# Which diodes are On?

Discussion in 'General Electronics Chat' started by ENG99, Sep 2, 2015.

1. ### ENG99 Thread Starter Member

Feb 13, 2014
37
0
Hi,
In this pic I'm only sure about D1 which is ON and for D2 and D3 I'm not really sure about them! So, please help me out how can I determine whether if they are on or off and how?

2. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,224
D1 is on
D2 + D3 is off

Oops .....Recalculating

Nope D1 on, D2 + D3 off.

Last edited: Sep 2, 2015
3. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
1,541
408
I get D2=off, D1 & D3 =on

But the more I look at it the less certain I am.

Last edited: Sep 2, 2015
4. ### kubeek AAC Fanatic!

Sep 20, 2005
4,851
859
Iterestingly enough my sim tells me that not all of them are ON. Sadly I don´t have the mental capability right now to figure out how to reliably come up with the result.

Edit: it seems there could be a bit of uncertainity coming from the assumed Vf of the diodes, which might turn D3 on as well. My sim seems to assume Vf of 0.56V at this sort of current with 1N4148 and states that D3 is off.

5. ### GLPro New Member

Aug 21, 2014
4
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D1 & D3 on............D2 off?

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,070
What assumption are you making about the forward voltage drop of a conducting diode?
0V or 0.6V or something else?

cmartinez likes this.
7. ### Lestraveled Well-Known Member

May 19, 2014
1,957
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The symbol is not for a schottky so I assumed a silicon diode with .7 Volt drop.

8. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,586
1,584
Ignore D2 and D3. Assume all diodes are perfect. Assume all diode's Vf are 0.6 V.

Loop current is 14.4 V / 30.2 K = 476.8 uA.

The node voltages are:

8.2K/12K = +5.49V. This is greater than 4.4 V, so D2 is off.

12K/10K = -0.232 V. This greater than -0.6 V, so D3 is off.

ak

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9. ### Lestraveled Well-Known Member

May 19, 2014
1,957
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AK, I used the same logic. Agreed (I am sure Mike is plugging this into spice as I type.)

10. ### ENG99 Thread Starter Member

Feb 13, 2014
37
0
Well you consider D1, D2 on and D3 off! Sorry it's wrong answer!

11. ### ENG99 Thread Starter Member

Feb 13, 2014
37
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you got them right! But how? I spent an hour trying to find how but i couldn't. plz show me how you found that, thanks

Last edited: Sep 3, 2015

Feb 13, 2014
37
0

13. ### ENG99 Thread Starter Member

Feb 13, 2014
37
0
Well, you got it, but how?!

Feb 13, 2014
37
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0V

15. ### ENG99 Thread Starter Member

Feb 13, 2014
37
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Sorry but the answer isn't correct!

16. ### Dr.killjoy Well-Known Member

Apr 28, 2013
1,234
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Try making the circuit to better understand..

17. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,224
Well isn't that special. You present an electrical puzzle and you leave out critical data. Then after several opinions are entered, you release the critical data at the same time you proclaim the winners and losers. Now where is that "Ignore" button I have been hearing about.

18. ### Dr.killjoy Well-Known Member

Apr 28, 2013
1,234
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I just a simulation on my phone and it say D1 on , D2 &D3 off..

19. ### ENG99 Thread Starter Member

Feb 13, 2014
37
0
This is NOT electrical puzzle this is example I found on google and I have been trying to solve it! And I shared it here so everyone can give me an idea how s/he got it. It's not my problem what do you think and how do you think! I asked question and I didn't write who is winner or loser YOU wrote it. And have fun with ignore button!

20. ### ENG99 Thread Starter Member

Feb 13, 2014
37
0
So, then Mesh analysis? I'll end up with 4 Loops? But how can I determine which one is on and which one is off?!