problems with H bridge

[mashersmasher]

so i thought about rigging some transistors to control some motors (which is apparently an H bridge) but any transistors i can find have so much resistance that a 250w computer power supply (for testing) has trouble lighting an led let allone a motor or even a relay. the number on the transistors i'm using are 2n 3906-338 and the motors are 2 that i pulled from a smashed up roomba i found in the trash (no battery though )

 

[NightFalcon90909]

250 watt?!
Optimally, get a few double AA batteries and some 2N2222's... together the whole deal might cost you three dollars or so... then try that and see how you go. Make sure you use the right transistors in the right places- NPN are not the same as PNP!

 

[mashersmasher]

ya it's an old 250w pc power supply and i'm using the 5v rail. so you think the transistors are the main problem? i admittedly only went to radio shack for components. even when i power them from a pin on my basic stamp (<6v and almost no current) it does the same thing. i power the motors from the power supply directly.

 

[NightFalcon90909]

I can't really imagine that it could be anything else. If you have the transistors all hooked up correctly, then they must be blown.

A watt is power which Volt time Amps- so if you have 5 volts, that is like 50 amps... your transistors are most likely rated for a few milliamps!

 

[mashersmasher]

hmm that's definitely a possibility. i heard one hiss and pop but i removed it. the others ran pretty hot but i figured as long as they do there job then the heat was not a concern. well i guess that's what i get for buying radio shack grab bags. i tried using a voltage regulator on the 12 v rail but same thing so i guess they are toast. thanks for the input! i might use one transistor to triger some high voltage relays that i have somewhere

 

[3v0]

You need to post the schematic.

 

[audioguru]

Why didn't you look at the max allowed current of only 200ma for the 2N3906. Then look at the max current (when it starts and when it stalls) for the motor.

If you don't know what is the max current of the motor then measure its resistance and calculate the current.

Attach your schematic. Maybe the motor's current is low enough but the base current for the transistor is too low.

 

[mashersmasher]

uhh i don't have a schematic. i basically just brainstormed the idea then put it together and hoped it would work ( it is wired correctly. i checked multiple times). a knowledgeable friend described it as an H bridge and after looking it up it appears to be exactly what it is. here is a schematic i got off the web just now

**broken link removed**

 

[audioguru]

The circuit from the web uses transistors rated at 800mA and a motor that has a stalled current of 360mA. it uses NPN and PNP transistors.

You are using transistors rated at only 200mA and you don't know the stalled current of your motor. You mentioned using only 2N3906 transistors, no NPN transistors, so you circuit is completely different.

Did you look up the pins on the transistors so they are not connected backwards?

 

[zanes]

And, that schematic is excellent when built with BC's.

 

[crust]

You are probably not supplying enough base current to get the collector / emitter current high enough. The other option is just use mosfets. There are literally hundreds of designs on the web and even on this site that show how to build a very reliable h-bridge with mosfets. The nice thing about them is that they have a built in body diode, so you don't have to worry (on small motors anyway) about inductive spikes destroying them and their on resistance is ridiculously low so you don't even have to worry about heating them up.
Having said that, the 3906 is probably too small to drive any motor without blowing up. The smallest I have ever used is a TIP31 which is in a TO220 and it was toasty even with a small motor and no load on it.

 

[mashersmasher]

thanks for the help! i think i'm going to rip up some dying servos and use their controllers in the mean time while i get some better parts.

 

[mneary]

Old (RC) servo motors can use as much as 1 ampere.

 

[Krumlink]

1 ampere? Mine at 5V (standard size) take up about 100ma.

 

[mashersmasher]

well they arn't really "old". the motors are fubar but the boards are working alright.