LED wired in parallel

[ALPETERSON]

I am experimenting with multiple LEDs in a parallel circuit. I bought some 2-pack Radio Shack high brightness white leds which are rated 3.3v+ forward voltage and 25 mA current on the package. Using an on-line led parallel calculator I deterimined that 5 of these LED's in parallel should require no resistor if supply voltage is in the 3.5v ballpark. In testing, I determined that they did indeed light properly at 3.5v on up to 3.8v. I did not want to burn them out by applying higher voltage. My supply voltage came from a 10.5v (1.5 amp) power supply hooked up to a 5k ohm pot.

Here are my three questions:
- 4 of the 5 LED's are bright and the 5th is noticeably dimmer. is this variation normal for products of this type?
- No-load voltage is definitely showing 3.5v to 3.8v. But hooked up to the LED's the supply voltage reads 2.56, no matter where the supply voltage was set (3.5 to 3.8). Is there a reasonably simple explaination for where the missing 1.2 supply volts went? Should I recalculate a higher supply voltage and stick a resister in for stability?
- Am I missing something in my intrepretation of `forward voltage'?

Thanks for any help you can give
Al (new member)

 

[MikeMl]

ALWAYS put resistors in series with LEDs!!!! Parallel or not.

 

[crutschow]

LEDs are not intended to operate in parallel without a resistor in series with each LED. The are current operated, not voltage operated devices.

The difference you saw in brightness is indeed due to normal variation in the forward voltage of the LEDs.

How did you determine that the "No-load voltage is definitely showing 3.5v to 3.8v". There is no such thing as no-load voltage with an LED.

The 2.56V supply voltage you noticed is the forward voltage of the LEDs. They are pulling the supply down from the 3.5V you have it set to. The LEDs will keep drawing more and more current at that voltage until they burn out. They do not operate like a light bulb.

Please read up on how LEDs operate.

 

[audioguru]

White LEDs are usually blue ones covered with a yellowish phosphor. Their forward voltage is 3.0V to 3.6V.
Maybe your white LEDs are two 1.3V IR ones in series and covered in a white phosphor.

 

[RODALCO]

Put a 47 ohm ¼Watt resistor in series with each led, and then connect the LED and Resistor strings up in parrallel as you had them before.

Then use your 5 k.ohm pot in series with your supply to keep the voltage to your led's to a safe level.

The led's should lit up almost equally bright as the resistors will compensate for minor voltage fluctuations between the led's.

The way you had them wired up before will cause the led with the lowest voltage threshold to take most of the current and burn out first.

You could then do a voltage measurement across the led and see minor voltage differences across each led.

A DMM with at least 2 or 3 digits after the decimal will give you the idea.

 

[ALPETERSON]

Thanks for your help

Thanks for the help. My no-load was the supply voltage not hooked to the circuit. I do have a DMM. I suspected that restistors were a major part of the curcuit, and will test that approach shortly. I am working with a 20 year old grab bag of various resistors bought from who-knows-where. Not enough of the proper size, so back to Radio Shack, or maybe order from DigiKey. I am still puzzled by the 2.56v but the package says `product may vary from depiction' so I will take that into account.

Thanks again.

I am experimenting with multiple LEDs in a parallel circuit. I bought some 2-pack Radio Shack high brightness white leds which are rated 3.3v+ forward voltage and 25 mA current on the package. Using an on-line led parallel calculator I deterimined that 5 of these LED's in parallel should require no resistor if supply voltage is in the 3.5v ballpark. In testing, I determined that they did indeed light properly at 3.5v on up to 3.8v. I did not want to burn them out by applying higher voltage. My supply voltage came from a 10.5v (1.5 amp) power supply hooked up to a 5k ohm pot.

Here are my three questions:
- 4 of the 5 LED's are bright and the 5th is noticeably dimmer. is this variation normal for products of this type?
- No-load voltage is definitely showing 3.5v to 3.8v. But hooked up to the LED's the supply voltage reads 2.56, no matter where the supply voltage was set (3.5 to 3.8). Is there a reasonably simple explaination for where the missing 1.2 supply volts went? Should I recalculate a higher supply voltage and stick a resister in for stability?
- Am I missing something in my intrepretation of `forward voltage'?

Thanks for any help you can give
Al (new member)

 

[Roff]

My best guess is that you are using the circuit below. The top circuit is my guess of how you are driving the LEDs. The bottom circuit is the Thevinin equivalent of it. This would explain the discrepancy between open circuit and closed circuit voltages.
I don't understand, though, how you would get significant light from 5 LEDs that are sharing approximately 1mA.

 

[ALPETERSON]

Led circuit

My best guess is that you are using the circuit below. The top circuit is my guess of how you are driving the LEDs. The bottom circuit is the Thevinin equivalent of it. This would explain the discrepancy between open circuit and closed circuit voltages.
I don't understand, though, how you would get significant light from 5 LEDs that are sharing approximately 1mA.

The top circuit is exactly the way I set it up. Note that I have never left it hooked up for more that 20 seconds or so because I was worried about burning up the LEDs. Looks like that was a good precaution. I am going to Radio Shack today to buy a 500 pac of assorted 1/4W resistors. Hopefully I will get at least 5 of the same in the range required. If I find 5-47 ohm, or 60 or 70 ohm, I will run the circuit through one of the on-line calculators and adjust the supply voltage accordingly.

Thanks for your help.
Al