Help me configure opamp resistance and voltage

Hey everybody,

i'm not ver used to creating circuits, so I'm not sure if what i made is going to work. Can anybody tha know better chek it out?

This is my circuit




I need to have a voltage of 12 in the 10k ohms resistance, which represents a little engine (with aproxximatly 80 ohms). The problem is that if i have 10k ohms resistance i get a 11.5 volts whic is ok, but if i use an 80 ohms resistance I get around 3V.

If you understood, can anybody help me?

BTW those two opamps are simply a multiplier, and an adder. All I'm trying to do, is to:

- If I have 3.3 in the source - I have 12V on the engine
- If I have 0V in the source - I have -12V on the engine(reverse)
- If I have 1,65 in the source - I have 0 V on the engine (off)

Re: OpAmp configurations

To drive a load of 80 ohms, you'll need an op amp with a low output impedance. Maybe 5 or 10 percent of your load impedance.

A typical op amp (741) puts out 20 mA.

Your load draws 11.5 / 80, or 144 mA. The op amp cannot drive that much load.

You'll need either a power op-amp rated for that much current, or you'll need to add a pair of push-pull transistors to your present op amp.

I also believe you can use lower volt levels as your supply. +12 and -12.

Re: OpAmp configurations

BradtheRad said:

To drive a load of 80 ohms, you'll need an op amp with a low output impedance. Maybe 5 or 10 percent of your load impedance.

A typical op amp (741) puts out 20 mA.

Your load draws 11.5 / 80, or 144 mA. The op amp cannot drive that much load.

You'll need either a power op-amp rated for that much current, or you'll need to add a pair of push-pull transistors to your present op amp.

I also believe you can use lower volt levels as your supply. +12 and -12.

Click to expand...

Ok, let me figure it out... so i was gonna use the 741, but instead I should use another one with a lower output impedance... I get it, but about the other resistences, do I need to change them? for example the 100kOhms on the right upper corner, wouldn't it drain all the current from my output??

I'm sorry for being very new to this... lol

Re: OpAmp configurations

A 100k ohm will carry very little current.

The high resistances are able to be used around an op amp by benefit of it having high input impedances.

Below is a working schematic for what you wish to do. It shows the op amp driving your load directly. It would need to be the type with low output impedance.

The dual oscilloscope traces show the response:
(a) 3.3 V in, +12V out.
(b) 1.65 V in, 0V out.
(c) 0 V in, -12V out.



There are other ways to do the job.

(a) Single 12V supply with an H-bridge. This is 4 transistors/mosfets arranged in a way so that it is possible to send current through your load in either direction.

(b) Bipolar 12V supply with NPN and PNP transistor in a push-pull arrangement.

However the above methods have their problems which must be solved.

Re: OpAmp configurations

Hi,It looks you want 10V with 80ohm and 12kohm.You can use LDO architecture to drive such a wide range of loading.
https://www.analog.com/library/analogDialogue/archives/41-05/ldo.html

juliopaveif said:

- If I have 3.3 in the source - I have 12V on the engine
- If I have 0V in the source - I have -12V on the engine(reverse)
- If I have 1,65 in the source - I have 0 V on the engine (off)

Click to expand...

If you want +-12V on the motor then the supply must be more than +-12V. +-15V or +-18V would be good. +-24V is too high; it is too much for a 741 and will waste power.

The circuit below is a bit more complicated than the one shown by BradtheRad, but is also more accurate. The transistors can dissipate up to 1W of heat, so should be mounted on heatsinks.

If you use a power opamp then you do not need the transistors, but the opamp should be mounted on a heatsink.

Re: OpAmp configurations

Seriously guys, thank you soo much for all your help, I'm very new here, and actually didn't even expect that much help.

Thank you ver much! I'll check all these circuits, and see the one that fits me the best!

Thank you.

- - - Updated - - -

I tried several configurations, and ended up using the H bridge! it seems to be the best solution for me, since i'm using a motor...

Again, I thank you all, for your help!