Do H-bridge resistor values need to be calculated, or can they be played by "ear"?

Community General Discussions
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Hi All,

I followed an H-bridgeschematic from a post by ignoblegnome and the resulting H-bridge worked perfectly.  I looked around online and saw some h-bridge schematics without resistors and also with resistors.  I wanted to know if resistors were necessary in h-bridges?  And I also came across a specific formula that is used to calculate the base resistor needed for a certain transistor.  Do these calculations need to be followed religiously? 

Please see the below diagram, it shows my findings when i changed R1 and R2 resistor values.  I was only using a 5V supply, and according to my tests, lowering the R2 value allowed the motor to respond to lower PWM values than if R2 was higher.  The response to the lower PWM values would make a 2-wheel balancing robot more responsive I would think.  If the base resistor calculations result in a resistor of higher value, why would i want to put it in the circuit if its going to make the motor less responsive?

However, I'm not sure what would happen to this H-bridge if it were using 12V and "current hungry" motors.  Would some part of the h-bridge burn out because I didn't use calculated H bridge resistor values?

In looking online, it seems as if the resistor values are calculated by taking the power supply voltage into account.  But what if I want to use a 3V supply at one time, and then a 12V supply another time, I can't keep unsoldering and resoldering resistors into the mix. I could perhaps use a variable resistor and tune it as necessary.  How is it that prepackaged motor drivers can accept say 5V to 12V motor supply voltages without the need to replace any resistors? Anyhow, thanks in advance for any insights that can be provided!  

My balancing robot project is on hold as I unknowingly damaged half of a TB6612FNG motor driver.  So what i'm doing now is salvaging h-bridge transistors from RC car PCB's that were laying around, and trying to make my own motor drivers.  They are not going to be as small and elegant as prepackaged ones, but they are a lot more fun to build!

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Yaay! After much research,

Yaay!  After much research, I’ve concluded that regardless of how the motor responds, I need to limit the current going through the transistors, to ensure they do not burn out.  

And depending on the transistors used, motor max current rating, and Voltage amount, I WILL ALWAYS have to do a transistor calculation and may have to settle for a less snappy (higher transistor saturation voltage required to turn on) circuit to ensure a smoke-free environment.

If anyone else stumbles across this post, here are the sites (of many) that I looked at for information:

Using a transistor as a switch - ermicro.com

Basics: Base Resistors on Transistors

Transistor Base Resistor Choices?

I guess at low PWMs, there will be some sort of “Deadzone” where the motor just whines and does not respond.

Oddbot’s tutorial Changing PWM frequencies on Arduino controllers will help eliminate the motor whine.

And this post gives a description of changing the pwm deadzone.

 

 

 

 

 

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A1129

viswesh713, LMR eagle eyed robot builder, noticed that your schematic has the A1129 emitters backwards. If built as drawn then you have a problem. Look at the original schematic. Again, thank viswesh713 for finding this!!

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Hey thanks! For the

Hey thanks!  For the schematic, I wired everything correctly in real life, and just replaced the words TIP120 and TIP127 with the NEC1129 etc. in the shown schematic.   So the symbols in the schematic are probably wrong, but I matched up the base, collector, and emitter in the actual circuit and it does make the motor go forward and reverse, when i “zap” the h-bridge input pins with PWM.  Thanks though to you and viswesh713 for the sharp eyes.  I need to pay more attention to detail in the future!