Newbie needs help in red LEDs

Hi, I'm new to this forum. I need some help with hooking up some LEDs.

I need to power 4 red "superbright" LEDs in series. I'm not sure about their ratings.I've picked up a free AC/DC transformer, the type that allows you to adjust voltage up to 9V. So i assumed each LEDs requires 2V. I adjust the transformer to 8V and hook it directly without resistors. It works fine and bright, slightly warm after a few minutes but using a multimeter, i measured 60mA flowing through the circuit. Have I done it right or done it wrong? 60mA seems too high. I read somewhere that LEDs should need 25-30mA. later, I need to hook up another 4 red LEDs parallel to this circuit. I'm still playing with it but need some advice from you cool people...!

Firstly, since you have a multimeter, check what is the actual voltage across the LEDs, each and as a whole. If it isn't within +-1V of 8V, you have a problem.

Either you have a fairly large output ripple or the transformer is not well regulated.

It is not uncommon for transformers to have fairly large ripples. Ripples mean that while the nominal voltage may be as stated, the actual voltage may rise as high as 1.4 to 1.6 times higher. So for 8V, the peak voltage may be closer to 12V. Unfortunately, the only way to tell about ripples is to use an oscilloscope.

Something which may work (I just though about it and need to check it out) is to measure the voltage output through a 1R or 10R resistor, then put a fairly large capacitor across the output and resistor and measure the voltage again. Large means 16V 5000uF or something. See if there is any major change in output voltage. If there is a large ripple, the capacitor will smooth it out and your multimeter may measure as much as 1.6 times more voltage than without the capacitor.

Checking for regulation is easier. Check open circuit voltage, then check voltage across a 1R or 10R resistor. Hopefully it will be within 1V difference.

Don't think of LEDs as "voltage devices"; think of them as "current devices"

I assume you are using T1-3/4 or 5mm LEDs - adjust the circuit by adding resistors to keep the current somewhere in the 20-25ma Range

Red LED's typically need about 1.8 volts.

for 4 this is 4 x 1.8 = 7.2 volts

you have 9 volts.

this is 9 - 7.2 = 1.8 volts too much.

you want a maximum of 30 mA, or the overdriving can damage your LED's.

To calculate resistor value that will allow 30mA to flow use Ohms Law:

Resistor value = Volts across resistor / Current Wanted

R = 1.8 / 0.030 = 60 Ohms

The nearest prefered value up from this is 68 Ohms.

The resistor needs a power rating = current x voltage

= 1.8 x 0.03 = 0.054 watts,

so a 1/8 watt or bigger will be more than suffice.

connect as follows :

battery positive --- resistor --- LED --- LED --- LED --- LED --- battery negative

Give this a try, and measure the current.

If it is above 30 mA this will be down to the LED vf being different from 1.8 volts, or your power supply giving more than 9 volts. Note : the simple wall socket outlet adapters normally give a higher output voltage than that stamped on them unless a high current is drawn.

If the current is greater than 30 mA try a bigger resistor.

Hope this helps, good luck, and have fun !

Hi, you guys are WICKED! Thanks for your advice. I'll go check my circuit out...