I have made a dual H-Bridge Board. My PIC16F84 doesn't have enough juice to Activate it. I had a Tip120 Darlington lying around but that wasn't enough either. What could I use to activate the H-Bridge Circuit?? (Straight 5 Volts works) thanks, Rick-
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.
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.
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.
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.