Basic Understanding of Resistors

[mark_3094]

I have recently posted a request for the community to check over my notes on basic electronics to see if I understood it well.

(see original post here:https://www.electro-tech-online.com/threads/basic-electronics.89284/#post699536)

In addition to this, I also have a document on resistors, if anyone would like to look over it.

Thanks

 

[colin55]

Here are the corrections to your document "Basic Resistors"

 

[mark_3094]

I can't see any changes to this document.
Has the wrong one been uploaded?

Thanks

 

[colin55]

Here it is.

 

[mark_3094]

Thanks very much for your help. It's much appreciated.

I have some questions about the changes:

In the section just after Figure 2, you have changed from 10mA to 200mA. Why is that? Is 10mA just not enough for a lamp?

If there is a resistor with five coloured bands, is the fourth one always tollerance, or is it the fifth one? if it's the fourth one, what's the fifth for?


Thanks again!

 

[mark_3094]

I have also done up this simple voltmeter as a sample circuit.
What do you think?
It covers really basic electronics, in my other post, and resistors in this one...

 

[colin55]

Here are the corrections

 

[colin55]

A globe will not illuminate until 50mA flows. Allow 100mA - 200mA for the example.





Reading "STANDARD VALUES" (on 5-band resistors)
5-band resistors are also made in "Standard Values" but will have different colours - and will be confusing if you are just starting out. For instance, a 47k 5% resistor with 4-bands will be: yellow-purple-orange-gold. For a 47k 1% resistor the colours will be yellow-purple-black-red-brown. The brown colour-band represents 1%.
The first two colour-bands for a STANDARD VALUE in 1% or 5% will be the SAME. These two bands provide the digits in the answer.
It's the 3rd band for a 5% resistor that is expanded into two bands in a 1% resistor. But it's easy to follow.
For a standard value, the 3rd band in a 1% resistor is BLACK. This represents a ZERO in the answer. (For 5-band resistors BLACK represents a ZERO when in the third band. This is different to 4-band resistors where black represents the word OHMS! If the third band is BROWN, the answer will be 1).
So the 4th band has to represent one-less ZERO and is one colour UP THE COLOUR CHART! In other words the 3rd and 4th bands (combined) on a 1% resistor produces the same number of zero's as the 3rd band on a 5% resistor!

 

[JimB]

There is a problem with the "Fig 2 - Resistor Circuit".
In calculating the current, the resistance of the lamp itself has not been considered.
The resistance of incandescant lamps is awkward as it changes with the current flowing through the lamp. The hotter the filament, the higher the resistance.
So, to find the resistance required to set a known current in a lamp circuit, you would first need to know the resistance/current characteristics of the lamp.

JimB

 

[JimB]

To expand on Colin55s comments on meters.
Basic meter movement respond to current.
Basic meter current ranges vary from 50microamp to 1milliamp (ish).
To convert a current meter to a voltmeter, put a high value resistor in series with the meter. This resistor is called a "multiplier".
To convert a basic meter into a higher range current meter, put a low value resistor in parallel with the meter. This resistor is called a "shunt".
Resistors used for multipliers and shunts are made as high accuracy resistors, usually they are wirewound resistors, as these tend to have more stable characteristics.

JimB

 

[colin55]

It would be a very poor movement that required 1mA. The worst multimeter I have ever seen had 2k per volt and this was mainly due to a shunt resistor. The actual movement was more sensitive than this.

Only shunt resistors are wire-wound as multiplier resistors are very high value.

Agreed, the resistance of the lamp in Example 2 needs to be included in the calculation. If the lamp is 3v @200mA, its resistance will be 15 ohm. Put this into the answer. (If you want to get technical, the original current for the 3v lamp will be higher than 200mA - about 500mA and now the problem gets quite complex).

 

[mark_3094]

Thanks for your help.

I think I may have been a bit misleading with what I was trying to do.
I'm just trying to provide a circuit that shows the basics of resistors / resistance, voltage and current. It's not really meant to be an explanation of how movement works.

The meter in the diagram was not meant to be an ammeter at all. The one I have here that I've been testing with is actually a voltmeter, so it doesn't need to be converted. Connecting a battery directly to it would give a correct reading, as long as it's in the correct range.

I based my work on a sample circuit in a website () which has the resistor networks in it.
Are they wrong, or is it just unnecessary?

I've made some changes, but not all. I wanted to clear up the misconception first. I've attached the document with some of the changes made. Let me know what you think, and I'll go from there.


Thanks very much for your help

 

[colin55]

You have got to stick to convention.
If you add resistors (multipliers) to a movement, you create a voltmeter. You cannot have a voltmeter sitting in the circuit and try to explain why you need extra resistors, if there are already resistors inside the meter.

This is not a magic Trick show.

 

[mark_3094]

Ok then. This may not be the best example.
Can you think of a sample circuit that shows resistors, voltage, current, etc?