# Circuits with diodes - III

Discussion in 'Homework Help' started by PsySc0rpi0n, Jun 30, 2015.

1. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3

Yes, I got that, but you said that was something missing... Was that that I was looking for!

Anyway, now I'm trying to workout a bit with zeners.

Let me know if I'm correct about zeners:
When we want to replace the zener by it's equivalent model, the attached picture is correct in terms of the batteries polarity?
That way I will use always a positive value for Vzener and the real sign will be determined by the batteries polarity.

And if I call Va, the voltage at zener's anode and Vk, the voltage at zener's cathode, can I say the follwoing:

Vk > Vz --> zener state
Va > Vk --> diode state (ON)
Va < Vk --> diode state (OFF)

• ###### aac1.png
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Last edited by a moderator: Jun 30, 2015
2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,605
396
hi,
Why don't you use the LTS lib zener models.?

Run these two sims and tell me if your definition is correct.

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3. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
Mr. EricGibbs, those 2 circuits are exactly the same, I guess!

4. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,605
396
hi,
One circuit has regular diode, which does not conduct when the Cathode is at higher potential than the Anode.
The other is a zener diode which is able to conduct in either direction. ie at the zener voltage and the diode voltage

A zener diode conducts when the positive voltage on the Cathode is equal too or greater than the stated zener voltage, relative to the Anode.
So a 3.3V zener would not start to conduct until the Cathode is 3.3V positive relative to the Anode.

It also conducts as a regular diode when the Anode is positive, by approx 0.7V relative to the Cathode.

A battery voltage equivalent to a zener voltage would not give the same result in a circuit path when subject to an alternating voltage.

Compare the two asc file plots I posted.

You should have zener models in your LTS library.?

E

5. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
MOD NOTE: When you want to start a new problem, PLEASE start a new thread! If you don't, then people will tend to look at the first post in the thread to see the original problem that you are presently discussing. It creates confusion and chaos.

6. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
ok. I'm sorry!

But if I need to ask help for a new circuit with zeners I should start a new thread?
This one was only to ask if my assumptions were correct!

I have now a simple circuit to try to understand if my assumptions are correct or not. Should I post the circuit here or on a new thread?

7. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
Yes, you should start a new thread. Again, someone looking at the last few posts in a thread often needs to look at the original circuit that is being discussed before they can form a meaningful response. Where do they look? The natural inclination is to look at the original post, where they see a circuit and they, quite reasonable, assume that that is the circuit that is being discussed in that thread. It creates chaos!

If you want to start talking about a new problem that is related to another problem, the start a new thread and, in the original post, put a link to the other thread that the new one is a take-off from.

8. ### ericgibbs AAC Fanatic!

Jan 29, 2010
2,605
396
Morning Psy,
If you are satisfied with information/answers already posted regarding the earlier circuit in post #1, then as advised start a new thread [ with a new title].

If you have any outstanding questions/queries regarding post #1 circuit, use this thread.

It is confusing in trying to determine which circuit you are referring too in your posts, we have to keep scrolling back and forth trying find a circuit that fits your latest question.

E

9. ### PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
3
Ok, sure...

I think this thread can be closed as I think I already know how to replace zeners by a correctly oriented voltage source!

10. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
Good! It just takes practice and perseverance.

We don't close threads unless they have become problematic wrt the ToS -- we just let them die a normal death.