LED use help

[Blue Shadow]

I received some LEDs from a friendly AKer without any specs on them other than they are 2.1-3 volt units.

So not knowing much about them, I hooked up three of them in series with a 15W resistor and powered this with a 9V battery. Lights and all the LEDs survived. I measured 82ma current draw through this combination. Does this tell me the information I need to know in order to use standard on-line LED resistor calculators to determine resistors for the different voltages?

These calculators have the working voltage, the circuit voltage and the forward voltage. I can measure the circuit voltage for any intended use, but from the above can the working voltage and forward current be determined?

[EchoWars]

You put 82mA of current through the LED's, and they survived?? Wow... Max current is usually about 20 to 30mA.

Anyway, read Post #7.
http://www.audiokarma.org/forums/showthread.php?t=146544

Make an assumption that the LED forward voltage will be about 2.5V or some such number. Calculate the resistor needed to limit current to about 10mA with your 9V battery source, then connect it up and measure the forward voltage to make a more accurate calculation for your final circuit.

[LBPete]

Here's a handy calculator (http://www.theledlight.com/resistancecalculator.html)for determining the resistor.

- Pete

[Blue Shadow]

Thanks guys. Doing a bit more research and checking the LEDs voltage drop I have across my circuit:

9V (-) to 15 watt resistor to yellow LED to green LED to red LED to (+) I get

1.18 volts across the resistor
2.55 volts across the yellow LED
2.97 volts across the green LED
1.68 volts across the red LED

and as stated an 82ma draw through the whole circuit. So I have 'normal' value LEDs, 1.7-3.0 voltage drops and somewhere around 20ma current drop for full brightness.

This means I can use the standard calculators and determine the proper resistor for the circuits I want to use them in. A couple plans are to light the tubes in my Millett Hybrid headphone amp/preamp and to light the door handles in one of my cars.

[electroking]

Your latest post is puzzling. You give four voltage drops that add up to
8.38, suggesting that your resistor and 3 LEDs are in series with the
9-volt battery as before. You said earlier that this circuit was drawing
82 mA. If it is so, you have the full 82 mA flowing through each of the
LEDs, which is too much as you were told. To calculate the resistor
you need to feed a single LED from a 9-volt battery, just assume
20 mA current and 2-volt drop at the LED. You need to have 7 volts
at the resistor terminals, yielding a resistance value of 7/0.02 = 350 ohms.
A 330-ohm resistor would be fine as a starting point.

Please note that a resistor is characterized first by its resistance (ohms)
and then by its power rating (watts), tolerance, temperature coefficient, etc.

[EchoWars]

...and as stated an 82ma draw through the whole circuit. So I have 'normal' value LEDs, 1.7-3.0 voltage drops and somewhere around 20ma current drop for full brightness.You seem bound and determined to fry those LED's. Once again, maximum current through most any LED is about 20mA!.

[chebishev]

Wholly Ohms Law Batman!!! I really suggest a basic electricity book from your friendly library. If you have the three leds and a resistor in series the voltages across them has to add to the battery voltage (as stated by electroking) . The current through the leds and the resistor is the same when connected in series. Current does not add when components are wired in series. If your circuit is in series and 82ma flows through the circuit this will make your leds light up very bright for a second or two and burn them out. (as stated by echowars) Blue Shadow your post seems to confuse current and voltage and also resistance and wattage. You may want to google ohms law and basic circuit theory and read up on them before you continue.

[Blue Shadow]

I'm not confused about this stuff, just the addition and reciprocal arrangements when using LEDs. I'm ok with resistors, capacitors and such but LEDs. What are they like check valves? I know fluid flow not electron flow and until it becomes visible (smoke) I don't get most of it.

I'm playing with these to see what is up, haven't left em hooked up long and surprisingly haven't blow one yet. I'll stop putting so much current through them and find the proper resistors to put 15-25ma through a single LED to see their brightness. I was told to enjoy working with them and learning. Maybe this is like skiing, you are not learning anything unless you fall a few times or in this case blow up an LED or two.

I don't think I have confused any of the resistance, voltage or amperage numbers in my post. I would be glad to learn where I did.

What I was trying to get was the numbers to plug into the different calculators for LEDs, knowing DC voltage from the measurement of the source, the current for the resistor was to be determined and you have all said stop!! then use about 20, and the voltage drop across the LED. These were mentioned as 2.1-3, but I measured them (or did I) across each LED.

So I'm going to use the voltage of the supply, the 20ma that seems correct and the value of 2.5 volts for each LED to get the resistor needed for the use of these LEDs. Then I can measure the current and the voltage drop across the LED and resistor and get the correct number for the LED, is that right? This will allow correcting the resistor for the circuit I use it in, varying the brightness by changing the current from say 15ma to 30ma.

Is this all messed up thinking?

So I just added a 220Ω resistor to the series and the LEDs look a lot happier. Current draw for the series in now about 13ma, so everyone feel much better about these LEDs lasting the day. The voltage drop across the LEDs measured both ways is now 2.0 for the yellow, 2.15 for the green and 1.6 for the red.

Is this voltage drop the one to use in the calculator?

I know this stuff really does not matter, using any voltage from 1.1-3.9 and and current from 5ma to 35ma will probably result in an adequate resistor for a circuit, but I'm trying to learn what can be determined with a cheap meter (my better one has sick leads on it right now) and a practice circuit. Thanks for working with me as I stumble through this.

[chebishev]

OK, it sounds like you want max brightness for a given LED and voltage source. Here is a way, buy a 2K potentiometer (or anything a few thousand ohms) and put it in series with your LED and current meter. Turn 2K ohm pot to max resistance (check with meter), connect voltage source then turn pot to get 20ma on current meter. Turn off and take your meter and measure resistance of potentiometer and now you have correct resistance value for given voltage source and LED to get 20ma of current. Good Luck.

[LBPete]

The specifications of the LED should tell you what the voltage drop is. Use that value, 20ma and the supply voltage in the calculator from my earlier post. It will give you the resistance value and the recommended wattage for the dropping resistor.

- Pete

[Blue Shadow]

I don't know that I am looking for max brightness, just playing with these LEDs to try to determine the digits as they came with NO digits. I have been instructed that 20ma is the max, ok I can do that.

I like the idea of the pot to determine the correct resistor for the results I would be looking for as this will allow adjustment to a brightness level, keeping the current below 20ma.

Thanks guys, I'm learning. Now to determine the characteristics of the projects I want to use these LEDs for.

[chebishev]

I like the idea of the pot to determine the correct resistor for the results I would be looking for as this will allow adjustment to a brightness level, keeping the current below 20ma.
Yeah, with the pot you could determine the brightness you want and get the exact value of resistor you need. I built a box with a pot and a built in meter and test points so I can read the resistance that I use for just that purpose. Besides it is more fun that using an online calculator.

[WopOnTour]

Sorry for the jack, but speaking of LEDs, do any of you guys have a diagram or link to a simple LED circuit for indicating cathode current for adjusting grid bias? I've always just added measurement "test points" to my amps and use a DVOM but I see a lot of newer amps using a couple LED indicators that turns "ON" in such a manner to zero in on your desired bias. I'm presuming they use a zener or something like that to make them accurate?
Anyone have???
WOT