please check this circuit

hello all
can anyone please check this simpe hbridge circuit and tell me if there is anything i have to change before going on ?
i intended to use it with a PWM pic output to drive 24v, 1amp motor
i have simulated it in proteus 6.9 and it seems to work fine.

thanks

Can't read it, as you posted it as a JPG - use GIF or PNG for perfect quality and a much smaller file size! - JPG is for photographs, NOT diagrams.

here it is again

ok here it is again
the transistors are TIP122 (upper) ,TIP 127(lower) and 2n3904(both sides for switching)

thanks

The bottom transistors are the wrong way round, and I would suggest you make the 10K pull-up resistors much lower - to try and drive the top transistors as hard-on as possible.

It's more normal to have the collectors of the output transistors together, rather than the emitters, this requires slightly more complicated drive circuitry - but minimises voltage loss. As long as you've got plenty of supply voltage you should be OK, because you're going to lose a good four volts with that configuration!.

It looks to me that you have theTIP127 transistors upside down.

Also, it may be a good idea to include protection diodes across the emitter / collectors of the transistors.

I didn't notice Nigel had already posted first.

Hi,

Agree with houseofwax and nigel compeltely. Check this one out:
http://www.armory.com/~rstevew/Public/Motors/H-Bridges/HBridge_NPN-PNP.gif

Also note the diodes that prevent the inductive motor kicking back voltages outside of the power supply's range, as houseofwax stated.

I recently built a full bridge with all NPN's (my order of PNP's didn't turn up) and that works beautifully...bit different from the above config though, obviously. I think, with houseofwax's corrections, it would be 'blow proof', at least as far as inputs are concerned. That is, with the twi inputs, theres no combination that causes the two left or right) transisitors to be on at the same time...very handy, and it saves a logic inverter being used.

Blueteeth.

hello
thanks for your reply
although i dont think what u said is true
i know most people wire TIP127 as upper transistors, but why not wire them this way ?
also the upper transistor wont both get connected at the same time as one of the transistors 2N3904 will ground one of them .
also the tip usually contanin a protection diodes already, maybe for higher frequncy i can install faster diodes.

am i correct ?

thanks

max_imum2000 said:

hello
thanks for your reply
although i dont think what u said is true
i know most people wire TIP127 as upper transistors, but why not wire them this way ?

Click to expand...

I've already explained that in the previous post.

also the upper transistor wont both get connected at the same time as one of the transistors 2N3904 will ground one of them .

Click to expand...

But, as I explained previously, your TIP127's are upside down, that's probably cosfused people into thinking they are NPN.

also the tip usually contanin a protection diodes already, maybe for higher frequncy i can install faster diodes.

Click to expand...

They contain diodes, but I don't know if they are well enough specified for this purpose, or merely a byproduct of the manufacturing process.

thank you all
i have already corrected the pnp transistor positions in the circuit but i posted the old circuit, so i didnt get what your saying at the start.

thank you very much.
i think i need to restudy transistor operation a lot