Voltage control with Ops Amp

AnalogKid

Joined Aug 1, 2013
10,987
Is this correct
No. Think.

Ignoring the wiper, R1 and VR1 form a 2:1 voltage divider. Therefore, the maximum voltage into the opamp is 12 V. With the opamp configured as a voltage follower, the max output voltage is the same. Based on your posts so far, you should be able to figure out what to do about this.

When you have an idea, rather than throw up a schematic and ask if it will work, post the schematic and explain to us why you think it will work. In working through the explanation, you will find and correct many of your own errors.

ak
 

Thread Starter

Dave Newbie

Joined Jun 12, 2005
23
No.
An opp amp attempts to keep the voltage difference between the inverting and non inverting inputs to zero. You want the gain of the opp amp (With it's associated components.) to be x 4. R2 and R3 form a potential divider. You can't calculate the actual value of R2 and R3 but you can calculate the ratio of their values. So if we say that R2 is 10 K calculate the value of R3. I am not giving you the answer as I am trying to get you to understand op amp circuits. I you have to think about how it works you will be able to work out how to do the calculations for different configurations.

Les.

Thanks for the lesson .

Volt into Ops amp is 6 volt we need gain of 4 so Rf is 30K and Rin is 10K.
1627947008117.png1627947008117.png
 

dcbingaman

Joined Jun 30, 2021
1,065
Hi, I am trying to control ops amp output voltage from 0 to 24 volt. Intent to use a digital Pot for VR1. Being a newbie at this like some advise.
what op amp to use to control 24V and if this schematic needs more attention.View attachment 244840
There are some simple rules that if you keep them in mind you can generate just about any op-amp circuit:
1. The input impedance to the + and - pins can be considered open.
2. The output impedance can be considered 0 ohms.
3. The op amp output will do whatever it has to do to try to get the voltage on + equal to the voltage on -.

Using these rules and nothing more than oms law V=IR. You can determine the output of the op amp for any given input. Recommend you try it on the two circuits I supply you with.

I am assuming you forgot to add the power to the opamp.

Questions to help:
How much current is going through R1 , R2 and VR1?
Being that the + terminal is at 24V what will the output be in order to hold the - terminal at 24V?
 
Last edited:
generally, you would start with a reference. 2.5 and 5V are common. The potentiometer goes across the reference. Then you would buffer the wiper of the POT. Now amplify to get 24V.

You do have to find an OP-amp good for 24V and good for the current you need. Since an OP amp can;t reach the rails, 0V is unobtainable without a slight negative supply. There is a chip that will do that, but I don't think it will work with 24V in.

I don't think you mentioned what current you need?

Another way, if you need the 24V for a different reason and not for power, you can make a voltage controllable current source across a resistor.
 
Remember that any op-amp (when working properly) moves its output in such a way as to try to make the + and the - inputs the same voltage. That is, if the +in is at a higher voltage than the -in, the output voltage will increase and if that causes -in to increase (or +in to decrease (such as the output driving a transistor which shorts the +in) ) then that's ok.
(Working properly means that supply voltage is correct, +in and -in are within the correct range, etc)
 

ronsimpson

Joined Oct 7, 2019
2,989
Did you check the input common mode voltage range for your amp. Many amps do not function of the inputs are too near the supply voltage. (plus supply is worse) Make certain it will function with inputs at zero volts.
 

dcbingaman

Joined Jun 30, 2021
1,065
Did you check the input common mode voltage range for your amp. Many amps do not function of the inputs are too near the supply voltage. (plus supply is worse) Make certain it will function with inputs at zero volts.
Good point. The AD8065 can operate at max 24V power single supply. If I am reading the datasheet correctly it appears this opamp can handle inputs at the supply range. If I am reading it correctly.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi Dave,
Could you tell us exactly what this 0v thru +24V at 40mA is going to be used for.??

So that we can stop guessing what type of OPA would be suitable.?
also confirm the Digi-pot type specification.?
E
 

Thread Starter

Dave Newbie

Joined Jun 12, 2005
23
hi Dave,
Could you tell us exactly what this 0v thru +24V at 40mA is going to be used for.??

So that we can stop guessing what type of OPA would be suitable.?
also confirm the Digi-pot type specification.?
E

DC selsyn system is widely used. as a transmitter, an indicator system. The intent is to replace the transmitter with a magnetic encoder. . Based on the figure below Convert magnetic output to Synchro output at A, B and C. Note system is 24 Volt. Resistance between A, B and C is 410 Ohm.

1628031832335.png

I have the output reference voltage at A B and C. So the ops amp is to provide that voltage to the transmitter based on AB C.
 

Thread Starter

Dave Newbie

Joined Jun 12, 2005
23
DC selsyn system is widely used. as a transmitter, an indicator system. The intent is to replace the transmitter with a magnetic encoder. . Based on the figure below Convert magnetic output to Synchro output at A, B and C. Note system is 24 Volt. Resistance between A, B and C is 410 Ohm.

View attachment 244992

I have the output reference voltage at A B and C. So the ops amp is to provide that voltage to the transmitter based on AB C.

So it's encoder referance to 3 digital Pot to 3 ops amp ouput.
 

LowQCab

Joined Nov 6, 2012
4,026
Now that we finally get to see what it is that You are trying to do .......
The next question is,
why are You using 3- Op-Amps instead of the special Circular Resistor Arrangement ?
and,
Are You using the 3-Phase "Motor"/ Indicator exactly as pictured ?
Or,
is there some other similar device (Indicator) that You wish to Power ?
Are You building your own DIY "version" of this device ?
In other words, do You actually need Plus and Minus 24-Volts for a particular reason, and
will the Resistances of the Windings still be ~410-Ohms ?

What is the entire Signal-Chain ?
What will this device be Indicating the position of ?
.
.
.
 

LesJones

Joined Jan 8, 2017
4,174
What kind of magnetic encoder do you plan to use ? If you mean an AC synchro transmitter then you will have to use synchronus rectification to convert the three AC outputs to DC signals with varying output and polarity. In the past I have used the DC selsyn systems for antenna position indicators but I used the original type of resistive encoder.

Les.
 
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