Help with EE homework.

Thread Starter

tinysteve

Joined Feb 27, 2020
3
Schematic.png
Here's my crudely drawn circuit. The question I need help with is simply, "Why are two 470 ohm resistors used in parallel when a standard value 240 ohm resistor is available and could easily be obtained? (Ignoring the 5 ohm difference)"
Thanks in advance.
 

Thread Starter

tinysteve

Joined Feb 27, 2020
3
Welcome to AAC!

There could be multiple reasons. We need more context.

Since this is homework, why do you think it was done?
This was done in a lab setting where we increased the supplied voltage in increments of 0.25 v and measured the voltage drop across the diode each increase.

The reason I am asking this question is that I really don't know the answer. I can try and guess that it has something to do with the voltage being dropped over two simultaneous resistors before reaching the diode but beyond that, I am lost. I really don't understand why you wouldn't just be able to replace it with a 240 ohm resistor in series...

Thanks for your response.
 

OBW0549

Joined Mar 2, 2015
3,517
The question I need help with is simply, "Why are two 470 ohm resistors used in parallel when a standard value 240 ohm resistor is available and could easily be obtained? (Ignoring the 5 ohm difference)"
As has already been stated, we need more context/information. Could be any of a number of reasons, none of which are obvious. Maybe the lab instructor just has a thing for 470 ohm resistors?

This was done in a lab setting where we increased the supplied voltage in increments of 0.25 v and measured the voltage drop across the diode each increase.
What was your supply voltage, at its maximum? At that voltage (minus whatever the diode drop was), what would be the power dissipation of a 240 ohm resistor? What is the maximum power the resistor is rated for? With a pair of 470 ohm resistors in parallel, how much power is each resistor dissipating at that voltage?
 

WBahn

Joined Mar 31, 2012
26,077
Imagine you are responsible for stocking components in a lab. Would you want to stock lots of different values, or just a few values knowing that people can easily combined them to get whatever value they happen to need at a particular time?

Go take a look at the standard values in the E6, E12, and E24 series.

Then consider power management. At the maximum voltage that you were to take your supplies, how much power is being dissipated by the resistor? How does that relate to the power rating of the resistor? If you are using the voltage across the resistors to determine the current, do you want the resistors to operate anywhere near their power ratings? If not, why not?
 

Hymie

Joined Mar 30, 2018
849
The most logical reason to use two resistors in a circuit (similar to that shown) is that should one of the resistors fail open circuit, there will still be a current flow through the diode (albeit at 50%).

Imagine a circuit where the shown diode (in the example) was the diode in an opto coulpler, the resistor values could be chosen such that with one resistor open circuit, sufficient current flows to switch the opto transistor on – thereby increasing the reliability of the circuit.
 

MrAl

Joined Jun 17, 2014
7,764
View attachment 200156
Here's my crudely drawn circuit. The question I need help with is simply, "Why are two 470 ohm resistors used in parallel when a standard value 240 ohm resistor is available and could easily be obtained? (Ignoring the 5 ohm difference)"
Thanks in advance.
Hi,

As others have been saying, there could be a lot of reasons for this. Power, resistance value, or even thy just want you to calculate the total value of two resistors in parallel with the same value, etc.

But if you are going to ask this question, then why not ask why there are two voltage sources as well. Do you understand why they are using two voltage sources one positive and one negative?

Is this a real life experiment or are you supposed to do this in spice?
 

WBahn

Joined Mar 31, 2012
26,077
But if you are going to ask this question, then why not ask why there are two voltage sources as well. Do you understand why they are using two voltage sources one positive and one negative?
I don't think he is using two power supplies. He stated that the drawing was crude. Note that, as drawn, the total output across the supplies is always zero. I'm guessing that he is using one output of a multi-output bench supply or perhaps using a dual output supply and either only adjusting one of them or that it is a tracking supply. The details would depend on their lab bench setup.
 

SamR

Joined Mar 19, 2019
2,426
Going out on a limb here but just what is the supply voltage and watt rating of the resistor? And if you know the supply voltage and have the resistor values you should know what the current across the diode is...
 

The Electrician

Joined Oct 9, 2007
2,780
This was done in a lab setting where we increased the supplied voltage in increments of 0.25 v and measured the voltage drop across the diode each increase.

The reason I am asking this question is that I really don't know the answer. I can try and guess that it has something to do with the voltage being dropped over two simultaneous resistors before reaching the diode but beyond that, I am lost. I really don't understand why you wouldn't just be able to replace it with a 240 ohm resistor in series...

Thanks for your response.
What was the highest voltage you applied to your circuit?
 
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