#### frankiem

Joined May 13, 2022
8
I am a first year engineering systems student. We were asked to make a circuit to convert the signal from an RTD into a voltage value of between 1 and 5 volts. My lab partner who has experience in electronics did most of the work on the construction of the circuit. I just don't understand how it was put together, and the logic behind it. I would massively appreciate it if someone could break it down for me.

#### Alec_t

Joined Sep 17, 2013
12,807
I just don't understand how it was put together, and the logic behind it. I would massively appreciate it if someone could break it down for me.
It has already been broken down into functional sections in the third image. Which section needs further explanation?
Welcome to AAC!

#### frankiem

Joined May 13, 2022
8
It has already been broken down into functional sections in the third image. Which section needs further explanation?
Welcome to AAC!
Embarrassingly all of it.

#### MrChips

Joined Oct 2, 2009
26,088
The first part with R1 and RTD constitute a voltage divider.

It would be a good exercise for you to derive Vout as a function of Vin, R1 and R2.

The next two portions are opamp circuit configurations.

This is your basic opamp circuit which you want to commit to memory.
Note that we draw the GND symbol towards the bottom of the drawing and not as shown in your drawing.

Using this as your starting point, a basic inverting amplifier is shown here:

This is a summing amplifier:

#### frankiem

Joined May 13, 2022
8
The first part with R1 and RTD constitute a voltage divider.

View attachment 267122

It would be a good exercise for you to derive Vout as a function of Vin, R1 and R2.

The next two portions are opamp circuit configurations.

This is your basic opamp circuit which you want to commit to memory.
Note that we draw the GND symbol towards the bottom of the drawing and not as shown in your drawing.

View attachment 267123

Using this as your starting point, a basic inverting amplifier is shown here:

View attachment 267127

This is a summing amplifier:
View attachment 267128

#### MrChips

Joined Oct 2, 2009
26,088
I should have added, the gain of the inverting amplifier is

Av = Vout/Vin = - R2/R1

See if you can derive this formula given the following constraints:

1) The voltage at the inverting input (-) is the same as the voltage at the non-inverting input (+) of the opamp.

2) Zero current flows into the inputs of an ideal opamp.

#### dcbingaman

Joined Jun 30, 2021
566
Do not forget that the input stage (R1/RTD) is not a voltage divider with just R1 and RTD but actually R1 in series with the RTD and R2 that happen to be in parallel. In other words the input signal is getting loaded down by the first stage inverting amp, this is OK you just need to take it into account when performing the calculations.

Last edited:

#### MrAl

Joined Jun 17, 2014
8,984
I am a first year engineering systems student. We were asked to make a circuit to convert the signal from an RTD into a voltage value of between 1 and 5 volts. My lab partner who has experience in electronics did most of the work on the construction of the circuit. I just don't understand how it was put together, and the logic behind it. I would massively appreciate it if someone could break it down for me.

View attachment 267093View attachment 267100View attachment 267097

Hello there,

Welcome to the forum.

From what you ask and your apparent scholastic goal i have to highly recommend that you learn a general circuit analysis method so you can start to understand circuits that are new to you without too much trouble. If you dont do that, you will always have to depend on finding solutions to every single circuit you run into by finding it in some book hidden somewhere in the back of some library or constantly asking someone else to help solve it. So even after years of this you will still only be able to understand those circuit which are exactly the same as the ones you had encountered previously in the years past. If you find a circuit that you almost know about but there is one resistor extra or connected in a slightly different way you would be lost again and that gets extremely annoying. That's fine if you know you will only encounter certain kinds of circuits but again when a new one comes along you will be lost again. If you learn a general analysis method you will be able to solve 99.999 percent of the circuits you run into without asking anyone else because you will have the basic tools to do almost any circuit.
It only takes a little more effort to do this too, that's the funny thing about it. You learn something like Nodal Analysis and then a few otehr things like four types of dependent sources and you are well on your way to solving a host of different circuits and gaining valuable information about them even with a circuit you've never seen before. To do any serious work in the field this is almost mandatory.
See everything else ends up being a "dumbed down" version of what you really should know about a circuit. If you learn how resistors, capacitors, and inductors work in an analysis you can solve a lot of passive element circuits and also active circuits like with the op amps. After a while it becomes almost too routine.
Another benefit is you get the solution for every node in the circuit with nodal analysis, not just the output.
For AC analysis it helps a lot to know how to deal with complex numbers and complex math. It's just a little more complicated than just working with real numbers and it is well worth the small extra effort.
Nodal analysis usually requires using simultaneous equations, but they are not much more than a single equation and once you do a few examples it becomes almost too easy.
What i mention above will get you quite far in the field you would be surprised. The circuit you mention here you would solve in about 5 minutes and have a good understanding of why and how it works.
If you want to go farther that would be up to you, such as getting into differential equations, but there are somewhat simple ways to handle those too, but even without that you'd get pretty far.

So a short list:
1. Nodal analysis.
2. Complex numbers, complex math.
3. Dependent sources.
4. Magnetically coupled circuits (kind of optional).
5. Differential equations, Laplace Transforms (somewhat optional depending on how far you want to go).

Of these the first three will get you quite far and you'd be solving circuits like the one you show here in your sleep

Give this some thought. This could be the best thing you could do for the advancement of your career.
I'd be willing to show you how to do most of this stuff if you like. I often help people on the web, in emails, texts, and in person. I end up learning from this process also and keep up with my own studies from the past.

#### frankiem

Joined May 13, 2022
8
Hello there,

Welcome to the forum.

From what you ask and your apparent scholastic goal i have to highly recommend that you learn a general circuit analysis method so you can start to understand circuits that are new to you without too much trouble. If you dont do that, you will always have to depend on finding solutions to every single circuit you run into by finding it in some book hidden somewhere in the back of some library or constantly asking someone else to help solve it. So even after years of this you will still only be able to understand those circuit which are exactly the same as the ones you had encountered previously in the years past. If you find a circuit that you almost know about but there is one resistor extra or connected in a slightly different way you would be lost again and that gets extremely annoying. That's fine if you know you will only encounter certain kinds of circuits but again when a new one comes along you will be lost again. If you learn a general analysis method you will be able to solve 99.999 percent of the circuits you run into without asking anyone else because you will have the basic tools to do almost any circuit.
It only takes a little more effort to do this too, that's the funny thing about it. You learn something like Nodal Analysis and then a few otehr things like four types of dependent sources and you are well on your way to solving a host of different circuits and gaining valuable information about them even with a circuit you've never seen before. To do any serious work in the field this is almost mandatory.
See everything else ends up being a "dumbed down" version of what you really should know about a circuit. If you learn how resistors, capacitors, and inductors work in an analysis you can solve a lot of passive element circuits and also active circuits like with the op amps. After a while it becomes almost too routine.
Another benefit is you get the solution for every node in the circuit with nodal analysis, not just the output.
For AC analysis it helps a lot to know how to deal with complex numbers and complex math. It's just a little more complicated than just working with real numbers and it is well worth the small extra effort.
Nodal analysis usually requires using simultaneous equations, but they are not much more than a single equation and once you do a few examples it becomes almost too easy.
What i mention above will get you quite far in the field you would be surprised. The circuit you mention here you would solve in about 5 minutes and have a good understanding of why and how it works.
If you want to go farther that would be up to you, such as getting into differential equations, but there are somewhat simple ways to handle those too, but even without that you'd get pretty far.

So a short list:
1. Nodal analysis.
2. Complex numbers, complex math.
3. Dependent sources.
4. Magnetically coupled circuits (kind of optional).
5. Differential equations, Laplace Transforms (somewhat optional depending on how far you want to go).

Of these the first three will get you quite far and you'd be solving circuits like the one you show here in your sleep

Give this some thought. This could be the best thing you could do for the advancement of your career.
I'd be willing to show you how to do most of this stuff if you like. I often help people on the web, in emails, texts, and in person. I end up learning from this process also and keep up with my own studies from the past.
Thank you very much for your reply. I found this this to be extremely helpful. I’d absolutely accept any help given as i am very much looking to progress my knowledge and understanding.

#### MrAl

Joined Jun 17, 2014
8,984
Thank you very much for your reply. I found this this to be extremely helpful. I’d absolutely accept any help given as i am very much looking to progress my knowledge and understanding.
Ok.

It will help if you can describe your math background as to what you have studied. For example, algebra, trigonometry, calculus, etc.

#### frankiem

Joined May 13, 2022
8
Ok.

It will help if you can describe your math background as to what you have studied. For example, algebra, trigonometry, calculus, etc.
I have studied trigonometry, algebra, vectors and complex numbers as part of my engineering course.

#### Papabravo

Joined Feb 24, 2006
18,408
I have studied trigonometry, algebra, vectors and complex numbers as part of my engineering course.
So, of all the circuit fragments that you still do not understand, which one would you like to tackle first?

#### MrAl

Joined Jun 17, 2014
8,984
I have studied trigonometry, algebra, vectors and complex numbers as part of my engineering course.
Oh ok great. You can work with a lot of circuits with algebra, using simultaneous equations.
Nodal Analysis is sort of easy to learn have you done anything with this in the past?

#### frankiem

Joined May 13, 2022
8
Oh ok great. You can work with a lot of circuits with algebra, using simultaneous equations.
Nodal Analysis is sort of easy to learn have you done anything with this in the past?
Apologies for the late reply. I haven’t done anything like that in the past.

#### frankiem

Joined May 13, 2022
8
So, of all the circuit fragments that you still do not understand, which one would you like to tackle first?
Which ever is most practical I guess. Potential dividend perhaps (left to right).

#### frankiem

Joined May 13, 2022
8
Which ever is most practical I guess. Potential dividend perhaps (left to right).
*divider

#### MrAl

Joined Jun 17, 2014
8,984
Apologies for the late reply. I haven’t done anything like that in the past.
Hi,

Ok, so how much algebra have you had so far?
For example, can you solve this:
y=x+2
for y if x=3
and can you solve the same:
y=x+2
for x if y=7
?

Then for simultaneous equations (which you would probably use software for anyway) can you solve:
5=a*2+b*6
7=a*3+b*8

solve this set of equations for a and b simultaneously?
You can use software for these too.

In much electrical work you would have to use software anyway because there will be far too many variables to solve by hand on paper.
You can do 2 or 3 equations by hand, but it starts to get harder with 4 or more variables.
You should probably learn how to do 2 equations by hand but also learn to use software that does symbolic calculations.
Using software you can do problems with a LOT of variables and then move to matrix methods which are not that harder to do. Matrix methods are used extensively in electrical work because they offer a convenient and systematic approach to solving large sets of equations which you would encounter in many different disciplines not just electronics.

I think that algebra (and included simultaneous equations) would be the least math prerequisite for doing extensive work in electronics. Without that background you have to constantly rely on formulas developed by other authors.

Last edited: