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? H-Bridge Output Too Low

Hi,

Several years ago I was trying to figure out a way to allow a motor to be controlled by a computer interface (to allow it to go both forward and backward by just applying a voltage on one of two lines). I designed a device that could (theoretically) do it using transistors. I later learned what the name of the device I had designed was. :)

Last night I finally managed to build an H-Bridge. I had previously tried it, but there were problems (mostly materials wise) that prevented it.

I made it last night and hooked it up to a DMM to test. I used a 9V battery as the external power source and gave it a run. It worked great, the DMM read about -9 when using one lead, and +9 when using the other. The problem is that it wasn't =B19V but rather =B19mV. The polarity was correct, but the output was about 1/1000 of the input. This makes it useless since I don't have a 9,000V power supply to drive the motor.

I'm guessing that the transistors I used are the problem since the rest of the device is just wires; that they are somehow also acting as resistors.

Here's the schematic I made of the H-Bridge I built:

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I'm using regular wires from various computer parts (the kind that connect buttons and LEDs on the front panel to the motherboard), and I got the transistors from the circuit board of a television. They are marked "C815 Y312" (I checked them with the diode testing function of my DMM, and they they all tested tested fine-and yes, I've got the pins correct.)

Like I said, the polarity works, but the output is far too low. Any ideas on what's going on and how to fix it?

Thanks a lot.

Reply to
Alec S.
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On Mon, 09 Oct 2006 12:03:33 -0700, Alec S. Has Frothed:

CHA CHING!!!!!!!!!!!

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794
Reply to
Meat Plow

So what's the problem and how do I fix it?

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

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but without the resistors.)

Reply to
Alec S.

You'll want to read up on basic transistor theory, that should clear things up a bit. Essentially, bipolar transistors are current-driven, that is in the case of the NPN type you show you need to have a voltage on the base which is positive in respect to the emitter with the current limited by a resistor to turn the transistor on. Often these days MOSFETs are used instead for applications like this which are voltage driven and a little simpler to work with. If you look up H-bridge there should be lots of examples out there you can simply copy, there's even single IC's which incorporate an entire bridge for you.

Reply to
James Sweet

Actually, I need more advanced transistor theory, I've already got the basics: a switching device that controls electric flow. What I need is more information on the different types, the internals (what's causing the drain), etc.

I'm aware of all the things you mentioned, but it won't help because I don't have the money or resources to buy anything. I'm using components that I'm scavenging off of old electronic equipment.

I'm currently looking around for some MOSFETs that I can salvage.

Reply to
Alec S.

That's all part of what I'd consider basic transistor theory, there's lots of information out there.

You know some companies will send you free engineering samples, Analog Devices is good about this, as is TI, National Semiconductor and Dallas/Maxim, just try not to abuse the system and request only what you will actually use. Companies that are pleasant to deal with and liberal with the samples I tend to recommend frequently to engineers at work, what goes around comes around.

If you want to salvage parts, old CRT computer monitors usually have a decent supply of power semiconductors in them.

Reply to
James Sweet

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