# Noob Power Jack Question

#### Videodrome

Joined Nov 12, 2009
63
So i hooked up a boost/bypass switch(3dpt?) to an led and then to a power jack and when i plugged the jack in and engaged the switch the LED didn't emit. Im a noob to this but is this because there is alot of other connections that i have yet to hook up that are causing this or did i get the wrong kind of jack. first is photo of my circuit with the jack plugged in, note the slight gap of the male end of the plug showing, the jack fits fine and snug but is not going in all the way. the second is of the original circuit im replicating and its jack, note how the gap is closed. Did I get the wrong jack? Why is the LED not lighting up?

#### Videodrome

Joined Nov 12, 2009
63
i just noticed that the orange cable from the LED isn't connecting to anything in that photo, i went ahead and fixed the error and it is still not working so im unsure of the problem still

#### SgtWookie

Joined Jul 17, 2007
22,210
I don't see an LED in the photos?

Did you use a current limiting resistor for the LED? If not, it may have instantly gotten fried.

Do you have the LED connected using proper polarity? Cathode (shorter lead) goes to the more negative voltage source, anode (longer lead) goes to more positive voltage source.

#### SgtWookie

Joined Jul 17, 2007
22,210
I forgot to give you the formula for calculating a current limiting resistor for your LED.
Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent

LEDs come with specifications for Vf (forward voltage) @ a given current.
Use the typical Vf @ current specification.
If you have to guess, guess low on the Vf. Don't use the maximum specifications.

Let's say that you have a red LED rated for a typical Vf of 2.2 @ 25mA, and a supply of 10v.
Rlimit >= (10v - 2.2v) / 25mA
Rlimit >= 7.8 / 0.025
Rlimit >= 312 Ohms.
Here is a table of standard resistance values: http://www.logwell.com/tech/components/resistor_values.html
Bookmark that page.
E12 values (yellow columns) are very common. E24 values (green columns) may also be available to you locally.
330 Ohms is the closest standard value that is >= 312 Ohms.

Let's see what that does to the current.
I = E/R, or current = voltage/resistance.
I = 7.8v / 330 Ohms
I = 23.64mA - close enough.

Now you need to calculate the resistor power requirement:
P = EI or Power in Watts = Voltage x Current
P = 7.8 x 0.02364 = 0.1844 Watts. (rounded off)
We double this for reliability, so 0.369 Watts. (rounded off)
You would need to use a 1/2 Watt resistor.

#### tkng211

Joined Jan 4, 2008
65
If there are 3 soldering tags in the female power jack, you may probably make a connection to the wrong tag (switching terminal), just try the another tag and check if the connected tags get some voltage between them after the power plug is inserted. You might also reverse the polarities of the power input so the led couldn't emit light.You can easily check it out with a multitester.