i built this DC motor power amplifier to drive the motor in both directions using an analog signal (from -1 volt to 1 volt via the potentiometer ) , the motor i use is rated for 24 volts and draws around 400mA at no load , however, when i tested it only one direction works fine ( the NPN TIP142 side) with no significant heat , the other side however doesn't drive the motor all the way to 10 volts and the PNP transistor becomes hot real fast, i replaced it with another tip147 but still got the same results, any idea what maybe causing this?

Update 1: I inserted a resistor in series with the PNP gate (5.1k) and it enhanced the motor's performance , however the oscillations still exist , should i also consider an RC snubber ??

 

Is C42 an electrolytic?

Update 1: I inserted a resistor in series with the PNP gate (5.1k) and it enhanced the motor's performance , however the oscillations still exist , should i also consider an RC snubber ??

What does 'enhanced the motor's performance' mean?
What oscillations? These are mentioned previously?

 

Why are there two Op-Amps in series when one would sufice?

 

Why are there two Op-Amps in series when one would sufice?

Agree, I have done just about the same with a single LM311.
Max.

 

Why are there two Op-Amps in series when one would sufice?

Agreed, but that's not the cause of the overheating problem.

 

I assumed it to be an incidental comment on the apparent over-engineering and not intended as a reason for the original OP's complaint?
Max.

 

I'm sure the circuit does not like driving that capacitive load. Try adding a 10 ohm resistor in series with the 1 uF.

 

Agree, I have done just about the same with a single LM311.
Max.

I would have just used basic 5 pin power Op-Amp like an LM1875 or TDA2003 (<$2) to drive the little motor.

 

I would have just used basic 5 pin power Op-Amp like an LM1875 or TDA2003 (<$2) to drive the little motor.

Yes, in my case it was a 240vdc generator field.
Max.

 

Agreed, but that's not the cause of the overheating problem.

The problem is from over complicating/engineering a simple system.

One pot, one power Op-Amp IC, and two resistors should be all that's required a .1 uf capacitor on each power lead and across the motor.

 

@AlbertHall 1- C42 is ceramic.
2- enhanced its performance means it increased the motor's speed, should've explained it better sorry about that.
3- a semi sinusoidal wave of around 5Vpp above any dc signal i apply to the circuit, measured it at the emitter and the base of the transistors.

@tcmtech i thought the first stage would act as a buffer ( obviously not :/ ) , i tried a TDA2003 at first but i always ended up overheating it and i couldn't know why , that's why i chose to do it with darlingtons.

@Ylli thanks for your feedback will try it and see if it works.

@MaxHeadRoom i know now it may have been made alot simpler , i just wanted to experiment with darlingtons :v

Should adding a cap from Pin 14 to Ground adjust the phase shift and remove the oscillations ?

 

i thought the first stage would act as a buffer ( obviously not :/ ) , i tried a TDA2003 at first but i always ended up overheating it and i couldn't know why , that's why i chose to do it with darlingtons

Did you have a properly sized heatsink on it?

A TDA2003 Power Op-Amp should have had no problems running your motor being it's rated for 3.5 amps continuous and 20 watts thermal dissipation with a properly sized heatsink which means that with your +- 15 volt power supply rail voltages it would take a solid 1.33 amps into a stalled motor to reach its thermal dissipation rating limit.

So given that, if you overheated it with a 400 ma motor load somethings not adding up here.