# New be resistor questions

I am very new to this and have some questions I am hoping you guys could answer. We are just getting into the Arduino life, we bought the kids for my son who has always wanted to build robotics.
My first question is can we use more than one resistor In a line to drop the voltage to the desired amount? This has not happened yet but I would like to know before it does. I did some looking online and found that some say it's fine to do yet another says that because the heat released by two resistors of different amounts that you get in accurate results, I don't understand this so if it's true could someone please explain it to me?
Next is how do I own the actual voltage things use things use, in the videos for example they say that the LEDs are 1.7 V, where does a person find out this info and is this the same thing as forward current?
When using resistors how close do you need to get to the target voltage, example – if the target resistor is 360ohms is 400 close enough and I'm isassuming you would rather go a little under with the voltage rather than over – is this correct?
My last question is I found this resistor calculator online http://ledcalc.com, how do I find out the forward voltage?
Thank you so much for your time, it's greatly appreciated! I want this to be as much fun and educational for my son as possible, and I'd prefer to look not look like a pool in the process

Forward voltage, current requirements and all other important stuff is found in the datasheet of the component you are using. Part name + datasheet will usually give you a good result on any search engine.

Yes, you can use two or more resistors in series, they add up. It they are heating up significantly you might need a larger one (not larger value, but rated for higher current and power). That being said, two resistors will probably heat up less than one equivalent, due to double the surface area through which they can radiate heat.

As for being close to required currents, when it comes to most low power LEDs, anything between 10 and 20 mA is usually fine. Check datasheet for specifics.

Also note that resistors are not meant to be used as voltage regulators. They regulate current.

ndevans2:
My first question is can we use more than one resistor In a line to drop the voltage to the desired amount?

Yes you can.

ndevans2:
Next is how do I own the actual voltage things use things use, in the videos for example they say that the LEDs are 1.7 V, where does a person find out this info and is this the same thing as forward current?

Normally when you buy a LED, the seller may give you that information (mostly on-line). If you don't have that information, you can assume 1,5V (for voltage drop) and less than 20mA for forward current. The forward current would affect the brightness of the LED. If you give the LED less current, the LED will be less bright that we could.

ndevans2:
When using resistors how close do you need to get to the target voltage, example – if the target resistor is 360ohms is 400 close enough and I'm isassuming you would rather go a little under with the voltage rather than over – is this correct?

Good question. It depends what is the application, but for almost the applications it's OK an error of some 10's or even 100's. The normal resistors have an error of 5%. So if you want 360 Ohms and you peak a resistor of that value you could have 378 Ohm. So, as you can see is not a big issue an error of a few Ohm's.

ndevans2:
My last question is I found this resistor calculator online http://ledcalc.com, how do I find out the forward voltage?

As I said, you must find that value in the "datasheet" of the LED you have buy, but you can assume a value between 1,5V and 2V.
I fill the form with the values:
Supply Voltage - 5 VOLTS
Voltage Drop Across LED - 1,5 VOLTS
Desired LED Current - 20 MILLIAMPS
How many leds connected - 1
and he gives me: 180 Ohm

If you don't have this value, if you add a bigger resistor the LED will be less bright. If you add a smaller resistor the LED will be more bright. You must to be careful when you drop the value of the resistor too much, because you can burn the LED.

Just to be clear, resistors will "drop" a certain amount of voltage for a given current, according to Ohms law, of Volts = Amps X Resistance (V=IR).

So if you have a 220 Ohm resistor from +5V to the anode of an LED and then the cathode to say ground, at 20 mA that will drop 0.02X220 = 4.4V. In fact it probably will not as the LED has a switch on voltage which will depend on what it is made of but may be between 1.2V and 2V. In effect regulates the current. So if you assume the full switch on voltage of the LED is 1.7 V, then you have 3.3 left for the resistor, so re arranging the formula I = V/R gives a current of 3.3/220 = 15mA (If you used a 330 Ohm you would get 10mA)

What you can't do though is use resistors as a voltage regulator.

Lets say you need to power an IC at 5V. No amount of resistance between 12V and the +V of the chip is going to get a sensible result you will just blow the chip.

Is it OK to use a resistor of a different value to the one you calculated? In most cases it is mandatory. Resistors are only made in certain values and for the most part you use the closest.

Just to add to the other answers: reading your question it seemed to me you are confusing voltage and current. Using water as example, the voltage is similar to the water pressure in the pipes, while current is similar to amount of water. When we say an LED has a forward voltage of 1.7V (it's just a generic value which varies with many factors including LED color) we say you need that amount of "pressure" to activate the LED: once that pressure is reached the LED (which works like a valve) will open up and let any amount of water pass through.
If you do not limit the amount of water flowing through the valve then you will damage the valve (LED) and break it.

A resistor works like a fixed pipe stretch, reducing the amount of water that can flow through it. How much water is allowed depends on the "stretchness" of the resistor and is ruled by Ohm's law.

This is without considering the water source capacity (Arduino pins) which is limited as well (abs maximum is 40mA per pin, but you should always stay within 20mA if you want your Arduino to live long).

Because resistors act like stretches in the pipes, when the pipe stretches the pressure is reduced as well as the flow, so voltage drops as well, but that is more a side-effect rather than the purpose. Because the stretch acts as mechanical brake, it warms up when the water flows through the stretch.

Hope this helps, if it doesn't, just scratch all this out.

What if the pipe breaks ? (hint: pipes only fail in one mode. while they CAN be blocked, they usually break or leak and the pressure is lost. A resistor has two failure modes, open or short or partial short (reduced resistance.)

If you want to drop a voltage for powering a device or circuit you definitely
don't use resistors. This is what a voltage regulator is for.

For reducing a voltage for measurement, or for limiting current to an LED
then a resistor-divider or single resistor is the appropriate method.

The equations you need to use all the time are
V = I R (Ohm's law)

P = V^2 / R = I^2 R (power dissipation)

V measured in volts
I measured in amps (ie 10mA is 0.01A)
R measured in ohms
P measured in watts

Just write them somewhere handy or memorize.

Also as mentioned resistance in series adds. In parallel the reciprocal of resistance
(called conductance) adds. Think of it like this:

In series the currents are all the same, the voltage across the resistors must add up.

In parallel the voltages are all the same, the currents through the resistors add up.

generally speaking, if you are withing about 10% that is good enough.

generally speaking, you just add values mathematically, 1,000 ohms is 1 resistor of 1k or 2 of 500 ohms or a couple 220 and a 100.... you do not consider any error or outside values when using multiples in groups.

as was stated, if you use them for power, then calculate the watts. many people would say that if you look at them for power, keep looking. there may be a much better way. the higher the power, the more you should look elsewhere.

the values seem odd at first, why 220 and not just 200 ? trivia. resisors values increase in geometric progression, each is just shy of 150% of the value of the one before.

dave-in-nj:
resisors values increase in geometric progression, each is just shy of 150% of the value of the one before.

dave-in-nj:
trivia. resisors values increase in geometric progression, each is just shy of 150% of the value of the one before.

Like 330, 360, 390, 430, 470 etc.. :-/

Thank you guys soooo much!!!

Wow. Now even -I'm- confused. Stretching pipes?

Resistors do not regulate anything. It is a strict relationship called Ohm's Law: Current equals Voltage divided by Resistance.

An LED does not obey this relationship, it is nonlinear. It is a simplification, but you can say that over a wide range of current, the voltage drop across an LED stays roughly the same. The exact voltage depends on the color and construction. In general, it increases as you go from infrared to red to yellow to orange to green. It doesn't hold for blue, UV, and white LEDs because they often use multiple LED elements, or a different color LED die with a phosphor that glows the intended color.

IR LEDs drop about 1.5V, red about 1.7V, yellow about 2V, green about 2.5V. But you should read the specifications for the particular LED if it is some other color.

Driving an LED from 5V means the LED drops, say 1.7V for a red LED, leaving the resistor with 5-1.7 = 3.3V across it. By Ohm's Law, Resistance equals Voltage divided by Current. So if you want 20mA, that would be 3.3V/20mA or 3.3/0.02 = 165 ohms.

That isn't a standard value, but you can simply choose one close to that. 180 ohms will give you about 18mA. 150 ohms will give you about 22mA.

Even on 10mA, most modern LEDs will light up pretty bright. So could simply use a 330 ohm resistor for 10mA of current. 3.3V/330 ohms = 10mA.

You -must- learn Ohm's Law, and Kirchoff's current and voltage laws if you want to do anything in electronics. The water analogy can be useful, but don't overuse it!

@polymorph the statement was meant as a visualization of the ohm's law, as in the following picture.

If we assume Ohm's law is a mathematical representation of a physical relation resistance (which is main property of a resistor) is an impediment in current flow, like a pipe section reduction is an impediment in water flow.

Does it make more sense now?

Obviously it's not meant to be a full or complete description of Ohm's law, but I believe it's easier to understand in it's basic principle this way.

But you were talking about stretching, as if a resistor changed resistance with voltage or current. Not a constriction in the pipe.

You are right, I picked the wrong word I should have used sqeeze or pinch.

+1 to rlogiacco (I just love the picture )

It sounds like you (and your son) should pick up a book on basic electronics and get familiar with the concepts of voltage, current, power and ohms law before trying to tackle something like robotics. Otherwise, it might prove to be too difficult to get any robotics project off the ground.

You don't need to master transistor current mirroring, but a good grounding in what the simplest parts do and how to choose the correct values will be immensely useful.