# Help me in This Circuit

Discussion in 'General Electronics Chat' started by khanz, Jun 15, 2008.

1. ### khanz Thread Starter Member

Mar 24, 2008
21
0
Hi All,
I am posting my circuit here on this website for some help. I am not using op amp for voltage addition. Please help me in adding these two voltages. The maximum output voltage is 10V.
If you have any question after looking at the circuit please post it.

Thanks

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2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
As you have drawn the circuit, the output measured from the lone 10k resistor to ground will be 12v.

That is because there is no ground reference supplied to the lower end of the two 10k potentiometers.

Once you connect the ground to the lower end of the two 10k pots, you will get the AVERAGE of the two output voltages.

Resistors are current limiters. If you use pairs of resistors in series, they are voltage dividers. You are summing the two voltages, but then dividing them by two using resistors.

You will need to use an opamp to multiply by two to get the sum.

3. ### khanz Thread Starter Member

Mar 24, 2008
21
0
Thanks for the suggestions. But the problem is this that I don't have any plus and minus voltages, and also I am not fully aware of op-amp funcationality. I have just studied it, so please if you help in building the circuit without using op-amp, it would be much appreciated.

Thanks once again

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728

Take a look here:

You will see a summing circuit using an opamp.
A 741 is a woefully outdated opamp. It's still useful for some things, but there are far better opamps out there. Look for one that is for single supplies and has rail-to-rail inputs and outputs.

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
For signal processing, a single supply op-amp adder would indeed be a better solution. Be sure the op-amp runs on a voltage greater than the expected sum of the inputs, since most op-amps do not run rail-to-rail.

If you are dead-set against op amps in this application, perhaps this will work for you:

I've used a similar circuit to drive high-side MOSFETS. (I prefer charge pumps, though.)

6. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Thingmaker, that would definitely work - however, there would be quite a loss of power in the resistors. If there were any load on the output, regulation would be nonexistant.

Seems that a pair of LM117/LM317s would be a more "clean" solution; it would also keep the supply impedance the same as that of the LM117/LM317. Make the ADJ ground reference for the upper LM117/LM317 the output of the lower LM317.

Of course, bypass caps are still needed on the output of both Vregs to keep them well-regulated.

But, we don't know what Khanzs' application is.

7. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
Indeed. Without knowing the application, it is hard to recommend a charge pump versus an adder versus a flux capacitor.