1st design
Diagram below shows a H-bridge using transistors ( from previous post). The diagram shows that you need 4 output pins to control them, but the transistors in adjacent can be connected to the same output, reducng the no of output to 2. (transistors Q8 and Q2; Q7 and Q4 can be connected to gether)
Component used
1. 2 PNP transistors
2. 4 NPN transistors
3. 6 resistors, each at the base of the transistors ( There are missing in the schematic, my bad)
4. 4 flyback diodes ( my bad again =))
Truthtable
Just assuming left side is A, right side is B
A | B | motor |
0 | 0 | Coastng |
0 | 1 | Clockwise |
1 | 0 | Anti-clockwise |
1 | 1 | Short circuit!!! |
**Coasting,both motor connections are not connected to anything, the motor is free to run
Advantages
No shoot thru when changing directions (??)
Can control motor speed with PWM
Disadvantages
Using 6 transistors, more costly
Not short circuit proof
Calculations need to be done in order to put transistor in saturated state ( to make transistor as a switch not amplifier).Read this for transitor calulation http://www.ermicro.com/blog/?p=423
2nd design
The 2nd design , using only 2 NPN and PNP!
Components used
1. 2 PNP transistors
2. 2 NPN transistors
3. 4 resistors, each at the transistors base
4. 4 fly back diodes
Truthtable
Just assuming left side is A, right side is B
A | B | motor |
0 | 0 | Coastng |
0 | 1 | Clockwise |
1 | 0 | Anti-clockwise |
1 | 1 | Short circuit!!! |
Avantages
4 transistors only = less cost
No shoot thru when changing directions (??)
Can control motor speed with PWM
Disadvantges
Not shortcircuit proof
Calculations need to be done in order to put transistor in saturated state ( to make transistor as a switch not amplifier). Read this for transitor calulation http://www.ermicro.com/blog/?p=423
3rd design
The 3rd design is using relays as the main component. 2 NPN transistors are used to control the adjecents relays.
Component used
1. 4 relays
2. 8 flyback diode
3. 2 NPN transistors
4. 2 resistors
Truthtable
Just assuming left side is A, right side is B
A | B | motor |
0 | 0 | Coastng |
0 | 1 | Clockwise |
1 | 0 | Anti-clockwise |
1 | 1 | Short circuit!!! |
Advantages
Relays are more rugged than transistors
Disadvantages
Relays have larger footprint, requiring more space
Abundant diodes needed 8 here, with only 4 with the 1st and 2nd design
Can't control motor speed with PWM, relay swithing speed too slow
Calculations need to be done in order to put transistor in saturated state ( to make transistor as a switch not amplifier). Read this for transitor calulation http://www.ermicro.com/blog/?p=423
4th design
This design in-corperates relays and mosfet in the H-bridge. I kinda thought of this design to overcome the disadvantage of all relay H-bridge no able to perform motor speed control with PWM. Well, the original design was to use 4 relays H-bridge, with a mosfet/ transistor controlling flow to the ground. The relay is just to control motor direction, MOSFET/ transistor to control motor speed with PWM.
However I stumble across Cytron's MD10 motor driver and guess that the schematic is as below. Very ingenius same concept , but only using 2 relays ( At the same time I'm implying that I'm slow, why didn't I thought of it!!! ). So, here we go, the 2 relay-1 mosfet H-bridge!
Component used
1. 2 SPDT relays
2. 2 NPN transistors
3. P channel mosfet
4. 2 resistors
5. 1 optocoupler
6. 6 fly back diodes
Of course, I think that it is not necessary to use an opto couple to control the MOSFET. A transistor, mosfet driver or another mosfet might be able to do the job? You can also replace the MOSFET with a transistor, if you like , I think it will still be able to do the job!
Truthtable
Just assuming left side is A, right side is B
A | B | motor |
0 | 0 | Brake high |
0 | 1 | Clockwise |
1 | 0 | Anti-clockwise |
1 | 1 | Brake low |
** motor can be electrically brake when its 2 terminals are connect together. Brake high, means motor terminals are both connected to Vcc; Brake low, motors terminals are connected to gnd.
Advantages
Relays are more rugged than transistors
No short circuit mode!!!
Can control motor speed with PWM
Disadvantages
Relays have larger footprint, requiring more space
Abundant diodes needed6 here, with only 4 with the 1st and 2nd design
Well , that's all for now, there are still H-bridge using MOSFET, we'll keep that for the next time =)
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