h-bridge - probably stupid question....

so i've made that h-bridge using 4 BD135 NPN transistors (http://www.fairchildsemi.com/ds/BD/BD135.pdf), but it doesn't work... schematic:

what am i missing?

motor is tested and working. there are no resistors whatsoever - circuit is exactly like shown on the pic. transistors are working - i've just tested all of them. i can turn on one of them using +5V from arduino.... so i'm guessing i need to add some base resistors to limit current, am i right?

Don't you usually use half NPN, half PNP?

You have to add base resistors, first of all, but schematic is design to work with +5V . To make it works up to +9 or +12V two more transistors required (at least). Look in : http://arduino.cc/forum/index.php/topic,67063.msg493189.html#msg493189

And : http://www.talkingelectronics.com/projects/H-Bridge/H-Bridge-1.html

AWOL: Don't you usually use half NPN, half PNP?

yes, but if you have 300 NPNs for less than 4 euro then why use PNPs ;)

yes, but if you have 300 NPNs for less than 4 euro then why use PNPs

I don’t know but maybe just maybe you might like it to work.

The alternative is to design the schematic properly.

transistors are working - i’ve just tested all of them.

How did you test them? Your skills with a transistor are not exactly legendary are they given your design of the H-bridge.

You need extra transistors to drive the top transistors, you need current limiting resistors in the base and you need four diodes to protect against reverse EMF.

ok. i’ve done some reading and i found this:

the omly mod. i’ve made was replacing single transistors with darlington pairs.

it seams to work in simmulation, but i want to ask u guys for opinion before i’ll start soldering :wink:

and yes - my skills with transistors are slim to non, but i do know how to plug them into tester in multimeter :wink:

but it doesn't work...

The problem with simulation is "garbage in, garbage out". I think the issue with using the NPN transistors on the H-bridge high side is that the transistor base current has to flow thru the motor and the low side transistor. I haven't seen any successful h-bridges based on your design.