H-bridge

[Electronics4you]

Hi there,

I have to run a 12V motor in both directions. I have build a prototype schematic below. The transistor used is the BD911 (PNP) and BD912 (NPN) with a max current through collector-emitter of 15A. I have some questions to the circuit

1. Do I need an ULN2003 with 12V to turn the PNP transistor ON/OFF, or can it be done in other (cheaper) ways from the PIC?

2. To get 5-6A through the bridge, do I then have to change the 1k resistor to a smaller, because of the transistors amplification on min. 25gg?

3. How do I calculate the total amount of current avaible to the motor concidering both hfe, base current,,...?

Thanks

 

[audioguru]

The BD911 and BD912 transistors are like most power transistor and need a very high base current to saturate with a high current load. For a load of 10A, the datasheet shows a max saturation voltage of 3V with a huge base current of 2.5A. Your 1k resistor will provide a base current of only 4.3mA, 580 times too small.

With less load current of 5A then the max saturation voltage is 1V with a base current of 500mA. The ULN2003 has a max output current of 500mA.
The ULN2003 needs a 12V supply to drive the PNP transistor through a current-limiting resistor and another resistor is needed to turn off the PNP.
Use additional darlington transistors to turn on the NPN power transistors.

You still need an H-bridge circuit that should include all this driving circuitry.

 

[Hero999]

Perhaps you aught to consider MOSFETs because they don't waste huge amounts of power when they're turned on.

 

[Sceadwian]

At 12 volts a MOSFET will work well. At higher voltages an IGBT might be a better sollution, it's basically a bipolar transistor with an integrated mosfet driver.

 

[Electronics4you]

Does that require a complex control circuit. Have you any circuitry showing how to control heavy loads with a PIC?

 

[Electronics4you]

I found this datasheet for the IRG4BC30K. It says that it has a Emitter-to-Collector Breakdown Voltage of 18V. So I can't use it for 20V motors?

 

[audioguru]

Its normal max supply voltage is 600V. When its supply voltage is backwards then its max is 18V. It is very expensive and has a fairly low current rating. Don't use a transistor backwards then this one isn't needed.

Instead, use a less expensive Mosfet that has a very high current rating.