# Voltage calculation - Help

Discussion in 'General Electronics Chat' started by imraneesa, Feb 16, 2015.

1. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
I found this circuit some where in the internet. I am trying to find how they got the value Vref as 10.5v. Can anybody please give me how they compute the vref as 10.5?

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Presumably by adjusting the 20k potentiometer until the required value was obtained .

3. ### MrChips Moderator

Oct 2, 2009
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The 20kΩ is variable resistor wired as a voltage divider also known as a potentiometer or pot for short.
You manually adjust the pot until you get the desired voltage,

4. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
If it is not the potentiometer and if we assume it as 20k resistor. Then what will be the Vref. Can we compute it?
If yes how?

5. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
You would then simply be tying the op-amp positive input to Vcc and the circuit wouldn't function as intended.
Perhaps you need to better understand the overall circuit operation.

6. ### MrChips Moderator

Oct 2, 2009
14,504
4,271
There is nothing to compute. You adjust the pot until you get 10.5V.

7. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
OK understood. I thought op amp is acting as comparator without any reference to the output. And if v+ is greater than v- then Vout=Vcc. If v- is greater then Vout=Vee. Then the current ic is adjusted by resistor (10ohm).
So by the way you all explained can I assume Vref as 10v instead 10.5v. And adjust potentiometer to get 10v.

8. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
If I use voltage divider.
Vref = (Vcc × 20k)/(20k+10)
= 11.99v if vcc is 12. Is it right?

9. ### MrChips Moderator

Oct 2, 2009
14,504
4,271
No. The opamp is not acting as a comparator. The output is fed back to the inverting input which makes it a unity-gain voltage follower.

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10. ### MrChips Moderator

Oct 2, 2009
14,504
4,271
No. RE is not part of the voltage divider.

The total resistance of the pot is 20k.
Part of the pot is R (let us assume the lower part). The upper portion is 20k - R.

Vref = Vcc x R /20k

11. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
Thank you so much for clearing my doubts. I really love this forum and it's people.

12. ### WBahn Moderator

Mar 31, 2012
20,221
5,747
It's hard to see how this thought is reasonable given that the inverting input of the opamp is tied to a node whose voltage depends on the output.

If that's the case, then what would the current be?

Why do you think you can assume it is 10V instead of the 10.5V that it is stated as being adjusted to?

If you want to adjust it so that it is 10V, then do so. That will result in 2V being dropped across the 10Ω resistor and, thus, 200mA of current in the LED.

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13. ### WBahn Moderator

Mar 31, 2012
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You can't just pick the first to resistances you see and throw them into an equation and expect the result to have any meaning whatsoever.

You need to understand what the voltage divider equation means and when it does and doesn't apply. It applies when you have two resistances that are in series. The 20kΩ pot and 10Ω resistor are not in series.

The potentiometer is effectively two resistors in series, one between the top of the device and the wiper and the other between the wiper and the bottom of the device. They are effectively in series because only a negligible amount of current flows in/out the wiper terminal. The sum of the two resistors is equal to 20kΩ. The position of the wiper merely determines how much of that appears above the wiper and how much appears below.

14. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
I was mistaken. I thought they are in seeies . iam in very learning stage.

15. ### imraneesa Thread Starter Member

Dec 18, 2014
198
4
I have design for buck driver for led. Can I use that for laser diode. What makes difference in design between led and laser diode if Vf and current requirement of led and laser diode is same?

16. ### WBahn Moderator

Mar 31, 2012
20,221
5,747
That's fine -- making mistakes is how we learn best and having our mistakes pointed out to us is how we learn best the fastest.