# Voltage Regulation Using Diodes Problem

#### alkozy

May 31, 2017
3
hi everyone
what i know about the problem: that the diodes have 0.7 voltage each as Vd
diodes 3 and 4 only will conduct if the input voltage Va is positive. in this case there will be an output voltage at both C and B ( C is that output before the 1k ohm R )
for a negative voltage diodes 1 and 2 only will conduct and for this case there will be output voltage at B only how much???
this is what i am asking for

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#### duke37

Jan 9, 2011
5,364
The diodes drop 0.7V each so 1.4V total.
When D1,D2 conduct you have the input voltage minus the diode drop across the 5k resistor so you can sketch the resulting voltage.
Have a go at this and then we can move on to D3,D4.

#### alkozy

May 31, 2017
3
The diodes drop 0.7V each so 1.4V total.
When D1,D2 conduct you have the input voltage minus the diode drop across the 5k resistor so you can sketch the resulting voltage.
Have a go at this and then we can move on to D3,D4.
I hope the pic is clear

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#### duke37

Jan 9, 2011
5,364
The picture is not clear, use a fatter pen.
The question asks for a sketch of a sine wave of peak voltage 5V so start with the voltage on a vertical axis and time on the horizontal axis.
You seem to have plotted the forward/backward characteristics of a diode. This was not asked for.

#### Laplace

Apr 4, 2010
1,252
The question asks for a sketch of a sine wave of peak voltage 5V so start with the voltage on a vertical axis and time on the horizontal axis.

See if you can sketch your answer on this as a background.

#### Electrical Engineer David

May 29, 2017
11
When the current reaches 0.1 mA the voltage across D3 and D4 will be 1.4 V. Assuming ideal diodes the voltage across D3 and D4 will remain at 1.4 V above 0.1 mA of diode current. Where will the current reach 0.1 mA? The answer is, when (Va-1.4)/(5000 + 1000) = 0.1 mA. Solving you find that when Va reaches 2 V then the diodes will be fully forward biased. So between 0 and 1.4 V on the input (i.e. Va) we can assume little to no current flow meaning Vb = Va and Vc = 0. At about 1.4 Va to 2 Va the diodes will begin to conduct. Above Va = 2 V the diodes will be fully saturated and what you have here can be thought of as a voltage divider output. So at Va=2V Vb will be 1.5V and Vc will be at 0.1 V. At any Va above 2 V the portion of the sine wave above 2V will be divided by (5000 + 1000)/1000 = 6. So any Va above 2 V will increase Vb above 2 V by an amount = (Va - 2 V)/6. Vc will increase from 0.1 V by an equal amount since the voltage across the diodes is assumed to be a constant 1.4 V.

During the negative portion of the sine wave D3 and D4 will be reversed biased and no current flows through the lower resistor. So again Vb = Va and Vc will be 0 V when Va is greater than -1.4 V. When Va goes more negative than -1.4 V D1 and D2 will begin to conduct. Here there is no voltage division so Vb will just be clipped and remain at about -1.4 V.

Vb:
To summarize, Vb will mimic Va between +1.4 Va and -1.4 Va. Between Va = 1.4 V and Va = 2.0 V, Vb will rise to 1.5 V. Above Va = 2 V, Vb will rise from 1.5 V to 1.5+(Va-2)/6. So for a 5 volt sine wave input the peak voltage will be reached at about 1.5 + (5-2)/6 = 2.0 VDC. Below -1.4 Va, Vb will remain at -1.4 VDC.

Vc:
From Va = 0 to Va = 1.4 V, Vc = 0 Volts. Between Va = 1.4 V and Va = 2.0 V, Vc will rise to 0.1 V. Above Va = 2.0 V, Vc will rise from 0.1 V to 0.1+(Va-2)/6. So for a 5 Volt sine wave input the peak voltage will be reached at about 0.1 + (5-2)/6 = 0.6 V. Below -1.4 Va, Vc will also remain at 0 V.

#### davenn

Moderator
Sep 5, 2009
14,198
When the current reaches 0.1 mA the voltage across D3 and D4 will be 1.4 V.

outstanding answer EED, but in future, please take note of what section you are posting in
this is a homework section so we avoid giving all the answers, rather we work with the OP to give hints on the way to get to the answer themselves

so pose questions back to the OP that get them thinking about what they are trying to do ....

cheers
Dave

Last edited:

#### Electrical Engineer David

May 29, 2017
11
n
outstanding answer EED, but in future, please take note of what section you are posting in
this is a homework section so we avoid giving all the answers, rather we work with the OP to give hints of the way to get to the answer themselves

so pose questions back to the OP that get them thinking about what they are trying to do ....

cheers
Dave
What is an OP?

Last edited by a moderator:

Sep 13, 2016
1,025
Original poster

May 29, 2017
11
sorry. im new

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,509
sorry. im new

Please go to the list of forums and then select the Homework forum. Click on the first post.

or just go here.

I understand that you may be looking at posts that are new rather than going in via the list of forums and browsing the posts. As such you may not be aware of the rules of the homework section.

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