[SOLVED] Opamp Oscillation Problem - Help Required

[hshah8970]

Hello everyone.

For the past few days, I've been struggling with my oscillating OPA549 opamp when I try to drive a simple DC motor with it.

Since I do not know the inductance values of the motor, I cannot employ an RC snubber circuit in parallel to the motor since I will not be able to calculate the R and C values - I tried trial and error but this was unsuccessful.

Please help with your suggestions to solve this problem and possibly offer an alternative to the RC snubber that will resolve the issue. Thank you.

Humza.

P.S. For those who are wondering what exactly the problem is:

I've made a simple noninverting voltage follower with my opamp to drive a DC motor. As soon as input voltage is applied, the motor starts off fine but quickly slows down to a halt and the output voltage at the opamp wanes. The opamp heats up.

 

[goldsmith]

Dear Humza.
Hi
Snubber network ? for what ? for a DC motor controlling ? it can't be necessary ( it is optional ) . by the way did you use freewheeling diode across your motor ? and what happened that you told the result was unsuccessful ? can you show me the results ? thus i can help you as well .
Best Wishes
Goldsmith

 

[FvM]

An OP has no problems to drive an inductive load because the closed loop output impedance is inductive as well. "RC snubbers", also called boucherot cell are usually supplemented to increase the phase margin with capacitive loads. It's a standard means for audio amplifiers.

The OPA549 datasheet mentions inductive loads separated by long cables as scenario where the RC snubber may be required. In this case, it's rather the cable capacitance that matters. In any case, there's nothing to calculate with motor inductance, in so far your question is missing the point.

You didn't yet report any observation of oscillations. Did you check the OP output with an oscilloscope?

Obviously this post should be appended as follow-up to your previous thread.

 

[hshah8970]

@goldsmith:
i added a snubber because i thought it would solve the problem i was facing with the motor. the problem and the results are simply that my motor starts out fine and then immediately slows down and stops. i've tried replacing the motor with purely resistive loads and found the voltage follower to work perfectly. i have also tested the motor separately and found that there is no fault in the motor. and yes, i did connect a freewheeling diode across my motor but to no avail.

@fvm:
i added this as a new post because i thought the discussion would turn into finding an alternative for a snubber circuit otherwise i would've definitely kept it in the original post. i now understand that my snubber approach is entirely wrong and not a solution to my problem. and no, i didn't check the opamp output with an oscilliscope - i have yet to do that. i read somewhere that the opamp heating up was a sign of oscillation which is why i concluded that to be the case.

all, please help in finding the problem. thanks to all who reply.

 

[goldsmith]

then immediately slows down and stops.

How much current your motor need ? perhaps the current that it need , is higher than allowed value of your opamp , and hence , it's current limiter will work ( as hiccup ) and it can be the reason of this effect .
Another thing that can be the problem , is , surge current ! perhaps it's transient current is a bit high . if , yes , you have to add a soft starter .
Best Wishes
Goldsmith

 

[hshah8970]

@goldsmith:
The opamp I'm using is OPA549 that has a maximum current limit of 10A thus the opamp's current limit is not an issue because the motor is drawing 5A at max. As for the 'soft start,' do you mean adding a capacitor in parallel with the power supply? I tried addding bypass capacitors with the +V_cc and -V_cc supplies to the opamp but that didn't work out. The results remained the same.

Humza.

 

[goldsmith]

How about inrush current of your motor ? can you predict how much is that ? i think more than 20A !

 

[hshah8970]

@goldsmith:
again, thanks for your response. i didn't know what inrush current was, as i'm new to dc motors and electronics. but i did a bit of searching and learned that the inrush current can be a lot times more than the normal operating current of the motor. it may very well be possible that the opamp is unable to deliver all that current and resultantly the volage drops at the output. any idea on how i could fix this, if this was the case?

 

[FvM]

Inrush current isn't a problem as long as the motor starts with the current delivered in current limit. But if the motor is blocked or a large torque is required to start it, the thermal protection will quickly reduce the output current.

To ask a trivial question, did you check that the supply voltage doesn't break down during motor start?

You ususally would want to check the required motor current with a variable lab supply before connecting the amplifier.

 

[goldsmith]

i didn't know what inrush current was, as i'm new to dc motors and electronics

Hi again
As you probably know , each motor , at start time , considered as a short circuit . because there isn't any inertia available , yet . hence , the start current will be higher than nominal current . By the way , you didn't tell why you selected this way to control your motor ? why not PWM and a simple mosfet ? it will be cheaper and more reasonable , of course !
Best Wishes
Goldsmith

 

[godfreyl]

For the past few days, I've been struggling with my oscillating OPA549 opamp when I try to drive a simple DC motor with it.

Please help with your suggestions to solve this problem and possibly offer an alternative to the RC snubber that will resolve the issue. Thank you.

Humza.

Hi Humza

You can improve stability a great deal by reducing the feedback factor. One way to do that is by configuring the opamp for a higher gain e.g. 10.

If you want to use it as a unity gain buffer, you could use the technique I showed here: https://www.edaboard.com/threads/255061/#post1093568. It is important that the source impedance driving the opamp is fairly low.

If you show the circuit you're using, we can give more detailed help. One thing that's important is what the opamp's input is connected to. i.e. Where does the input voltage come from?

I'm tempted to suggest putting an RC filter at the input but I'm not sure if an electrolytic capacitor could be used. That would depend on whether the motor is ever run in reverse, or only in one direction.

Regards - Godfrey

 

[FvM]

You can improve stability a great deal by reducing the feedback factor.

The possibly misleading point with this thread is that the existence of oscillations hasn't been yet verified. OPA549 isn't particularly wide bandwidth, not more than 1 MHz. There may be an oscillation problem though, but many others as well.

 

[hshah8970]

@goldsmith:
A PWM would be beyond the scope of my project. I need a simple motor driver with my voltage follower. Do you think that by adding transistors to the output of the follower, I could get the required inrush current? If yes, please share a schematic.

@godfreyl:
Thanks for your response. I'm going to give your schematic a try and see if reducing the feedback factor works.

@fvm:
You're right. I haven't yet had the chance to confirm the oscillations with an oscilliscope. I shall do that today. If the opamp was oscillating, what kind of a waveform will I see on its output? Moreover, the voltage supply works fine. But there were two instances when it broke down or apparently, 'tripped.' I'm going to conduct more careful observations today and shall update.

Thank you to everybody who is replying and trying to help me out.

 

[goldsmith]

Hi again . of course , because it is kind of linear circuit you can put a transistor in series with your motor but pay attention to the specifications of your transistor ( Current gain/CE breaking voltage , allowed current .... ) ( however , it will be pretty warm because of high value of dissipation ! and perhaps you can use it as an electronic cooker ! )
Good luck
Goldsmith

 

[LvW]

The possibly misleading point with this thread is that the existence of oscillations hasn't been yet verified. OPA549 isn't particularly wide bandwidth, not more than 1 MHz. There may be an oscillation problem though, but many others as well.

hshah89770, please excuse me for my silly question: Are you sure the dc motor is OK? That means: Did you try to operate the motor with a simple dc source (without the opamp)?

 

[hshah8970]

@goldsmith:
I understand.

@lvw:
Yes, the motor is fine. The first thing I did after this problem began is to check the motor directly by my DC power supply.

 

[LvW]

The opamp I'm using is OPA549 that has a maximum current limit of 10A thus the opamp's current limit is not an issue because the motor is drawing 5A at max. .

Hshah8970, I suppose you are aware of the necessity to provide a resistor R(CL) to set the current limit capability?
For example R(CL)=7.5 kohms for a limit of Imax=5A .

 

[FvM]

Hshah8970, I suppose you are aware of the necessity to provide a resistor R(CL) to set the current limit capability?
For example R(CL)=7.5 kohms for a limit of Imax=5A .

Apparently he does. It's the bad with cross posting... https://www.edaboard.com/threads/255149/

 

[hshah8970]

@lvm & @fvm:

Yup, I'm aware of the current limiting resistor for the opa549. And again, I'm sorry for this being split into two posts.

 

[hshah8970]

Guys I found a simple enough driver configuration. Any suggestions for which transistors I should buy?

Also, since I'm a noob in electronics, help in how this circuit works would be appreciated.