H-bridge blowing my transistors

Thread Starter

Code39

Joined Aug 20, 2011
10
I'm currently designing a H-bridge circuit for controlling a DC-motor.
I based my design of this one: http://www.jfunk.org/extras/labs/BipolarHBridgeSchematic.gif

I'm using 2N4403's as the PNP transistors and 2N4401 as the NPN transistors, and 1N4004 diodes.

Here is the schematic with my components:


(I excluded the base-resistors for now, since they are not connected yet.)

I currently have this circuit on a breadboard and using a ATX power supply to supply 12V DC.

The problem is that once i connect the power supply and power it on all of the transistors become very hot and the top-left PNP blows after about 10 seconds.

Does anyone have an idea about what I'm doing wrong here? Could it be a bad transistor or something like that? I've checked all of the diodes and they're OK.
 

praondevou

Joined Jul 9, 2011
2,942
You need to post the base drive circuitry.

What is the motor rating?
Was the motor connected when you blew up the transistors?
Did you make sure that NEVER both transistors of one leg are ON?
 

Thread Starter

Code39

Joined Aug 20, 2011
10
The motor is a 12V DC motor and it was connected while testing. Each of the transistors have a 100 ohm resistor connected to the base, and these resistors are then connected in a cross so that the base resistor of the top-left transistor is connected to the bottom-right and the the top-right is connected to the bottom-left. None of the transistors bases where powered during testing, so the motor was not running.
 

mhastie1234

Joined Feb 10, 2012
29
How are you driving the transistors? They should not all get hot at the same time unless your turning them on at the same time. The diodes are only there for voltage spikes when you turn off the motor. Are you driving the transistors with 5 volts and only turning on q1 and q4 or q2 and q3 at the same time
 

mhastie1234

Joined Feb 10, 2012
29
If you didn't have any of the transistors bases powered it should not get hot. Are you sure everything is wired correctly? How are you driving your h bridge
 

Thread Starter

Code39

Joined Aug 20, 2011
10
If you didn't have any of the transistors bases powered it should not get hot. Are you sure everything is wired correctly? How are you driving your h bridge
The bases are not connected to anything except the resistors which then are connected in a cross as described in the last post. So technically the circuit should be idle. These are the datasheets I used while installing the transistors:

http://www-inst.eecs.berkeley.edu/~ee105/sp12/labs/2N4401.pdf
http://www-inst.eecs.berkeley.edu/~ee105/sp12/labs/2N4403.pdf

The transistors on each side are connected collector to collector as shown in the schematic in the first post.
 

Thread Starter

Code39

Joined Aug 20, 2011
10
Please draw a schematic showing what the 100 ohm resistors were connected to.
I think it was like this so all the transistors were turned on all the time so of course they get hot:
This is the current schematic



I think I may have used to small resistors, since mine are only 10 ohms and I was planning to use 100 omhs, due to bad lighting in the room while I was reading the color codes. Would replacing the 10 ohms resistors with 100 ohms be sufficient or should I use higher values?

I have done some simulation in Multisim and found that even with 100 ohm resistors I get too much voltage loss over the resistors to give the motor even close to enough voltage.

I would like to end up with about 6-10V over the motor, more would cause it to move my table assembly too fast.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,249
All your transistors are turned on and are shorting the power supply. So of course they get extremely hot.

You control the speed of a DC motor with pulse-width-modulation, not with the hoped resistance for transistors that are or might be partially turned on.

When a DC motor starts then its current is very high. You are trying to add a resistance in series with the motor which reduces its torque so it might not start running.

The voltage across a running motor depends on its load. Without a load it runs fast with 3V. With a heavy load (and when it starts) it runs not too fast with 18V.
 

Thread Starter

Code39

Joined Aug 20, 2011
10
All your transistors are turned on and are shorting the power supply. So of course they get extremely hot.

You control the speed of a DC motor with pulse-width-modulation, not with the hoped resistance for transistors that are or might be partially turned on.

When a DC motor starts then its current is very high. You are trying to add a resistance in series with the motor which reduces its torque so it might not start running.

The voltage across a running motor depends on its load. Without a load it runs fast with 3V. With a heavy load (and when it starts) it runs not too fast with 18V.

So your suggestion is to add resistors with higher resistance to keep the transistors from turning each other on?
 

Audioguru

Joined Dec 20, 2007
11,249
So your suggestion is to add resistors with higher resistance to keep the transistors from turning each other on?
No.
You must turn on only two crossed transistors and turn off the other two transistors so that they all do not short-circuit the supply voltage.

This circuit is similar to yours. Only "forward" or "reverse" is high at one time.
Your circuit had no control so both "forward" and "reverse" were both high all the time.
 

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