H-bridge Problem

[ahmad-al3omar]

dear all;
I have power transistors (tip122 -npn- ) and (tip126 -pnp-) in order to make a high rating H-Bridge. I tried to use the four npn transistors in one design and to use two npn and two pnp in another one but there allot of problems, the motor rotates in a very low speed and one of transistor burned. How can I come over these problems without using a single H-Bridge chip.
thank you
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A.T.N

[Ttelmah]

Difficult to know, without having a lot more details of your bridge. First, what voltage is the bridge running off?. Second, how are you driving the transistors?. Third, have you got the trap diodes present to prevent the flyback spikes when the bridge transistors turn off, from destroying the circuit?. Fourth, what current are you actually controlling?.
If your voltage is low (5v), then you really should consider using FETs, rather than bipolar transistors (probably should anyway...), but the big problem at low voltages is the typically 0.6v 'drop' across the transistors, which on a 5v circuit, will result in a 'best case' across the motor, of only 3.8v, and the transistors having to dissipate at least 1/3rd the total power delivered to the motor...
With any H bridge, you also have to be very careful about what happens when things change state. Either having to design the hardware to ensure that drivers turn 'off' faster than they turn on, or building delays into the software (which is why the proper H bride drive outputs on some of the PICs have programmable 'deadband' timings). How are you driving the bridge in this regard?.
Come back with some real details (possibly a circuit diagram of what you have tried), and we may then be able to help.

Best Wishes

[Guest]

Hi,

Firstly, can you post your circuit?

We the need to look at the code to see what you are doing to the circuit.

Also indicate motor ratings etc.

Regards,