? H-Bridge Output Too Low

Normally you'd use P-channel devices or PNP transistors on the top there... Also your GND side transistors are wired backwards.

P-channel devices or PNP transistors on the top

I'm guessing that that's not what I'm using, how can I tell and what's the difference?

Oh, right. I made the drawing earlier, and it's not quite accurate. The one I used while hooking it up (because it was clearer) is this one:

On 09/10/2006 the venerable snipped-for-privacy@synetech.cjb.net etched in runes:

I suggest that before you pick up a soldering iron, you first learn to use Google. Type in "H Bridge" and you'll get more than 81 million hits. The first two are here:

They are both excellent descriptions and show you what is wrong with your circuit.

On 09/10/2006 the venerable Alec S. etched in runes:

If you haven't used base resistors your transistors will be smoke. As Alec said you should use PNP transistors in the top legs. There's an excellent treatise on H-bridges here:

There is one 'key' problem. The top transistors are running as emitter followers. Typically the voltages on the emitters will be about 0.6v below the voltage on the base. The bottom transistors, are running in common collector mode. As such, the current drawn into their bases, will rise sharply, when the input voltage rises much above 0.6v. The input voltage will therefore be clamped to only a little above 0.6v, and there will be almost no output swing... Though the circuit will still be limited, simply adding a resistor in the feed to each base of the two 'lower' transistors, will allow it to start working.

Best Wishes

What soldering iron? I made it on a makeshift breadboard.

million hits.

I suggest that before making condescending posts, you learn to properly quote; full-quoting is rarely a good idea. Seriously, you sound like you are reprimanding me for doing a sloppy job on some customer's equipment or something. I am just doing this for fun as an experiment at home. If you're so great at electronics, then why didn't you just give me a straight forward answer instead of scolding like you're my electronics teacher, for not being as great as you? I'm sorry for polluting YOUR Internet with questions; maybe you can yell at everyone until it gets completely cleaned up.

Once I had learned what the name of what I was designing is, I was able to research them and I did what you suggested quite a long time ago. I'm familiar with the basics of H-Bridges and have even seen one of the sites you referred to. However, most of those 81,000,000 hits do not mention the step down that I am experiencing, so I was unable to find an answer. Perhaps you could suggest a better search query? I've already seen many pages that discuss H-Bridges and seen different designs for them. The problem is that I have neither the resources, nor the money to buy a premade H-Bridge (that would be a waste for what I need it for anyway) or parts to build my own as per the specs on such pages. I am using whatever components I can strip off of old equipment.

The first two are here:

circuit.

The first one is not bad. I will see if I can scavenge some MOSFETS (I think I've seen a couple on the TV's circuit board.)

Ah, thanks. I was wondering why all the schematics I saw had resistors in them (if not lots of other things.)

I'll add them, but what resistance should they be?

The easiest way to tell what you are using is via the part number of the transistor.

The TIP120 is an npn darlington pair transistor.

You don't indicate where you were had the DMM hooked. Was it across the motor?

If so, with no load on the motor you probably won't read a very high voltage.

You would be better off hooking the DMM in series with the motor and putting in the Current mode rather than the Voltage mode. Then see what the current through the motor does as you grab the shaft and put resistance on it (wear gloves!!!!).

Did the motor actually turn or not??

tim ab0wr

I'll see if I can scrounge up some PNP and NPN MOSFETS today but the transistors I used are still fine. I didn't apply a huge voltage to them, so they wouldn't be smoke. In fact, I found that simply touching their bases, caused them to open and the circuit to complete (it's a good thing I was testing with a low voltage-a AA battery-at that time since I was accidentally opening both sides by merely touching them!)

I didn't hook up the motor at all at right away. I first hooked up the DMM to see if the polairty was actually reversing or not. When I saw that it was, I tried the motor instead, but it did nothing. I then hooked up the DMM again to read the voltage (I had just looked at the polarity the first time), and saw that it was 1/1000 of the 9V that measuring the battery alone gave.

Shaft? Gloves? It's a tiny, little toy motor.

No, there wasn't enough voltage coming out.

What's wrong with using the transistors and resistor values on the link that you posted as having built?

You want ~15 mA base current, based on the datasheet to saturate the 2SC815. At 5 volts, use a 330 ohm resistor. In addition to the problems outlined in other posts, the sad news is, that's a really wimpy transistor. Keep rummaging around on that TV circuit board - maybe you can find some PNP's and some different NPN's.

Ed

Did you hook the emitter of the top transistor to the collector of the bottom transistor to complete the circuit? (i.e. jumper over the motor)

If not, I'm not sure what voltage you thought you would see since you don't have a complete circuit for the collector-to-emitter current to flow through.

If you did connect them together then I'm not sure what you are reading with the DMM other than the voltage drop across the wire between the emitter of the top transistor and the collector of the bottom transistor. That's going to be a pretty small voltage since the wire will have a small resistance. My guess is that this would also let the magic smoke out of the transistors since there is no load limiting the current through them.

tim

There is a much bigger problem than that; the circuit isn't an H-bridge at all! The two "top" transistors are in parallel and the the two (backwards) "bottom" transistors are in parallel!