# multiple voltage followers with individual offsets.

Discussion in 'The Projects Forum' started by Soggy Canuck, Dec 13, 2015.

1. ### Soggy Canuck Thread Starter New Member

Dec 12, 2015
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I'm working on a machine that has a master drive that outputs a 0-10 Vdc signal based on machine speed. The machine has 40+ secondary motor drives grouped in quantities of 4 to 8. Each group will take the base 0-10 Vdc signal and an additional adjuster signal to allow for group speed adjustment.
Given the limitations of the current sourcing of the main drive signal I had planned on using a series of Op-amp non inverting voltage followers with NPN transistors to drive the roughly 70mA of current that the 8 ganged drives will require.
So far so good, where I'm drawing a blank is how to handle the individual group offsets. I was thinking about a potentiometer tied to the Op-amp supplies to give me a +/- signal fed into the Op-amp input as a summing signal.
Question 1: I'm guessing I need to drop a small signal diode and resistor on the the master drive analog signal to prevent circuit cross talk?
Question 2: How would I ensure the offset voltage doesn't load the master drive signal if the offset signal is lower than the master drive signal?
Please forgive the format of the questions, I'm a practicing controls electrician whose electronics training is 30+ years out of date. I was originally trained on vacuum tubes if that gives anyone here an idea how out of date I am.

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,143
768
The best language in the world for electronics, is a Drawing!, draw a circuit diagram of what you want any chips, relays, etc...then we can help you further.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
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Here is one way: The input V(in) is between 0V and 10V. The offset input that is summed in V(offset) goes from -2V to +2V. The gain between V(in) and V(out) is unity. See the yellow trace. The plot shows the transfer function, V(out) vs V(in) at different values of V(offset).

The offset at V(out) goes from -1V to 1V even though V(offset) goes from -2V to +2V because the gain of the summer is -R1/R4 = -10K/20K =-0.5. It can be made any relative gain depending on how big you want the offset. Note that the current booster cannot offset in the negative direction... (well it can with a modification).

Because the summer inverts, U2 has to be there to undo the inversion caused by U1.

Last edited: Dec 13, 2015
4. ### Soggy Canuck Thread Starter New Member

Dec 12, 2015
4
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Old dog, new tricks. all of my schematics are hand drawn. I'm not up on all the new fangled CAD software.

5. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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The way I did it, there is no interaction between your "master drive" and the offset voltage. The offset voltage can come from a pot as easily as from a voltage source...

6. ### Soggy Canuck Thread Starter New Member

Dec 12, 2015
4
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Thank you for your input MikeML, I'll build up a breadboard sample and try it out. I'm sorry I was not responding to your first post. I was off drawing a diagram as requested by the previous poster. If you don't mind me asking, what did you use to draw up your schematic. It looks like something I should invest in.

7. ### AnalogKid Distinguished Member

Aug 1, 2013
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1,297
That is LT Spice, a simulation program based on SPICE, and free from Linear Technology at linear.com.

What is the range of the +/- offset voltage?

ak

8. ### Soggy Canuck Thread Starter New Member

Dec 12, 2015
4
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Not wanting to expose an unbelievable level of ignorance....but SPICE?
As to your question, I won't know for sure until I test on the machine, but if I had to guess, I would say +5/-3V. That will be my starting point at any rate.
Would LM324 op amps work in your circuit, I have a good stock of them to play with.

9. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
LTSpice. Freebie at Linear.com. It has a bit of a learning curve to it. It is not the best drawing package; its power is the simulator.

10. ### Veracohr Well-Known Member

Jan 3, 2011
559
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It's an acronym which means "Simulation Program with Integrated Circuit Emphasis", and has been around for 40 years. LTSpice is pretty easy to use, but like anything, if you're new to it there's a learning curve. Plenty of people here know it enough to help with any questions you have. I also suggest the LTSpice User's Guide for knowing which parameters which are available for editing/defining.

11. ### crutschow Expert

Mar 14, 2008
13,479
3,367
To further elaborate, SPICE is an simulation program that does a reasonably good job of simulating analog circuits consisting of common analog components and ICs such as transistors, op amps, passive parts, etc.
There are also models available on the web for LTspice that allow simulation of simple digital ICs such as the CD4000 series of CMOS digital circuits.
I would never build even a simple circuit without simulating it first.
It's amazing how many design errors I've discovered doing that.