H Bridge motor Driver using Darlington..........Plz. Help...

[nikhilsigma]

Hi Guys............

I am trying to design a H- Bridge Motor Driver...............
here is the circuit which will remain activated at one time...(i.e. these two darlington pairs are opposite diagonally, if we place the them in normal H form..)

**broken link removed**

here i have given 5 volt input supply to ensure logic 1 at the input..

and the 3 ohm resistor is the load i.e. motor in this case...........
the problem is you can see that i am not getting the sufficient output across the load... and a lot of voltage is dropping across the darlington pairs i.e. TIP122, TIP127..

this means that the darlingtons are not working in saturation region, but i have solved all the mathematical equations for the circuit... and had calculated the value of resistor such that darlingtons should remain in SATURATION region........

I am not getting the problem...........
according to me it should be working fine......
but it's not....working that way.........

Please help me out with this..............

Nikhil.....

 

[abbeyromy]

Re: H Bridge motor Driver using Darlington..........Plz. Hel

saturation voltage of a Darlington transistor is one VBE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon.

You need to check the values of your resistors used.
Also in H-bridge is a circuit which enables a voltage to be applied across a load in either direction. How are you achieving this?

 

[nikhilsigma]

Re: H Bridge motor Driver using Darlington..........Plz. Hel

first of all these two darlington pairs are of diagonals of the bridge,
and in a bridge two diagonal switches activates at same time so as to complete the circuit, thus i had taken only two diagonal pairs...which will be activated at same time......other two will be the replica of the above...... :-D

n yes i want to have a potential drop of Vce sat about both the darlingtons to ensure maximum voltage about the load..............

and one more thing,,, that if i decrease the value of load(i.e inc. the value of load resistance) then the potential drop across the tarnsistor goes more towards saturation...

but why these darlingtons are not going in saturation with the bigger load....


Please help me out...........

 

[FvM]

You should check the transistor datasheets. 1.1 - 1.3V is a usual Vcesat value for darlington transistors. For low voltage drop, use MOSFET.