# Help determining where does current flow through diodes

1. Mar 26, 2012

### oujea

Hello

Can anybody help me determine where does current flow in the circuit on the picture below:

How it goes when Uul is >0, and how when Uul is negative (<0). There is inverting OP AMP, and Uop will be >0 when Uul is <0, right? And how it goes then through diodes? Zener diodes here are not important for me, because they are only limiting output voltage.

2. Mar 26, 2012

### earlofwessex

the zeners are important as they are you only path for feedback. looks like a comparator and rectifier, why would you want to do that?
where did the circuit come from, what does it do?

3. Mar 26, 2012

### Staff: Mentor

A couple clarifications about the circuit. There should be an explicit connection dot above the resistor Rp connecting it to Uizl. And the polarities shown for +/-Um are backwards...

4. Mar 26, 2012

### oujea

I have to analyse how it works for different voltage Uul values. I have to find output voltage for all possibilities of the Uul voltage

5. Mar 26, 2012

### earlofwessex

hi oujea, what have you tried to do, and where are you getting stuck?
did you look up rectification? plenty of websites have good explanations of those four diodes in that arrangement.

6. Mar 26, 2012

### oujea

Unfortunately I didnt find anything helpful (maybe because my english is bad, so I cant search for the right thing). I tried to analyse this, and I found that all diodes (D1, D2, D3, D4) are conducting when Uul is negative. but I'm not sure if I'm right.

7. Mar 26, 2012

### oujea

the circuit on the picture is actually op amp limiter. here is it's characteristic:

8. Mar 27, 2012

### earlofwessex

hi Oujea, here is a simple site explaining that diode arrangement.

it was the second hit on google (search full wave rectifier circuit) below wikipedia.

are you supposed to have anything connected to the ±Um nodes? what analysis methods are you supposed to use?
the only way all four diodes would be forward biased is if you had something active there.
perhaps if you show step by step what you have tried, it will be easier to point you in the right direction

9. Mar 27, 2012

### oujea

This is not rectifier, and link you posted did not help me because it shows 4 diodes in Graetz circuit (and it is rectifier), but circuit on my pic is voltage limiter.

10. Mar 27, 2012

### earlofwessex

ok, well the principle is the same for those diodes.
lets back track a second, I'm making some assumptions here.

the first thing you do with op-amp circuits is look at the feedback path, which should give you an idea of whats going on. the feedback path in this case is from the output, through those zeners and to the inverting input (negative feedback).

there is another possible path, but only under certain conditions which are
1. there is (as Berkeman says) a connection between Uizl and the top of Rp. this seems likely, as the dot is missing from several other places obviously connected, but the drawing is a little ambiguous
2. there is some current path between -Um and Um, which also seems likely as they have the same Ident, and are drawn like outputs.

are these two conditions true?

if so, then another feedback path exists, but changes its route based on the difference between VUop and VRp.
its nature is described by the rectifier circuit if you model the unseen connection between Um and -Um as the DC load, and the source as VUop to VRp.

does what i'm saying make sense?