Motor Control Circuit Questions

[webzila]

Hello,
My group and I built the following dc motor control circut (that I got help with in another thread regarding transistors on this forum):


I replaced the SK100 transistor with NTE262 and the SL100 with TIP110

We have a high speed 9-18V DC Motor (18,000 RPM / 1.98A max).
When we plug a 12 V DC power supply directly to the motor it spins very fast but when we use the circuit to drive the motor then it seems to spin much slower.

We supplied the circuit with 12V power...we are scared to increase the voltage because we dont want to burn out any components (we already had to replace several transistors because we broke the leads).

With the circuit that I posted above, would it be safe to increase the voltage to ~18 V......Or is there another reason why the motor operates slower than normal when driven by the circuit?

Additional note: With power supply set to 12V, we just measured to voltage going to the motor and it says 1.5V .... what is wrong?

 

[Exo]

Yes, it is safe to increase the voltage. But 1.5V on the motor with a 12V supply is realy low.

where exactly did you measure that voltage, straight over the motor's connections when it is running?

what are you using as a power supply? if it can't supply enough current it'll go 'trough it's knees' and voltage will drop.

 

[webzila]

We unplugged the motor and plugged the multimeter in its place to see how much voltage is coming out.

With the circuit activated (A high, B low ... or vise versa) we can only get 1.5V out -- even when we put the power supply to 20V

The power supply that we are using is BK Precision DC Power Supply

 

[Exo]

Try measuring the voltage on the motor when it is running that'll give a better picture.

The voltage will (should) be +-4V less then the supply voltage then. This voltage is dropped over the transistors wich is normal and explains why it runs slower. Just raise supply voltage until you have enough on the motor.

If however the voltage is still realy low when the motor is connected then something else is wrong...

I'm not familiar with a 'BK precision' supply, does it say somewhere how much current it can deliver?

 

[webzila]

The BK Precision allows you to set the current...
We had the power supply set to 2A and 12V and when measuring the motor it would say something like 0.6V (and the power supply changes from 12V to 2.6V when under load)

Then when we set to current to 0.4A and changed the power supply to 35V then the motor started running faster and now measured at 1.5V

This is weird.

 

[webzila]

Additional note:

We just hooked the circuit up to a 9V Alkaline Battery instead of the power supply. The DC motor is still spinning slow and when measuring its voltage it says 0.9 V

So it must be the circuit...
Are the NTE262 the correct transistors to replace SK100

Do you see anything else that might be wrong here?

 

[Russlk]

The NTE262 is a PNP darlington, rated 100 volts and 8 amps. It is OK for your application. The problem is that Q6 and Q3 never get turned off. You need seperate drivers for the PNP and NPN transistors.

 

[webzila]

Could you explain how to do setup seperate drivers for the PNP and NPN transistors?

We tried to check the circuit to make sure everything is plugged in correctly. On the circuit diagram, when you look at the TIP110 and NTE262 transistors...the side with the arrow on it is always the emitter and the other side (without the arrow) is the collector?

Also, does anyone know if and where I could buy a pre-build/tested dc driver circuit...something like the one we are trying to build and something cheap..

Thanks

The NTE262 is a PNP darlington, rated 100 volts and 8 amps. It is OK for your application. The problem is that Q6 and Q3 never get turned off. You need seperate drivers for the PNP and NPN transistors.

 

[Russlk]

H- bridge circuits are probably available, do a Google search. Borrowing an idea from the MOSFET circuit, this should work:

 

[webzila]

Would the problem that I described be fixed if we do the following to the circuit:

> Swap Q3 with Q4 and Q5 with Q6. The emitters of Q3 and Q4 should be
> connected and drive one side of the motor, Q5 and Q6 similarly driving > the other.

Thanks

 

[Russlk]

What makes you think that would work? You need to do some serious studying up on transistors.

 

[webzila]

The reasoning behind it is as follows (when looking at the original circuit, as it seems that we dont even need the two 1K resistors that we added)

One side of the H-bridge is as follows:
npn collector to V+, pnp collector to 0V
npn, pnp bases joined together, driven by Q1 collector (or Q2 for the
other side of the H-bridge)
npn,pnp emitters joined together, this is the center of the half-bridge
and connects to the motor (and the diodes, as originally drawn).

-------+------- +9V

|
/ c
+--| npn
| > e
| |

| +--------- to motor/diodes

| |
| < e

from Q1 collector ---+--| pnp

\ c
|

-------+------ 0V

this is basically a complementary emitter follower - the emitter voltage
is always 1 diode drop below the base voltage, which is either 0.2V (Q1
on) or 9V (Q1 off)