H bridge transistors heat up

[I14R10]

I built H-bridge with MJE2955 and MJE3055 using this schematics **broken link removed**. It works, kind of. The motor spins one way and other way as it should be. The problem is that the voltage across the motor is only 1V. This H bridge is powered from 9V supply. And along with that problem, the transistors really heat up. When I measure the voltage across the power supply, it's also 1V, so I suspect there is a short circuit. But I can't find it after few hours of experimenting. I'm almost ready to give up and go with relays. Is there a mistake in that schematics?

 

[crutschow]

Is the 9V from a 9V transistor battery?
What is the current draw of the motor?
Does the motor work okay if you connect it directly to the power supply?
You may just be drawing more power than the source can supply.

 

[I14R10]

The motor does work OK when I connect it directly to power supply. 9V is from ac dc adapter. I played around with the base resistor value and it seems that my resistor was wrong. I had to put 400 ohm resistor to the base of every transistor. Now it works. Sorry, I always spend 5 hours trying to make it work, and then when I write a thread asking about what's wrong, I always find a solution within 10 minutes of posting.

 

[crutschow]

As a rule-of-thumb a BJT requires a base current of at least 1/10th of the collector current to be fully on (saturated).

 

[FvM]

The circuit is simply rubbish. With open switch, both transistors are turned on, shorting the power supply.

 

[d123]

(Q3 + Q2) or (Q4 + Q1) might be a nicer combination, but not both switches pressed at the same time.

 

[I14R10]

Yes, I already fried my finger on one transistor . I plan to use it with microcontroller and always keep one or both inputs low.

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Is this circuit any better http://www.talkingelectronics.com/projects/H-Bridge/images/H-Bridge-6.gif ?

 

[FvM]

Is this circuit any better

Looks better to me. Take care not to activate both inputs simultaneously.

 

[I14R10]

Or I could simply move the switch from first schematics so it's not toward the ground. Instead I could put it after R5 and R6. That way the inputs are always connected to the ground.

 

[d123]

Hi,

Moving the switch would only change the NPNs always being on to the PNPs always being on. The second schematic is a better choice. As you are using a microcontroller, I understand you can program in a little dead time between forwards and reverse so the two inputs never coincide.

 

[Gorgon]

Is this circuit any better http://www.talkingelectronics.com/projects/H-Bridge/images/H-Bridge-6.gif ?

It looks better but you should add some pullup and pulldown on the transistor bases. Pullup on the PNP, and pulldown on the NPN. In addition to that you should add some deadtime delays between the switchings.

 

[Audioguru]

The circuit is simply rubbish. With open switch, both transistors are turned on, shorting the power supply.

The horrible circuit text says it has been tested. The website in India is owned by a graduated engineer.