Motor driving

[patroclus]

I also tried things
But I tried with the first 2 transistors, not yet with Q3. I builded the circuit as it is, the same thing, except Q3. When Q1 is ON, it is in active region, and Q2 in full saturation. Vce in Q1 is around 4.9 V.
Well, i supose it's ok, isn't it? I supose Q1 can never be saturated, can it?..

 

[patroclus]

Nigel, I don't understand I don't know if I'm dumb or something. Do you understand something? What about the last circuit which link I posted last, it is the same thing! (in the H bridge.. look at those 4,7ko base resistors)

 

[Nigel Goodwin]

I also tried things
But I tried with the first 2 transistors, not yet with Q3. I builded the circuit as it is, the same thing, except Q3. When Q1 is ON, it is in active region, and Q2 in full saturation. Vce in Q1 is around 4.9 V.
Well, i supose it's ok, isn't it? I supose Q1 can never be saturated, can it?..

Q1 doesn't need to saturate, in fact it must never saturate, if it did it will probably blow the BE junction of Q2.

As for the H-Bridge circuit, yes that seems even worse - I can only imagine they are using very low power motors?.

 

[patroclus]

Nigel, that's what I thought, but no. :lol:
Better laugh than cry. The author says that the desing is suitable for motors of up to 1A or even more. Theoricaly, 1.5A !!!
This is really good now, because the first circuit was for small motors of up to 300-400mA...

It must be something we are missing??

 

[Nigel Goodwin]

It must be something we are missing??

I think all we are missing is burning our fingers on the hot transistors on the very slow moving robot :lol:

 

[patroclus]

Well, not. Nigel, take a look at this test I did...

I used a BD135 (same as BD140 but NPN), and conected a DC motor, that at 3V takes 320mA. I set Vcc to 3V.

Motor + to Vcc
Motor - to colector
Emitter to ground
Base to Vcc across 330 ohm

As now Vbe is 0.8V, not 5V, Ib is 0.8/330 = 2.4mA (instead of 2.27mA from the BD140 circuit). Ok?? take a look at this.

With that base current, the transistor achieved full saturation... Ic = 310mA, and Vce=0.19V
Using a larger base resistor, 1k, (just 0,8mA Ib) the motor did not start until Vcc was risen to 4.5V. then it turned ON in active region, not far from saturation (Vce=1.1V) but take a look at THIS :

Ib = 0.8mA
Ic = 310mA !!! (measured by multimeter)
This lead us to a hFE of 380!!

NOW I'm really confused

 

[Nigel Goodwin]

As now Vbe is 0.8V, not 5V, Ib is 0.8/330 = 2.4mA (instead of 2.27mA from the BD140 circuit). Ok?? take a look at this.

Slight miscalculation, the base current is 2.2/330 = 6.67mA, giving an Hfe of 46.

For the 1K resistor it will be 2.2V/1000 = 2.2mA, which gives an Hfe of 141 - assuming the collector current is still 310mA, but if the saturation volage has increased, it's likely the current has dropped (less voltage to tne motor).

 

[patroclus]

Slight miscalculation, the base current is 2.2/330 = 6.67mA, giving an Hfe of 46.

No.. it is a common emiter, and emitter is grouned. Base is at 0.8V (I checked it with multimeter too), so 0.8/330...

Remember I use a NPN, and builded a equivalent circuit.

 

[Nigel Goodwin]

No.. it is a common emiter, and emitter is grouned. Base is at 0.8V (I checked it with multimeter too), so 0.8/330...

It makes no difference, the current is dependent on the value of the resistor, and the voltage across the resistor (not the Vbe of the transistor, which is pretty well constant). With your 3V supply and 0.8V Vbe, the voltage across the resistor is 2.2V.

 

[patroclus]

Damn, you're right...
I'm almost to give up. I'm waiting for the author of the robots to answer my question...

no one in this forum has worked with this kind of circuits??

 

[Nigel Goodwin]

Damn, you're right...
I'm almost to give up. I'm waiting for the author of the robots to answer my question...

Have you considered building things and seeing what happens?.

 

[patroclus]

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.

 

[Nigel Goodwin]

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.

 

[toowie2uk]

parallel them up?

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

 

[patroclus]

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

 

[Nigel Goodwin]

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).

 

[falleafd]

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.