# Finding values of resistors in circuit with diode

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
So, I did this practical where the circuit has an x and y terminal that connects to a box that has a three resistors and a diode.

There are places you can connect the circuit, A, B and C.Illustrated here, https://i.imgur.com/0UBelxh.jpg, the top is the circuit and the box is the box. So I did all possible connection and collated my results, here, https://i.imgur.com/6XoSZ4j.png.

I'm trying to ultimately find the value of R3, the voltage across the conducting diode was given to me and is 0.70 V. I'm to consider to voltage between A and C when A is positive and calculate:

1. The voltage across R3.
My table says 4.48 V, so I just subtracted 0.7 V from it.
2. The current through R2 and R1.
I think when X is B and Y is A will give me the current for R1 at 22.19 mA and when X is C and Y is B gives me the current for R2 at 88.9 mA.
3. The value of R3
I think the law is current splits up in each strand, so I can use the combined current of R1 and R2 and divide the voltage in question 1 by it and get the value of R3.

I'm not 100% certain on question two and three, I'd appreciate if anyone could help.

#### dl324

Joined Mar 30, 2015
12,871
• JohnInTX
Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
Cropped, edited photos for posterity.
Power source:
View attachment 145088
Circuit:
View attachment 145087
Table:
View attachment 145086

What is the resistance of the current meter? What is the voltage of the source?

Is this really homework?
I was told neither, the only information I was given voltage across the diode, the circuit was set up for me and I just had to use it. It's not really homework, it was a mock test.

#### dl324

Joined Mar 30, 2015
12,871
I was told neither, the only information I was given voltage across the diode, the circuit was set up for me and I just had to use it. It's not really homework, it was a mock test.
Unless the current meter is ideal, it's impact on the measurements needs to be considered.

What is the voltage reading on the power source when it isn't connected to the circuit?

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
Unless the current meter is ideal, it's impact on the measurements needs to be considered.

What is the voltage reading on the power source when it isn't connected to the circuit?
I did this practical in december so I can't remember and I don't remember it being mentioned by my tutor.

#### MrChips

Joined Oct 2, 2009
23,541
The only information you need are lines 2, 3, and 6.

Line 3 will reveal R1.
Line 6 will reveal R2.
Line 2 will reveal the current through the diode and R3.
Knowing the voltage drop across the diode, you can determine R3.

#### dl324

Joined Mar 30, 2015
12,871
I did this practical in december so I can't remember and I don't remember it being mentioned by my tutor.
Would it be correct to assume that you don't know or remember the correct answers?

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
The only information you need are lines 2, 3, and 6.

Line 3 will reveal R1.
Line 6 will reveal R2.
Line 2 will reveal the current through the diode and R3.
Knowing the voltage drop across the diode, you can determine R3.
So 3.78/0.0=40.3 ohms?

Would it be correct to assume that you don't know or remember the correct answers?
Yes.

#### MrChips

Joined Oct 2, 2009
23,541
Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
Show all your work.
I made a mistake there, meant, R = V/I = (4.48-0.7) / (53.5/1000).

#### MrChips

Joined Oct 2, 2009
23,541
I made a mistake there, meant, R = V/I = (4.48-0.7) / (53.5/1000).
What is your reasoning behind this calculation?

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
*
What is your reasoning behind this calculation?
4.48 V is the drop across the diode and the resistor, the diode has a voltage of 0.7, so I subtracted that for the resistors voltage and current is the same across each component in series

#### MrChips

Joined Oct 2, 2009
23,541
*

4.48 V is the drop across the diode and the resistor, the diode has a voltage of 0.7, so I subtracted that for the resistors voltage and current is the same across each component in series
Yehbut, you still have R1 and R2 connected to your circuit. You need to determine R1 and R2 first.

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
Yehbut, you still have R1 and R2 connected to your circuit. You need to determine R1 and R2 first.
ah, how do I do that?

#### MrChips

Joined Oct 2, 2009
23,541
Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
Read what I wrote in post #6.
Hey, what about this method
resistances of r1 and r2 found through ohms law
Current through r1 and r2: 4.48/(r1+r2) = i1
Current through R3 = 53.5 - i1 = i2
Resistance through R3 = (4.48-0.7)/i2

Last edited:

#### MrChips

Joined Oct 2, 2009
23,541
Hey, what about this method
resistances of r1 and r2 found through ohms law
Current through r1 and r2: 4.48/(r1+r2) = i1
Current through R3 = 53.5 - i1 = i2
Resistance through R3 = (4.48-0.7)/i2
I cannot tell if your answers are right or wrong.
Show ALL your work.

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
I cannot tell if your answers are right or wrong.
Show ALL your work.
First, are you sure R1 isn't B to A meaning line 4? When its connected through A to C, it's like a circuit that has a p.d. of 4.48 volts and current of 53.5 mA.

R1 = 4.48/(22.19/1000) = 201.892744479 ohms
R2 = 4.48/(64.9/1000) = 69.0292758089 ohms
Equivalent resistance: 201.892744479 + 69.0292758089 = 270.922020288 ohms
Current through R1 and R2: 4.48/270.922020288 = 0.01653612355 Amps
Current through R3 = (53.5/1000) - 0.01653612355 = 0.03696387645 Amps
Voltage through R3 = 4.48 - 0.70 = 3.78 Volts
Resistance of R3 = 3.78/0.03696387645 = 102.262001798 ohms

#### MrChips

Joined Oct 2, 2009
23,541
Yes, I meant line 4.

Where are you reading 4.48V?

Realistically and for expediency, stick to 4 significant figures.

Thread Starter

#### akthrowaiz

Joined Feb 3, 2018
10
Yes, I meant line 4.

Where are you reading 4.48V?

Realistically and for expediency, stick to 4 significant figures.
In line 2 the voltage is 4.48, I was asked "when A is connected to C and A is positive, to work out the voltage of R3, the currents of R1 and R2 and the value of R3." and was given a diode value of 0.7 V.