220 Ohm resistor. But 1W, 1/2W, or 1/4W ? Help me spend $18

[TheRealVictor]

I'm just trying to play with some leds. It seems the most common resistor for this application was the 220 Ohm resistor so I went to buy some and there is a bunch of different wattage sizes. Nobody ever mentions this.

I have some 150 Ohm resistors. Could these work? I also have some that are 320 Ohms.

I would like to buy a decent parts list for capacitors and mosfets bu t I don't Know what to buy.

So I have $18.  How would you spend it? I am going to be hooking up sensors and a recharge shield.

Please and thank you!

[larryd]

Try the 330, see if it works to your standards.

Wattage = Voltage X Current

Buy logic level MOSFETs, search this site for discussions

eBay has assortments available. here

Jameco has assortments but is expensive. here

Amazon has assortments here

Dipmicro, but watch quantity pricing here

[TheRealVictor]

LarryD thank you very much I'll go with the 330s. I cant seem to do the simplest thing without a million details.

I have a lot to learn. Here is your total beginner question of the day. Is the current what the led is rated for, or what flows out of the battery? I don't know where to get the numbers for the equation W= V * C

Thanks for taking the time to respond.

[Grumpy_Mike]

For normal 20mA LEDs get the lowest wattage and hence the cheapest you can find. That is normally quarter or eighth watt.

The numbers to use are the current through the resistor, which in this case is the same as the LED and the voltage across the resistor which is the supply voltage minus the voltage across the LED.

[DVDdoug]

I have a lot to learn. Here is your total beginner question of the day. Is the current what the led is rated for, or what flows out of the battery? I don't know where to get the numbers for the equation W= V * C

The current is calculated using Ohm's Law  (Current = Voltage/Resistance).

The basic power formula and Ohm's Law can be combined algebraically to get Power = V²/R.

[CrossRoads]

Many LEDs have a max continuous current rating of 20mA.
If the source voltage is 5V, and Vf of the LED is 2.2V, then the resistor to set that up is:
(Vs - Vf)/current = resistor
(5V - 2.2V)/.02A = 140 ohm.
The power dissipated in the resistor is P=IV. V = IR, so sub in: P = I*IR, so .02A * .02A * 140ohm = .056W, 56mW
Alternately, Vr = Vs - Vf = 5V - 2.2V = 2.8V.  .02A * 2.8V = 56mW

Say you only had 220 ohm resistors available, how much current would flow?
(Vs - Vf)/resistor = current
(5V - 2.2V)/220 = .0127A, 12.7mA
And .0127 * .0127 * 220 ohm = 35.4mW

Does that help?

[larryd]

In Crossroad's example:
35.4mW < 1/4 watt         this is a good thing
i.e.
.0354W < .250W  

It will be my 'guess' most of your resistors would be 1/4 watt.

Size comparison:
Resistor Wattage


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[CrossRoads]

Less than 1/8W also, 0.125W.
I see 1/10W resistors too, 0.1W. Smaller size.

[larryd]

[TheRealVictor]

Many LEDs have a max continuous current rating of 20mA.
If the source voltage is 5V, and Vf of the LED is 2.2V, then the resistor to set that up is:
(Vs - Vf)/current = resistor
(5V - 2.2V)/.02A = 140 ohm.
The power dissipated in the resistor is P=IV. V = IR, so sub in: P = I*IR, so .02A * .02A * 140ohm = .056W, 56mW
Alternately, Vr = Vs - Vf = 5V - 2.2V = 2.8V.  .02A * 2.8V = 56mW

Say you only had 220 ohm resistors available, how much current would flow?
(Vs - Vf)/resistor = current
(5V - 2.2V)/220 = .0127A, 12.7mA
And .0127 * .0127 * 220 ohm = 35.4mW

Does that help?

It does help a lot. Except I don't know what Vs stands for in the equation above. I found this for the other symbols, but Vs was not there. http://www.allaboutcircuits.com/textbook/direct-current/chpt-2/voltage-current-resistance-relate/

For normal 20mA LEDs get the lowest wattage and hence the cheapest you can find. That is normally quarter or eighth watt.

The numbers to use are the current through the resistor, which in this case is the same as the LED and the voltage across the resistor which is the supply voltage minus the voltage across the LED.

Thank you that let's me buy something specific now. I have piles of resistors I'll never use it would seem.

I need to sit down and read a while. That kind of reading is hard for me but I know I got to do it. I have a lot of questions coming Thanks to everyone who contributed an answer.

[larryd]

Vs (Source Voltage)



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[Paul__B]

Well now, the formula for power is V²/R.

Considering resistors placed directly across 5 V, V² is 25, so a 100 Ohm resistor would dissipate ¼ W, and a 220 Ohm resistor an 11th of a Watt.

Clearly, a tenth Watt rated 220 Ohm resistor is safe in any circuit powered at 5 V.

[TheRealVictor]

Well now, the formula for power is V²/R.

Considering resistors placed directly across 5 V, V² is 25, so a 100 Ohm resistor would dissipate ¼ W, and a 220 Ohm resistor an 11th of a Watt.

Clearly, a tenth Watt rated 220 Ohm resistor is safe in any circuit powered at 5 V.

That's a great walk through. I have no idea why I was making it so complicated. I mean I hate math, but c'mon. It's just simple division? Why do they go and make it all complicated looking? I hope there isn't a ton of math in this. I know it's hard to be smart without knowing math but I can write code, and I thought that was what I would be doing. Why do I always get sidetracked like this? Just to be safe, I'll dig out a calculator.

LarryD what do the red dots represent? they look like switches. I usually look up stuff like that but googling "red dots" might not turn out so well.


So I got adventurous and won an auction for $20. How did I do?:
20x Resistor 220Ω
 10x Resistor 1kΩ
 10x Resistor 10kΩ
 3x   Potentiometer
 1x   Photoresistor
 4x   Red LED
 4x   Green LED
 4x   Blue LED
 1x   RGB LED
 1x   LED bar graph
 1x   LED Matrix
 4x   Push Button
 2x   Diode
 2x   NPN Transistor
 2x   PNP Transistor
 1x   Active Buzzer
 1x   Passive Buzzer
 1x   Relay
 1x   Motor
 1x   Servo
 1x   Joystick
 1x   LCD1602
 1x   L293D
 1x   74HC595
 1x   MPU6050 Acceleration sensor
 1x   LM35 Temperature Sensor
 1x   Breadboard
 65x Jumper Wire M/M
 10x Jumper Wire F/M
 1x   Battery Holder

Apparently there is a 155 page book of tutorials.

[gpsmikey]

Understand that if the actual power is 1/10 watt then 1/4 or even 1/2 watt (same resistance) is just fine.  I mention it this way because depending on who you are getting parts from, you may find the 1/2 or 1/4 watt resistors (or whatever) are significantly cheaper than say the 1/10 watt resistors. Be sure and check the quantity pricing - often times, 10 or even 100 of the same value are MUCH cheaper than the individual prices.  Get common values like 100, 220, 330, 1k, 4.7k and 10k in bulk to have them on hand (the first 3 are typical values for current limiting for LED's etc, the last 3 are more common for pull-up or input protection.

[larryd]

The red dots are just connection points which are joined to other parts in your circuit.
They may or may not have numbers assigned to them for lead reference.

You can think of them as the point where you solder a wire to.

I put the black dots there to say:
"here is where you measure the voltage".

More reading for you:
https://learn.sparkfun.com/tutorials/how-to-read-a-schematic

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