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Topic: Finding component resistance (Read 232 times) previous topic - next topic


Hi all!

Just got my Arduino starter kit a few days ago. I've been working through the projects in the book and so far really enjoying it.

I'm just trying to understand how / when to use resistors and capacitors. From what I understand, you use a resistor when you have unused voltage in your circuit?

Regarding capacitors, my understanding is that you use this to have a stored voltage in parallel with a component that is going to cause a large voltage drop. That way, the component can draw extra power when it needs to from the capacitor?

Finally, I guess my question is - how do I work out what the requirements are for my   components, in terms of resistors and capacitors etc? 

Thanks in advance. Hopefully my question makes sense!


Welcome to the group!

Google has many links to 'basic' electronics.

For example Google:  resistor LED

Also, YouTube has many tutorials on the topic.

Arduino links of interest.

Listing of downloadable 'Arduino PDFs' :
Either Google   >>>- - - - >       arduino filetype: pdf

Listing of downloadable 'C++ PDFs' :
Either Google   >>>- - - - >       C++ filetype: pdf

Arduino cheat sheet:

Troubleshooting common errors:

Watch these:
Arduino programming syntax:

Arduino arithmetic operators:

Arduino control flow:

Arduino data types:

Some things to read



Language Reference:


How and Why to avoid delay():

Demonstration code for several things at the same time.

Useful links:

Arduino programming traps, tips and style guide:

Jeremy Blume:

Arduino products:




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Resistors are used to limit current to LEDs (they drop voltage linearly dependent on current) or some other devices with similar properties, and to limit current into the gate of a FET which could otherwise briefly exceed the rated current for an output pin. They are used as pullups or pulldowns to keep a pin in a defined state when not connected to anything else, or for protocols that require them (like I2C), or to keep a MOSFET off. They can also be used as part of a filter or RC timing circuit, and have myriad uses when working with opamps. They can be used as a voltage divider to level shift a voltage lower (for example to measure the voltage at the input of a regulator, which would otherwise exceed Vcc and hence not be measurable with the arduino ADC - ex to measure the state of a battery). Very low value resistors can be used as a "shunt" to measure current. There are LOTS of uses for resistors.

A capacitor stores energy when different voltages are applied to the two sides of it; it can also be thought of as acting against any change in the voltage across the capacitor. Capacitors are used for filtering on the power rails (to help maintain a stable voltage in the face of sudden changes in load) - in this role they're mandatory for regulators to maintain a stable output voltage. Small ones (typically 0.1uF ceramic ones) are used for "decoupling" on power pins of digital ICs (the power consumption of these, even if low in absolute terms, can change so rapidly that the inductance of the wires to the IC results in a glitch that can cause undesired behavior - putting a decoupling cap right next to them will stabilize the voltage on the part during this time preventing that. Virtually every digital IC needs these, and 0.1uF is almost always appropriate).  They are also used in filters, in RC timing circuits, as "loading capacitors" on crystals. There are LOTS of uses for capacitors too.

As to how to work out what you need, it all depends on what application you're using them for - as noted, there are a great many uses for both of them, some quite simple, and some very complicated.
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Jan 23, 2019, 11:09 pm Last Edit: Jan 23, 2019, 11:27 pm by DVDdoug
Resistors serve several purposes, but before you get into that...

When you study electronics the first thing you learn is Ohm's Law (Current =Voltage/Resistance).

Resistors "resist" current flow.   The higher the resistance, the lower the current (with a given voltage).     With infinite resistance (and "open circuit") no current flows.   With zero resistance (a "short circuit") 'unlimited" current flows unless there is some resistance elsewhere in series and "bad things" can happen with a short.

A wire has essentially zero resistance.   Current flows freely through the wires in your house with essentially zero voltage drop across  the wires, so all of the voltage is delivered to the load (a light bulb, or hair drier, or whatever is plugged-in). 

In house wiring, the resistance of the load determines the current.   A light bulb has less resistance than a hair drier.    If you short-out the power outlet with a wire, excess current flows and you blow a circuit breaker. 

A turned-on light switch is a short-circuit in series with the light bulb.   All of the voltage "appears" across the infinite resistance of the switch so there is no voltage across the load.   

A turned-off light switch is an open circuit and no current can flow.   Since the resistance is (nearly) zero, (nearly) no voltage is "dropped" across the switch and all of the voltage appears across the load. 

There is a water-flow analogy where water-flow represents electrical current flow.   Water pressure represents voltage.   A skinny pipe represents high resistance and a fat pipe represents low resistance. 

The difference is, if you cut a water pipe water flows out all over the place.   If you cut a wire you get infinite resistance and no electrical current flows.  And, generally nothing bad happens with zero water resistance (except maybe for a flood ;) ).

Resistors with LEDs are "unusual"
because the resistance of the LED is non-linear...   The resistance of an LED changes drastically with voltage.    If you increase the voltage above the LED's operating voltage it's resistance drops.  That means we can't directly  apply Ohm's Law to the LED.*  But, we can apply Ohm's Law to the resistor to figure-out the current & voltages.

With a series resistor the voltage divides between the LED & resistor.**    The voltage across the LED "magically falls into place" and the remaining voltage is dropped across the resistor.    Now that we know the voltage across the resistor we can use Ohm's Law to calculate the resistance needed for the desired current through the LED & resistor.


* Ohm's Law is a law of nature (with man made units-of-measure) so it's always true and it's true for the LED.  It's just hard to use because the resistance of the LED is unknown until we to know the current & voltage. 

** The way that voltages & currents combine or divide in series & parallel circuits is described by Kirchhoff's Laws.


Hi all,

Thanks for the great replies! Very helpful.

@DVDDoug - to clarify regarding your LED example..

If we don't know the LED resistance / voltage drop, how does the project book know what Ohm level resistor to use to ensure voltage is distributed evenly?

E.G for the RED LED they say to use a 220ohm resistor? 


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Jan 24, 2019, 11:59 am Last Edit: Jan 24, 2019, 12:01 pm by MarkT
And its probably worth realizing we do three basic things with electronics:

1) control power for something
2) process an analog signal
3) process digital information

For 1) its all about not wasting power and being able to handle the power without some component failing,
and these days this means switching circuits using inductors and capacitors for short-term energy storage.

2) is about manipulating a voltage or a current that represents something, and the components are
used because of their mathematical properties (resistors relate a voltage to a current, capacitors
relate a rate of change of voltage with a current, etc).  Linear amplification is very important too as
passive components can only decrease the signal's power.

3) is logic gates, processors, every signal is HIGH or LOW.

2) and 3) are interlinked by ADCs and DACs, so you often hop between analog and digital as
some things are much cheaper to do digitally.

And for all three things we need to provide suitable power (usually at constant voltage, occasionally
with constant current).
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Each component have datasheet, you can find it on the manufacture website!
I will point some info to make it easier to you to know which components you should pick for your project.

Resistance specification
The resistance is obviously the key resistor specification. The value of the resistance is required by the calculations for the particular application in which it is to be used.

It is always best to use preferred values as these are easier to obtain. There are several series of resistor values that are used. These are referred to as the E-series. E3 has three values in a decade, i.e. 1.0, 2.2 and 4.7. Values of 10Ω 22Ω 47Ω are available in the tens of Ohms decade, 100Ω 202Ω 470Ω are available in the hundreds of Ohms decade and so forth.

It is always preferable to use as few values in a circuit design as possible as this reduces the number of different types required for any one design. Other series are also available, E6 with six values in each decade: 1.0, 1.5, 2.2, 3.3, 4.7, 6.8. There are also E12, E24, E48 and E96 etc values available, although their costs can increase marginally and they mean that many more component types are needed in a given design.

Power dissipation specification

Although the resistance is the key parameter for any type of resistor, another important parameter in the resistor specification is the amount of power it can dissipate.

When current passes through a resistor power is dissipated and this manifests itself in the form of heat. In turn this cases the temperature of the resistor to rise, and if too much current passes through the resistor, the temperature rise can be too great and it can cause the resistance to change, or in extreme cases it can cause damage to the resistor.

The power dissipated in a resistor is easy to calculate. The basic equation for power is:

   W = power in watts
   V = voltage in volts
   I = current in amps

It is often easier to combine this equation with Ohm's Law to create a more useful equation which calculates the power dissipated from a knowledge of the resistance and the voltage across it:


   R = resistance in ohms.

All resistors have a power dissipation rating specification. This is the maximum power that they are designed to dissipate. The resistor type should be chosen so that this power level is never exceeded in operation. In fact good design practice dictates that the maximum power dissipation should be well inside this. Many electronics design companies operate a practice where they state that the maximum actual dissipation should never exceed around 60% of the rating of the particular type of resistor. By doing this, the reliability of the circuit is improved.

Power de-rating specification

The resistor specification for power de-rating can be important when components may be expected to run at higher temperatures.

Under these circumstances the resistor will be running hot and it is necessary to ensure that its capability is not exceeded.

Typically the same power dissipation will be quoted up to a given temperature, after which the derating is applied. Typically this is a linear curve above the given temperature.

Temperature coefficient specification
In certain circumstances the resistor specification for temperature coefficient is of importance.

The temperature coefficient specification is the parameter that indicates the change in resistance with changing temperature. The resistor specification for the temperature coefficient will be very dependent upon the type of resistor, and it may also vary from one manufacturer to another. IT is therefore important to check the resistor specification for the temperature coefficient to ensure the particular resistor is suitable for the given application.

The temperature coefficient is the change in value of the resistance over a given temperature change. Normally it is expressed in term of parts per million, ppm, per degree Celcius, i.e. ppm/°C.In other words a 100kΩ resistor with temperature coefficient specification of 1000ppm/°C for a 10 °C temperature rise would change would change by 1000 /1 000000 * 100 * 100 000 Ω = &10Ω. This could be quite significant in some circumstances.

Maximum temperature specification
The resistor specification for temperature needs to be adhered to. Above certain temperatures the resistor may function outside its specified operating parameters. Also under extreme conditions damage could result and the overall circuit may cease to function.

If resistors are operated well above their rated temperatures for extended periods, the value of the resistance can permanently increase, and this could cause the overall circuit to malfunction.

A further reason for operating below the rated temperature is overall reliability. Resistors, and all other components are more likely to fail if operated outside their specified ranges. Often components are operated inside their specification with a good margin to ensure that the reliability is maximised.

Resistor specification for maximum voltage

Resistors are designed to operate up to a certain voltage. Above this voltage there is the possibility of breakdown as a result of the electrical stress applied to the component.

As a result of this resistor datasheets will contain a resistor specification for the maximum voltage that should be applied.

The actual value will depend on a variety of factors including the physical size of the resistor, its structure, the technology used, and a variety of other factors.

Typically it is not good practice to run a resistor close to its rated voltage specification. Often design standards recommend running a resistor at a maximum of 60% or even les of the maximum rated voltage to ensure reliability is maintained.

These resistor specifications are some of the more commonly seen resistor specifications and parameters. Other exist and manufacturer datasheets should be consulted before settling on a given type.

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