Hi Guys, it's me again

These circuits seem to work, but inconsistently. (files attached)

The input to the opamp is 0V or 5V (digital outputs from NI's Data Acquisition Card). The motor controller I am trying to run from the output of the opamp runs on 24V @11.6mA (0V o/p for 0V input and 24V o/p for 5V i/p). PLease please tell me if I am on the right path. I have these circuits soldered and they seem to work fine once or twice but not consistently. Please help... I am on time constraints and would really appreciate it if I could do something with this opamp.

Thanks!

V

 

What is the part number of the op-amp you are using?

 

Generally fine for simple gain of 10 amp. But some details depend on the op amp, the range of input and the input and output. In particular you may have trouble reaching the rails. What op amp are you using, how well is the power supply bypassed. What is the input and the expected output?

 

oh darn.... i forgot to mention the part number.... It is LM324 that I am using ,.... I need output of 24V but current limited to 11.6mA with input of 5V and 0V for 0V input.....

Thanks a ton!!

V

 

The biggest problem I see is ground. As a class op amps do not handle being to close to either power supply rail very well, neither on the input or especially the output.

 

The biggest problem I see is ground. As a class op amps do not handle being to close to either power supply rail very well, neither on the input or especially the output.

Actually, the LM324 input range includes gnd, and the output will go within a few millivolts of ground with a 10k pulldown to gnd.

 

Yea! now that I think about this, I seem to be getting a constant output of 22.638V at the output even when the input should be zero. I am assuming that the zero is technically a few hundred millivolts. Is that right?

VJ

 

Are you sure the 24V input needs to be *limited* to 11.5mA?

Most devices that take a switched 24V control signal are internally limited; it likely draws 11.5mA when connected to 24V.

Also, try putting a pulldown resistor from the none-inverting amp input to 0V, say 10K or even 1K depending on what is driving it.

You could also add a signal diode inline from the 5V control to the amp input, so the 5V signal has to exceed 0.6V before the amp input lifts from 0V.

If I was building this, I'd use two transistors; something like a 2N7000 or a section of darlington driver to give an open collector output, then a resistor to the gate of a P channel FET with the source connected to 24V (and another resistor between source and gate to set the correct G - S voltage for the device).

The drain is then the 24V switched output.

 

thanks rjenkins.... yes, the motor controller manual says Digital inputs should be 11.2mA @24V DC.....

Yea, I thought so too about the device drawing its required current when connected to a 24V supply because most digital inputs have high input resistance. But, I dont know how to verify this. Can I just measure the resistance from A digital input with respect to the ground using a multimeter?

Regarding pull down resistor, I have to place a 1k or 10k from non-inverting input to ground right?

Diode i tried. 5V -> -|>|- ->op-amp ... sometimes, the input voltage to the diode fluctuates..Dont understand why. The output from an NI DAQ card is 5V @ 5mA that drives the op amp.

I tried many circuits, but I thought an op amp is the simplest for now. Will look into your circuit at the same time.

 

Do you have a manual or part number of the motor controller that you could post a link to? Sometimes that can make it easier for people to make recommendations...

 

The motor controller is Allen-Bradley PowerFlex 70 Adjustable Frequency AC Drive from Rockwell Automation.
Thanks!

 

I think you will be better of with a votage comparator setup.

 

I looked at the user manual, page 1-16, table 1.H. The LM324 should work. Instead of using negative feedback, connect a voltage divider to the inverting input, 100k from the inverting input to+24V, and 10k from the inverting input to ground. This will give you a threshold of 2.2V, which is good for any 5V logic system you are getting your control signal from.
You don't need any pull up or pull down resistors on the output, and the LM324 can provide the required 11.2mA while giving you adequate voltage margins for both high and low level requirements. The requirements are Von(min)=19.2V, Voff(max)=3.2V (from Table 1.H). The LM324 should provide Von≈22.4V, Voff≈10millivolts.

 

I think you will be better of with a votage comparator setup.

I would normally recommend a comparator, but our OP already has the LM324, and it should work fine.

 

Thanks Ron! Just a question from my side in understanding...

So, no feedback at all, Just 5V into Non inverting input and voltage divider 2.2V at inverting input. this gives the voltage, I get that.

How would I know if the current is within 11.2mA? Correct me if I am wrong. I am connecting an ammeter and measuring current from the output to ground. Now that is short circuit current right? Thats the only current I can measure. But under load of the digital input, the current may be different. How to know what current enters the Motor controller? should I connect an ammeter in between the output of opamp and motor controller and check?

Thanks a ton!!

V

 

Thanks Ron! Just a question from my side in understanding...

So, no feedback at all, Just 5V into Non inverting input and voltage divider 2.2V at inverting input. this gives the voltage, I get that.

How would I know if the current is within 11.2mA? Correct me if I am wrong. I am connecting an ammeter and measuring current from the output to ground. Now that is short circuit current right? Thats the only current I can measure. But under load of the digital input, the current may be different. How to know what current enters the Motor controller? should I connect an ammeter in between the output of opamp and motor controller and check?

Thanks a ton!!

V

The input is basically equivalent to a ≈2k resistor to ground. I wouldn't fret about measuring the current. If you want to, connect the ammeter between the op amp output and the digital input. It should read zero when the input is zero, and about 11 mA when the input is high.

 

@ Ron... Thanks a ton! You have clarified a lot for me. Regarding your circuit, I plan to implement in the next phase. Right now, I am on negative feedback non inverting op amp and it seems to work well. I have pull down resistors at the output to make sure the current is limited to within the range and I measure it as well. So for now, I am using the above with a 2.2K pull down resistor at the output and then feeding the signal to the motor controller. Please let me know if I am doing things right at the moment.

Thanks!

V

 

@ Ron... Thanks a ton! You have clarified a lot for me. Regarding your circuit, I plan to implement in the next phase. Right now, I am on negative feedback non inverting op amp and it seems to work well. I have pull down resistors at the output to make sure the current is limited to within the range and I measure it as well. So for now, I am using the above with a 2.2K pull down resistor at the output and then feeding the signal to the motor controller. Please let me know if I am doing things right at the moment.

Thanks!

V

A pull down resistor goes from the output directly to ground. I think you are describing a 2.2k series current limiting resistor. This will form a voltage divider with the ≈2k input resistance of the controller, and you will get less than 12V on the input. This is not enough to switch the unit on.
You don't need to limit the input current. It will only draw as much as it needs (≈11mA).

 

Oh no no... I think I didn't describe it clearly. I changed the 2.2K current limiting resistor to a pull down resistor. The 2.2K is connected to ground and the output is given directly from the opamp to the motor controller.

btw, how did you know that the motor controller has an input resistance pf 2k ? is it mentioned in the manual?

Thanks!

Vj

 

Oh no no... I think I didn't describe it clearly. I changed the 2.2K current limiting resistor to a pull down resistor. The 2.2K is connected to ground and the output is given directly from the opamp to the motor controller.

btw, how did you know that the motor controller has an input resistance pf 2k ? is it mentioned in the manual?

Thanks!

Vj

You don't need a pull down resistor. The op amp has active drivers, and pulls up and down without the need for external resistors.
I said the input resistance is ≈2kΩ because 24V/11.2mA≈2kΩ.