Hi Guys,
I am trying to make an H Bridge with 4 BC 547 like the one on the picture. The switching works well, but the problem is that the motor spins very very slow.. I am guessing that there's not enough current, is it true?
Also any suggestion on what transistor should i use?
I know that I can use an IC like L293, but i want to make the bridge from transistor..
THANKS A LOT 
One half of your bridge should really be constructed from complementary (PNP) transistors.
Rb= (VPORT – VBE) / IB = (4.2 – 1.4) / 0.001 = 2800 Ohm, use 2K2 Ohm resistor ,Also don't forget to add Free wheeling diodes in order to eliminate the back EMF.
Okay I will try using pnp
amrmuhammadelsaid:
Rb= (VPORT – VBE) / IB = (4.2 – 1.4) / 0.001 = 2800 Ohm, use 2K2 Ohm resistor ,Also don't forget to add Free wheeling diodes in order to eliminate the back EMF.
Where should I put the resistor? Isnt 2k2 too big?
What is the supply voltage "+V"?
MarkT:
What is the supply voltage "+V"?
The supply voltage is 9v
It can be done with just NPN bipolars.
But the basevoltage on the upper pair is the supplyvoltage minus what the motor drops.
So they might not saturate with the same resistors as the lower pair does.
BC547 is a 100mA transistor.
If the voltage to A (or to B) is 5V then the max available to the motor is approx 4V.
You'll get slightly better results if you replace the BC547s by BC337s, and omit the base resistors on the upper transistors. But as RP says, the maximum voltage you can get at the motor in this arrangement is about 4V. And you need to add flyback diodes.