# DC Servo Problem

Discussion in 'The Projects Forum' started by Sommer, Aug 21, 2013.

1. ### Sommer Thread Starter New Member

Jun 3, 2011
4
0
Hello
Im making a simple DC servo.
But i have a problem with the output of my DC Servo.
I always have 14 mV DC on the output no matter what i do.
It's a bit like it is all created at the opamp. but it shouldn't do that.
If anyone have some answers that could help me have no DC on my servo i would really appreciate it.
I have made a simple Schematic you can see. Of course the power supply is not added but i thought it would confuse some. But both opamp have + - 12 V DC.

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2. ### jegues Well-Known Member

Sep 13, 2010
735
45
That's a pretty confusing diagram.

Why are there blue + and - symbols drawn on the opposite terminals of each of the op amps? What voltages are you using as the positive and negative power supplies to the op amps? What type of signal are you injecting at the signal terminal?

3. ### Sommer Thread Starter New Member

Jun 3, 2011
4
0
I see the confusing part of it now.
Dont look at the V+ and V- in the opamps.
The opamps is correctly supplied.
The blue + and - is the input to the opamps.
The red numbers is the values i have measured.
As far as i understand the circuit it will work as i will write underneath:
e.g.
If there is +1 V DC on the signal (lets say a signal generator gives a sinus plus +1 V DC)
I want to remove the DC value.
I have my amplification that is 2x the signal.
That will give +2 V DC to the DC servo.
The DC servo should send out - 2 V DC. At the output of the Servo there is a voltage divider that halves the output so the output send to the signal is -1 V DC.
+1 V DC from the Signal Generator minus the -1V DC from the DC servo should give 0 V DC.
With the 499K resistors and the capacitor at the DC Servo OPAMP the DC servo should have a big delay. this is so it will not oscillate.

Where am i doing something wrong or where have i misunderstood something?