# How to design a voltage divider circuit that has 2VDC, 5VDC, and 8VDC

#### mattrrrrrr

Joined Feb 24, 2019
4
Hello,
For my lab, I am supposed to design a voltage divider circuit. I have done a lot of searching and I can't seem to understand.

I don't even fully understand what the question is asking
Question:
You have a 10VDC source available. Design a voltage divider circuit that has 2VDC, 5VDC, and 8VDC available

Thanks

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
If you have a 10 VDC source, 2VDC represents 20%, 5VDC 50% and 8VDC 80%. You can't have the three voltages coming out a single voltage divider array. See image IMG1.

Is the question actually referring to a single voltage divider for each voltage as shown in IMG2?

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#### mattrrrrrr

Joined Feb 24, 2019
4
If you have a 10 VDC source, 2VDC represents 20%, 5VDC 50% and 8VDC 80%. You can't have the three voltages coming out a single voltage divider array. See image IMG1.

Is the question actually referring to a single voltage divider for each voltage as shown in IMG2?
I am not exactly sure what you're saying about IMG1.

To me personally, it makes more sense for it to be only one circuit. When it says to "design a voltage divider circuit" it is singular not plural. So I would assume one circuit.

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
If you try to divide 10V in three voltages, the sum of the three voltages must be equal to the total voltage source, in this case 10V. Having said this, 2 + 5 + 8 = 15V, and your source is only 10V. So you can see is not feasible. You can, for example, have 2V, 5V and 3V, and the sum is equal to 10V. Before designing the circuit, i.e, choosing the resistors, you need to understand this. Otherwise, I suggest you to review the voltage divider theory.

For instance, see IMG2 in my previous post. If we were to design a single voltage divider for the 5VDC, i.e, 50% of the 10V source, you generally choose R1 = R2., and that would be the design equation.

This is what I can infer from your question. If it's not convincing, let's wait for someone else to help.

#### WBahn

Joined Mar 31, 2012
27,478
If you try to divide 10V in three voltages, the sum of the three voltages must be equal to the total voltage source, in this case 10V. Having said this, 2 + 5 + 8 = 15V, and your source is only 10V. So you can see is not feasible. You can, for example, have 2V, 5V and 3V, and the sum is equal to 10V. Before designing the circuit, i.e, choosing the resistors, you need to understand this. Otherwise, I suggest you to review the voltage divider theory.

For instance, see IMG2 in my previous post. If we were to design a single voltage divider for the 5VDC, i.e, 50% of the 10V source, you generally choose R1 = R2., and that would be the design equation.

This is what I can infer from your question. If it's not convincing, let's wait for someone else to help.
Uh, look at your own IMG1 to see that fallacy of this claim. What if R2 and R3 were each 100 Ω and R1 was 800 Ω. What would V1, V2, and V3 be? What would their sum be?

If the voltages are all measured relative to the negative terminal of the source, than you can have as many as you want using a single voltage divider chain as long as no single output is more than 10 V.

Now this is completely leaving aside what happens to the other voltages when you actually try to draw current from one of the "outputs", but I think this lab is just to get the open circuit voltages to be close to the values given.

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
Uh, look at your own IMG1 to see that fallacy of this claim. What if R2 and R3 were each 100 Ω and R1 was 800 Ω. What would V1, V2, and V3 be? What would their sum be?

If the voltages are all measured relative to the negative terminal of the source, than you can have as many as you want using a single voltage divider chain as long as no single output is more than 10 V.

Now this is completely leaving aside what happens to the other voltages when you actually try to draw current from one of the "outputs", but I think this lab is just to get the open circuit voltages to be close to the values given.
Yes, you're right. When I said R1 = R2, I just referred to single voltage divider based on two resistors. It won't be the same case for three resistors. As you clearly said: you can have voltages as many as you want using a single voltage divider chain as long as no single output is more than 10 V.

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#### eetech00

Joined Jun 8, 2013
3,418
Hello,
For my lab, I am supposed to design a voltage divider circuit. I have done a lot of searching and I can't seem to understand.

I don't even fully understand what the question is asking
Question:
You have a 10VDC source available. Design a voltage divider circuit that has 2VDC, 5VDC, and 8VDC available

Thanks
Hi

A voltage divider does what it sounds like.....(hint...hint....hint....)
Use basic Ohms law to get started. Then review Kirchoffs law.

First try dividing a voltage in half..

eT

#### WBahn

Joined Mar 31, 2012
27,478
Hello,
For my lab, I am supposed to design a voltage divider circuit. I have done a lot of searching and I can't seem to understand.
Just how much searching is "a lot"?

If you Google "voltage divider circuit" your get page after page after page of hits.

I don't even fully understand what the question is asking
Question:
You have a 10VDC source available. Design a voltage divider circuit that has 2VDC, 5VDC, and 8VDC available
It impossible for us to know what you've been taught so far unless you tell us.

Presumably you know about Ohm's Law and KVL/KCL. Are those reasonable assumptions?

Presumably you know about series circuit made of multiple resistors in series? Is that a reasonable assumption?

Presumably you've been shown what a voltage divider is and at the very least the equations that govern it (or at least it is in your lesson material in case you missed class that day). Are those reasonable assumptions?

#### mattrrrrrr

Joined Feb 24, 2019
4
Just how much searching is "a lot"?

If you Google "voltage divider circuit" your get page after page after page of hits.

It impossible for us to know what you've been taught so far unless you tell us.

Presumably you know about Ohm's Law and KVL/KCL. Are those reasonable assumptions?

Presumably you know about series circuit made of multiple resistors in series? Is that a reasonable assumption?

Presumably you've been shown what a voltage divider is and at the very least the equations that govern it (or at least it is in your lesson material in case you missed class that day). Are those reasonable assumptions?
I searched for multiple instances of voltage divider circuits and I couldn't find any examples that were similar to my use case. This is for my DC-Circuits class and it is my first electrical class.

After deciphering your Engrish I was still left confused about what you were trying to ask with the "presumably" stuff.

I don't really understand how me having prior knowledge or not about electricity is important here. I posted the exact word-for-word problem that I could not figure out in an attempt to get some help. If you are unsure of what I know, you can solve that by using Layman's terms while describing what the problem was asking and how to solve it.

#### mattrrrrrr

Joined Feb 24, 2019
4
Hi

A voltage divider does what it sounds like.....(hint...hint....hint....)
Use basic Ohms law to get started. Then review Kirchoffs law.

First try dividing a voltage in half..

eT

#### WBahn

Joined Mar 31, 2012
27,478
I searched for multiple instances of voltage divider circuits and I couldn't find any examples that were similar to my use case. This is for my DC-Circuits class and it is my first electrical class.

After deciphering your Engrish I was still left confused about what you were trying to ask with the "presumably" stuff.

I don't really understand how me having prior knowledge or not about electricity is important here. I posted the exact word-for-word problem that I could not figure out in an attempt to get some help. If you are unsure of what I know, you can solve that by using Layman's terms while describing what the problem was asking and how to solve it.
The problem, as stated, should be perfectly understandable by someone that is near the beginning of a first course in electrical circuits. That is why it is relevant to know where you are at -- and it sounds like you are not at the point you should be for the course you are taking.

If you don't know Ohm's Law, then it is pointless describing how to approach the problem using Ohm's Law.

If you don't know either Kirchhoff's Voltage or Current Laws, then it is pointless describing how to approach the problem using either of those.

Furthermore, since this is NOT the Homework Done For You forum, we will not tell you how to solve your problem -- it is YOUR problem to solve. The objective of the problem is not to turn in a correct solution, but for you to demonstrate that YOU can arrive at a correct solution. If we just give you the solution, you will likely THINK that you know how to get it, but come the exam you will discover that you really don't.

What we are MORE than happy to do is to HELP you get past specific difficulties you having so that you can move forward. If that is not the kind of assistance you are willing to accept, then this is not the forum for you.

Can you answer the question I asked the other responder, namely: Look at IMG1. What if R2 and R3 were each 100 Ω and R1 was 800 Ω. What would V1, V2, and V3 be?

If you can't, then you aren't ready to design a voltage divider circuit yet and you need to take a step back and come up to speed on the basic fundamentals that you are already falling behind on. Continuing on without doing so will just dig the hole you are in deeper and deeper until it swallows you whole.

If you can solve that problem, it gives us a good starting point to guide you toward crafting a solution for the assigned problem.

If you can't, then show your best attempt and let's get you past that hurdle first.

#### crutschow

Joined Mar 14, 2008
30,822
Yes.

So how many series resistors in the divider do you need to get the three voltages?

After that determination, a simple way to determine the resistor values is to assume a current through the divider and then calculate the resistor values from that.
For example, you can use a normalized current of say 1A through the resistor divider string from the 10V supply to ground.
Then, using Ohm's law, you can calculate the normalized resistor values, starting from the bottom of the string, to get the voltages you want.
For the voltages given, you can do those calculations in you head without a calculator, or even a pencil and paper.
After you get those normalized resistor values you can rescale them to any resistor or current value you might want.

But if you don't know Ohm's law, or Kirchhoff's Voltage or Current Laws for those calculations, then you need to learn those first.
Then come back with any questions you may have.

#### Xavier Pacheco Paulino

Joined Oct 21, 2015
728
As @WBahn said, this lab should simply be about getting the open circuit voltages. I'm going to attach IMG1 again and let's try to answer @WBahn question. If R2 and R3 were each 100 Ω and R1 was 800 Ω, what would V1, V2, and V3 be?

If you apply KVL (Kirchhoff's Voltage Law), you have:
-10 + VR3 + VR2 + VR1 = 0.
By Ohms' law, V = I*R
-10 + I*R3 + I*R2 + I*R1 = 0
Basic math will tell you that I = 10 mA

Now you have the current.
V1 = VR1 = I*R1 = 10mA * 800 = 8V
V2 = VR1 + VR2 = ?
V3 = VR1 + VR2 + VR3 = ?

I have to admit that my previous post (post #4) was not well expressed. I hope this one clarifies things a little bit more. As you can see, you must understand Ohm's Law and Kirchhoff's Laws before attempting to design your lab homework. If you understand this, I'm sure you will not have any kind of issue in your homework.

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#### MrAl

Joined Jun 17, 2014
9,555
Hello,
For my lab, I am supposed to design a voltage divider circuit. I have done a lot of searching and I can't seem to understand.

I don't even fully understand what the question is asking
Question:
You have a 10VDC source available. Design a voltage divider circuit that has 2VDC, 5VDC, and 8VDC available

Thanks
Hello there,

Do you know what a voltage divider is?
Also, did you ever design a voltage divider that just has to put out one voltage like say 4v from a 10v source?

As others have shown, a voltage divider is a set of resistors where the values are chosen so you get the voltage you need at the junction of the resistors. Now maybe you can look it up again and make some sense out of this problem.

If you have never encountered a voltage divider before, you need to first find out what that is and then go from there.