, etc. Then there are the all-in-one bridges like the L293, but they're not as fun because you can't readily see what's happening inside.

-- Gordon Author: Constructing Robot Bases (Forthcoming) Robot Builder's Sourcebook, Robot Builder's Bonanza

Rick Cassidy wrote:

It might work, but only barely. The 3055s have very little gain, especially at high current. If you can get some 2955s, you can improve your circuit like this:

Steering transisters tr1 and tr2 are what you need to add. tr3 and tr4 provide current limiting in case you enable forward and reverse at the same time.

The upper transistors, being NPN's, are working in emitter follower mode, so while they can have high current gains, they can have no voltage gain. In fact, they won't even have a voltage gain of 1. Their output will be at least the base-emitter drop lower than the micro's output.

So if your micro can muster say 4.9V into that base leg, and you loose another, say, .2V through that current limiting resistor (hard to guess without knowing values), then the base-emitter drop of the top transistor of .6 - 1.2V, you might only get 3.5V applied to the top side of the motor. Your lower transistor will also drop .2 to 1.1V collector-to-emitter. So you will be applying something like 2.5V out of your 9V supply, which will be no better than if you applied 5V, 6V, etc., to it.

Without a level shifting circuit, using a higher power supply on the motor drive circuit won't result in any additional voltage out of that H-bridge circuit.

If you check out

they have a Dual H-bridge driver that drives from a 5V TTL/CMOS source and will turn handle 2.5 Amps / bridge up to 18 vdc or so.

-Kip

Replace with darlington transistors, or a LMD18240 H bridge IC

Does the LMD18240 H bridge IC work with driving voltages less that 10 V? This seems to be a common problem with the LMD18xxx H-bridge drivers from what I understand.

AFAIK, there's no such chip. Dan must be confusing the LMD18200 and the LMD18245, both excellent chips that don't work below 12V.

Not so much of a "problem" per se than a design criteria. It is a characteristic of the DMOS transistors used in the LMD18200 and chips like it. A bridge like the LMD18200 is designed for automotive or industrial control, and it would be extremely rare to encounter voltages under 12 volts in these applications.

Like so many chips, the '18200 was probably designed for a specific large customer, and the datasheet specs reflect the needs of that customer. It's then released as an open product for everyone, and in the case of the LMD18200, there is but very minor notation of the minimum voltage requirement.

-- Gordon Author: Constructing Robot Bases (Forthcoming) Robot Builder's Sourcebook, Robot Builder's Bonanza