Can someone please tell me what the various type of resistor wattages mean w.r.t the calculations I made earlier? Based on calculations and guidance from you folks we concluded that I need to use a120 Ω resistor.

Radio Shack had several types of resistors -

• 1/4-watt resistors
• 1-watt resistors
• 5-watt resistors
• Power resistors
• Surface-mount resistors

How do I calc which type to use wattage to use?

 

A resistor is made from materials that survive a high temperature so a resistor can be hot enough to char a pcb or to melt a nearby plastic capacitor. The resistor can also burn you if you touch it. that is at its maximum dissipation rating.
But if you do not allow maximum dissipation and use only half the maximum rating then it is much cooler.

The dissipation in a resistor is the voltage squared divided by the resistance or the current squared times the resistance.

You have 120 ohms and it has a typical current of 18.75mA. Then it dissipates (18.75mA squared) x 120 ohms= 0.042W so you can use a 1/4W, 1/8W or 1/10W resistor.

 

I picked up a copy of the MAKE Magazine's Electronics: Learning by Discovery and it's been invaluable.

One thing I want to make certain I understand is the exact reason I need a resistor for my LEDs. The reason I am asking is b\c when I simply insert LEDs into my breadboard w\o a resistor and apply power the LEDs light up. Subsequently, when I insert the resistor into the breadboard the LEDs also light up and don't see any obvious difference at least nothing that is obvious to the human eye.

I guess what I am doing when not using the resistor is shorting the lifecycle of my LEDs.

It is then safe to say that the reason I need a resistor, based on calculations w.r.t the the vF and related figures, is b\c LEDs are fussy about the amount of power they receive?

Furthermore, I assume there is no PS in the world that could deliver exactly what every LEDs needs in terms of voltage and therefore the resistor plays an important role in taking the power from the PS and allowing just enough through to let the LEDs operate at their optimal operating range.

Thanks

 

I need help on understanding where I pace my LEDs, Resistors, etc on my breadboard. I cannot seem to understand the logic used to decide placement other than recreating the schematic drawing on your BB.

In the photo at the very bottom, how to decide where to place my LEDs and Resistor? I thought I would place them in the Terminal Strips and apply Voltage via the Bus Strips. However, I can only get them to light up in the configuration as depicted directly below.

Then I see this image online somewhere and to me placement looks totally haphazard. I fail to understand how voltage is directed first to the Resistor and then on to the LEDs.

 

A description of a breadboard and it's correct use begins on page 65 of your book. You may have thought that there was some internal connection between the bus strips and terminal strips; there isn't. You have to make that connection with a jumper wire. Try building the circuit from experiment 3 on your breadboard. The larger circuit of 5 LEDs in series is just an expanded version of the experiment 3 circuit with 4 additional LEDs, a higher power supply voltage and a different resistor.

 

You MUST use a resistor, or the resistance of the wiring or the resistance of the battery or power supply to limit the current of an LED or it will burn out VERY soon.

The actual voltage used by an LED is different for each LED (yours might be 1.5V to 2.4V or anything in between) so you cannot simply apply a voltage directly to an LED.
Also, the voltage used by an LED is reduced (then its current becomes higher) as it warms up.

Your circuit has a resistor in parallel with your LEDs so the resistor is doing nothing but getting warm. All your LEDs can easily burn out.
The resistor must be in series with each LED to limit the current.