Let's Make Robots!

Another H-Bridge Post

Hey hey.

So, it's my turn for an H-Bridge post.

It's a simple one too - I'm looking for a circuit that I can build without confusing the heck out of myself.

I've gone through about 6-7 different articles (with schematics and the like) about them, and understand the concept behind them. I've even tried to put a few together, but have had zilch in the way of voltage flowing out the other end.

This circuit is the one I've tried three times so far, as it's the only one I can find that specifically uses a breadboard.


I'm still having trouble visualizing all the connections in a schematic to get everything to work properly, so this post helped a lot.

However, I still can't get it to work.

I could break down and buy an L293D, which I will soon, but I'd like to build the bridge from scratch to help me with learning more about circuits.

Anyway, if you know of anything that might help, I'd be greatly appreciative. I'm heavily considering just going back to the beginning and finding a book that will teach me from scratch while putting off the robotics until I get a better handle of electronics. 

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What transistors are you going to use? What voltage do you want to run the motor at? And what voltage will you use to control the h-bridge? This may be the same as the motor voltage.

 All these factors will influence the design of the h-bridge.

 Every type of transistor has a particular pin-out, i.e, which pin is the collector, which pin is the emitter, which pin is the base. (This assumes you will be using BJT type, not mosfet). Not getting this right for your particular transistors will mean your h-bridge will not work.

Getting hold of a good basic electronics text is a good idea. But no need to give up robots while you learn! Building simple robots using basic electronics helps reinforce what you are learning. You may spend some time going, "Aaah, I don't get it!", then suddenly, "AHA!!", and you understand. (Later on you forget again, like me, and have to join an online robotics forum to help you remember from long ago. :P )

Ah - those are questions I don't know the answers to yet.

However... I think what the problem is - is that there's not enough voltage coming out the emitter of my transistor to power the motor - so I need to amp that up a bit. I'm only using 3V in, since that's all I've got at the moment... I could actually use the 5V from my Arduino more than likely.

Anyway, I went back to the basics and started testing out a single transistor, making sure I knew exactly how to use it, and I do. I put an LED in place of the motor, and my single transistor circuit works just fine, I just think I need to power it up a few notches.

I picked up one of my robotics books, and now that I actually have parts to compare the book to, it makes a whole lot more sense and I can actually build the circuits inside. I like these simple schematics... I understand these :)

Anyway, thanks for the encouragement - I guess it's all just one step at a time! 

Ha! Actually.... the resistor I was using between the base and the input to the base was just too high of a value. It was 22K ohms - but now that I just removed it, it's running my tiny motor just fine.
Did you replace it or remove it altogether? In almost all cases you'll need a resistor somewhere between the transistor signal source and the transistor's base, otherwise the current flowing into/out of the base may be high enough to damage the transistor. Plus it's a waste of power, but that's normally not such a big deal compared to blowing a transistor =)
While experimenting I took it out. I'll be replacing it later though. I just need to go through my resistors and figure out what value I need. Something less than 22K ohms at least :)