H bridge with pot for DC motor driver

[Lawrey]

I'm trying to make a driver for a DC motor with pot as the direction and speed control. I have simulated my circuit however, I encounter problem with the increment of the output voltage and current of the driver. the increments are good from pot percent of 5% - 95% however when it reaches either 0% or 100% the output greatly increases (i.e 90% - 0.5V, 95% - 0.6V, 100% - 2.5V). I'm just worrying that this will result to a sudden RPM boost as it reaches either 100% or 0%. I'll attach a picture of the circuit. Thanks! 🙂

[Lawrey]

Somehow I find a solution regarding my problem. It seems that I was also varying the supply of the transistors in my H bridge which seems to be the reason of the problem and know I fix it to 15V. I've also added a cascading transistor just to increase my output current and voltage. I'll attach the resulting circuit which is already good as far as the output is concern although I don't know if this enough as a DC motor driver. I would be glad  to hear any suggestions and comments for the resulting circuit 🙂

[johnsold]

Lawrey,

Your circuit cannot work with 2N222A transistors. The simulation might run, but if you build it all you will get is smoke and foul odors!

The maximum continuous collector current rating for the transistor is 800 mA. The power rating with an inifinite heat sink is 1 W.

The values shown in your image exceed those limits. Consider the case shown where the pot wiper is at one end. While not completely accurate, assume that Q1, Q5, and Q4 are conducting and all the other transistors are off. With a 15 V supply and 3 V across the motor that leaves 12 V across the transistors. With 1 A flowing the total power dissipation in the transistors is 12 W.  No matter how it is divided, the dissipation in at least one of the transistors far execceds the limit. Next, the hFE of a 2N2222A at 500 mA is >=30 and probably lower at 1 A. For a collector current of 1 A and hFE of 30 the base current will be 33 mA. That requires the voltage at the positive end of R4 to be almost 34 V.  So, this circuit cannot drive Q4 to anything approaching 1 A.

Simulations are great tools but you must understand the limitations of the models. Often the best way to do this is to analyze the circuit manually in some simple cases and then check the simulation against that result.  The model you are using apparently does not accurately represent the reduction of hFE at high currents, nor does it consider the maximum ratings of the devices.

Lytnn

[johnsold]

Here is a quick analysis of one half of the H-bridge with the pot all the way at one end.

R1 and R4 are connected to +15 V.  I have assumed a 3 ohm resistor as the load in place of the motor (based on the voltage and current readings in your image).

The base current of Q4 is approximately 14 mA ~ (15 V - 0.7 V)/1000 ohms.  If hFE4 = 30, then Ic4 = 420 mA.

Ic5 ~ Ic4 = 420 mA. For hFE5 = hFE4 = 30 and hFE1 = 300 then Ib1 ~47 uA.  That makes Vb1 = 15 V - 47 uA*1000 ohms = 14.95 V. Then Ve5 = Vb1 - 1.4 V = 13.6 V. Vc4 = Ve5 - Ic4*Rmotor = 13.6 - 0.42*3 = 12.3 V. The power dissipation in Q4 is PD4 = 12.3 V * 420 mA = 5.18 W. The dissipation in Q5 is (15 - 13.6)*420 mA = 588 mW.

Transistor Q4 will fail almost immediately. Since transistors often fail shorted, Q5 will fail as soon as Q4 shorts.

Lynn

[Lawrey]

Ooh. It seems like I still have a lot of things to learn when designing. I guess I need go back to the drawing borad and compute and find the right component for my circuit. I didn't pay attention to the ratings because I thought that multisim can automatically detect and notify me of any malfunction that can happen. Thanks anyway for your help. I have only asked twice here at this forum and I believe you answer both of them. Thanks Lynn 🙂

[johnsold]

Lawrey,

Learning to design circuits is a process which takes some time.  I have been at it for 40+ years and am still learning.  Simulators are really analysis tools.

I think of design as an iterative process. Start with a specification. In your example you apparently want to drive a small motor with a continuously variable magnitude (speed, torque, power?) and direction. You do not have any feedback. Next you select a circuit configuration, such as your H-bridge. With such a configuration you can do some simple analyses (before selecting transistors) of maximum voltage, current, and power dissipation at each transistor. This would be similar to the analysis I posted above, but somewhat simplified (ignoring base currents and hFE for example). The values determined from this preliminary analysis can be used to select suitable transistors. You know the current rating has to be larger than the maximum motor current and the power rating will be larger than that of the devices you initially selected.

At this point you have a preliminary design with some specific transistors selected. Performing an analysis like mine will likely show that single transistors in each leg of the H-bridge will not work and that you need a Darlington configuration in all four positions (unless you want a 10 ohm, 20 watt pot).  So now you need four more transistors. A few calculations on resistor values will give you a complete circuit.

At this point you run the simulation. Check the results, first to verify that it performs as you expect, and, second, carefull,y to see if any component has exceeded its ratings. Many simulation models do not accurately represent the ratings limits. For any part of the circuit which does not perform well or which exceeds ratings you modify the design and then repeat.  Just think about how much more difficult this was before simulation software existed and all those calculations were done manually.

Lynn