What kind of op-amp is that?

NascentOxygen

I believe you realise this is so not right that you need to make a fresh start.

Hint: you know V₊ so determine V_.

Femme_physics

Taking your criticisms into consideration, here is my new idea...




;)

I like Serena

Looks good...

Femme_physics

Ah..the legendary Klaas enthusiasm persists I see...
Thanks.


How?

If I use the Voltage Divider, it gets nullfied. Should I just use KVL to try and find it? Seems like a longer route, but I guess I can... unless I'm not seeing something?

NascentOxygen

Alternating between 2 threads is probably half the trouble, you are not giving yourself a chance to get to grips with either circuit. Let's stay with this one until you get a few fundamental misconceptions sorted out.

The op-amp inputs draw no current (so we say), so the op-amps can be ignored when it comes to affecting any circuit you connect to their inputs. You have a resistor divider here. The op-amp (-) input has no effect on the divider currents or voltages, so you can forget about it and just concentrate on the resistors.

EDITED: corrected

So V_ is set by the 12v and the resistor ratios.

What is V₊ set to?

Femme_physics

V+ is ground.

How did you figure V- is necessarily 12V? Which formula or principle did you use?

NascentOxygen

By V₊ I mean the voltage on the op-amp's non-inverting input, often denoted V₍₊₎.

V_ is what you worked out here. http://www.physicsforums.com/showpos...2&postcount=21

Except you wrongly equated it to 0. It's value is a fraction of the op-amp's output, and the specifications of the problem tell you the op-amp output voltage is +12v.

Femme_physics

Now I understand. There's still voltage at V-, but no current. I keep forgetting that,that's the op-amp properties.


Yes, Vout is 12
V- = a fraction of the output
V+ = 5 V (Due to the zener diode)
So V- = 7 V


Wait, are you telling me that for any op-amp no matter what Vout = (V-) + (V+) ?

Anyway, here's the new refined solution :)



I hope!

NascentOxygen

Correct for V₊. But you are forgetting the important thing about op-amps (on which I expounded at length in your other thread) that V_ = V₊ when operating as an amplifier. So V_ is not 7v.

In light of what I've just written, perish the thought!

You are not using your time to best advantage, Femme_physics. You have gone on and done further work, futilely, without waiting until you have had the answers here confirmed. So now you have to recalculate.

And don't forget that you already have worked out the value for the resistor feeding the Zener. I think it is Ra. So you are slowly getting there.

But as you will note, we haven't mentioned Rx yet.

Femme_physics

Actually I did use some time to my advantage, and I thought I got Rx, but when I tried calculating it turns out I got the same equation despite the fact I picked two different loops....my equations cancel out Rx therefor!




Oh right, as long as they're not used as comparators! I remember that explanation!

So V- = 5 V!


Great. I can redo the calculation setting 5 instead of 7, but am still stuck with Rx...

NascentOxygen

There are a number of theories accounting for the inclusion of Rx in this circuit. Fortunately, we don't need to understand any of them in order to determine what value the designer must have chosen for Rx. We can come back and consider the finer points of his design philosophy later.

You have noted there is a potential difference across Rx of 13v. Good.

The current that flows from the 25v supply through Rx must get to ground somehow, otherwise no current would flow. 3mA of it goes into the output of the op-amp. I can see that you account for about another 2mA thorough the zener, to keep the zener's voltage firmly at 5v. Any other routes for Rx's current to get to ground that you can see?

Once you have accounted for all the current through Rx you can use Ohm's Law to determine what value Rx must be.

Good luck!

Femme_physics

I think I got it... full solution here:





What do you think?

I like Serena

Flowers...
Or...
More flowers...

Femme_physics

Yea?!? I got it?! YESS!! YSES!!!!!!!!!! WEEPIE!!!! I love my life. Really was a great walkthrough. I really appreciate your help, this feels orgasmic to finally get it done!! Now, there's that other annoying exercise to conquer and hopefully this is the last op-amp that would give me troubles!

THANK YOU! Nascent, you're great. ILS, thanks a bunch as usual!

I like Serena

Now can I get a drawing?
Preferably with some flowers? And Or?

I like Serena

Femme_physics

Oh yea, you'll get 2 flowers, and 1 Or! Tomorrow after a good sleep. It's 1 A.M. and Or doesn't look so great after a marathon of HW. :)

EDIT: OOOOOOOOOH! Thanks :) I'll do one myself too.