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OP Amp for Digital Pot

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rayhall

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I have this circuit to vary the voltage from 6 to 22 volts. The problem is it does not work. I get voltages around 6 volts all the time.

**broken link removed**

I believe I need an OP Amplifier and have the circuit like this. Can some one suggest an OP Amp to use and is this going to work.

Ray.

**broken link removed**
 
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There's no gain so of course it won't work.

You need to configure it like a non-inverting amplifier.

To get 0 to 22V, you need a gain of 4.4.

To get 6 to 22V, you need to connect the lower side of the pot. to another resistor and vary the voltage between 6V and 22V it's a little more difficult.
 
I have this circuit to vary the voltage from 6 to 22 volts. The problem is it does not work. I get voltages around 6 volts all the time.

The main objective is to make it function as well as how a variable regulator functions, correct? Why bother with the additional components? All you need is the pot and R1. Pardon me if I'm not seeing it from other standpoints as well.
 
It's a digital pot. which means its maximum voltage rating is equal to the digital supply voltage which is 5V in this case.

I was wrong earlier, it's not difficult to get 6V to 22V, there are two easy solutions:

  1. Connect a resistor in series with the lower side of the pot. as I said above, you need to configure the op-amp with a gain of 4.4. What I didn't way is that the LM317 needs to be in the feedback loop of the op-amp. The value for the resistor in series with the pot, needs to give 6/4.4 = 1.36V when placed in series with the pot. then the voltage from the pot. will vary from 1.36V to 5V which multiplied by 4.4 to give 6V to 22V.
  2. Set the LM317 circuit to an output voltage of 6V and connect the op-amp's output to the usual 0V point of the LM317 circuit. Now you want the op-amp output to vary from 0V to 22-6V, 0V to 16V when the input is varied from 0 to 5V so you'll need a gain of 16/5 = 3.2.

I would choose #2 myself as it's probably the easiest solution.
 
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I see. Digital pot... that's quite new to me.


What's the difference between a manually operated and a digitally controlled pot?

Do I watch the wrong movie here?

Boncuk
 
Well... I only have the capability of visualizing mechanical pots.. you know.. twist and get the resistance varied.. digitally? Would that mean there would be ohmic resolution issues?
 
Just to note that the output voltage stability of the circuit with the op amp will now be determined partially by the +5V supply voltage, not just the 317 reference.
 
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Can I ask a quick question, I was trying to do something similar to this using a 741 with a gain of 10 to control the duty cycle of a 555 timer. This was my attempt to make a servo from scratch. The problem was, I could not get the 555 to work the H-Bridge properly. Both inputs on the op-amp had a pot attached, one would "sense" what the motors position was, and the other was to control the motor. Can someone tell me why something so simple didn't work?!?!
 
You should really start your own thread and post some schematics.

You can vary the voltage on pin 5 to change the duty cycle. It will be inverted, a higher voltage will be a lower duty cycle, so you need an inverter either before or after the circuit.
 
2. Set the LM317 circuit to an output voltage of 6V and connect the op-amp's output to the usual 0V point of the LM317 circuit. Now you want the op-amp output to vary from 0V to 22-6V, 0V to 16V when the input is varied from 0 to 5V so you'll need a gain of 16/5 = 3.2.


I would choose #2 myself as it's probably the easiest solution.

Thanks Hero99...

I have used your #2 option and it works in software. I will now test it in real hardware with the digital pot in the circuit.

Here is what I ended up with.

Ray.
**broken link removed**
 
I thought you wanted 6V to 22V?

That will go all the way down to 1.25V.

Here's a circuit which will go down to 5.7V and up to just over 22V
 

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  • 6V to 22V digipot&#.png
    6V to 22V digipot&#.png
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Yes I was after 6v to 22v. My circuit went lower then 6 volts, but I was willing to settle for that. Thank you for the new circuit design, I will use it.

Ray.
 
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