Have you considered building things and seeing what happens?.

 

Well, I already builded up the cricuit in protoboard. The first 2 transistors work just fine and as they have to, but Q3 does not saturate until Ib is at least 6.75mA for a 3V@330mA motor, which is FAR away from Ib in the circuit...
Now I wonder if I should do things as they are in the schematic, or I shoud do thing as I think they have to be.

 

I should try it how it is, and see what happens.

 

you can parallel a couple of the darlingtons up in the ULN2003 I think. Should give you a bit more. Heat sink might be useful! I do admit to having a crater in my bread-board where I left a darlington driving a huge stepper on test

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I finally found a clue.
these circuits are to control low current servo motors of up to 120-130 mA. For them, it works just fine. For more current demanding motor, it need more base current, so both Nigel and I were right about that.

What really made me not understand all this was the use of transistors which can hold up to 1.5A, but the circuit could not manage more than 130mA. well, I'm gonna get with my desing

 

That's fairly good practice, by using over rated components everything will run cooler and be more reliable. Don't forget, under stall conditions the motors will take a lot more power. The kit robot Cybot relies on the use of non-rechargeable batteries to limit the stall current - with nicad's the stall current is likely to blow the driver transistors (they only use TO92 2A ones).

 

Oh sorry, I didn't follow this topic from the beginning. But may you tell me why you want to build such a complex circuit to drive only 130mA motors? Will you circuit is smaller or cheaper than using a driver IC such as TA7288P or ULN2003 as toowie said?

Many times, I've asked myself why to use a complex circuit or a driver IC! And a prof of IOWA university answer that because of the prices of simple IC are always cheaper than complex circuits, and for mass products, they use driver IC in stead. However, is there any advantages as you use your circuits? Pls explain meeh. I built some motor drivers upto 5A circuits, because I didn't find the suitable IC works at that current. But why you use it for a small motor? I cannot distinguish the differences of control character between these two cases, but only the prices, its weight , and the complex as making circuits.

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