H-bridge

[adrianvon]

Hi,

If i use the attached h-bridge circuit to drive a 12V 2A motor, will it work fine? What is the maximum current that it can handle? and can i drive it directly from a PIC18F microcontroller?

Thanks in advance.

 

[Reloadron]

If you read the data sheets for the TIP122 darlington and the TIP127 darlington you will see their max continuous forward current which looks fine to me. You might want to heat sink the tabs.

I also suggest you give this thread a read to gain an understanding of what the required base current in your darlington pairs would be based on their gain. Your PIC will need to supply that much base current so can it? Check the PIC data sheet.

Ron

 

[audioguru]

Your darlingtons are emitter-followers, not switches. Then the inputs must swing from 0V to +12V. The outputs of the PIC swing only to +5V so your H-bridge WILL NOT WORK.

When the PIC output goes high to +5V then the emitter of the TIP122 goes to only +2.8V. Both TIP127 darlingtons will be turned on ALL THE TIME since their inputs must be +12V to turn them off!

 

[adrianvon]

Hi,

Thanks for your replies.

When the PIC output goes high to +5V then the emitter of the TIP122 goes to only +2.8V. Both TIP127 darlingtons will be turned on ALL THE TIME since their inputs must be +12V to turn them off!

If i connect them as shown in the attached diagram, will it work? Instead of supplying 5V, the PIC is driving two bc546 which will supply the TIP122 and the TIP127 with 12v at their base.

 

[Reloadron]

Thank you AG, as I looked at the drawing and just saw H bridge and nothing beyond, like the configuration. Very bad on my part for not looking at the obvious and assuming canned circuit of H bridge. So while the darlington transistors chosen would handle the current the configuration was wrong.

You may want to give this link a look. Then compare your configuration.

Ron

 

[ronsimpson]

If i connect them as shown in the attached diagram, will it work? Instead of supplying 5V, the PIC is driving two bc546 which will supply the TIP122 and the TIP127 with 12v at their base.

When I look at your last drawing all the TIPs have their bases tied to VCC.

 

[audioguru]

Your transistors are connected wrong.

The 220 ohm resistors are not doing anything since the darlingtons are connected as emitter-followers.

My 330 ohm resistors have a voltage drop of about 0.66V and the darlingtons have a base-emitter voltage drop of about 2.2V so the 12V is reduced to +8.9V on one side of the H-bridge. The other side of the bridge has the 0.2V saturation voltage of the transistor plus the 2.2V base-emitter voltage of the darlington.
Then your 12V motor gets only 6.5V.

You should use Mosfets (they have a very low voltage loss).

 

[adrianvon]

You should use Mosfets (they have a very low voltage loss).

So if i use the IRF4905 and the IRF3707 MOSFETS, will the h-bridge work? Attached is an h-bridge using these transistors.

 

[audioguru]

The circuit with the Mosfets will work perfectly.

 

[adrianvon]

Can i control the last attached h-bridge directly from the microcontroller? and do i have to change the 1N5401 diodes, or they can handle high current?

Thanks in advance for your help.

 

[Reloadron]

The 1N5401 diodes serve as flyback diodes. What high current? The diode used is a 3 amp 100 PIV diode. Do you have a dislike of looking up and using data sheets? As AG stated, the circuit will work just fine for your 2 amp motor.

Ron

 

[kinarfi]

I'm not familiar with PICs, but if their output is only +5, will they get the FETs turned on? And is a 74 amp FET a good choice? I think not, do some digging and you'll find some better matches to you project.
Kinarfi

 

[ronsimpson]

I'm not familiar with PICs, but if their output is only +5, will they get the FETs turned on? And is a 74 amp FET a good choice? I think not, do some digging and you'll find some better matches to you project.
Kinarfi

If you are going to turn on a FET from 5 volts you need to find a logic level MOSFET.

 

[adrianvon]

audioguru, regarding the voltage drops which are shown in the diagram you have attached in reply number 7, can you please tell me how they were calculated since i built the same circuit in ISIS proteus and it is showing different voltage drops.

Attached is a screenshot of the design on proteus.

 

[audioguru]

I used the voltage drops called "Collector Emitter Saturation Voltage" listed as the maximums on the datasheets of the darlington transistors. Your simulation program uses "typical" values but you cannot buy "typical" transistors, you get whatever they have.