LED problem

[rodkings]

Hi there this is my first thread in this forum.I am making an LED project using super bright white LEDs and super-bright amber LEDs but the problem is that it seems that once I connect (in parallel) the amber and the white the white immediately turns off...I tried connecting two white LEDs and the light up but placing an amber and a white makes only the amber light up. It may be possible that the current is not enough to drive the white LED correctly once the amber LED "steals" current...but if that so how come the white does not light at least with a dim light? And if that is the case is there a way to solve it and supply it with more current.
Thank you for your time have a wonderful day.

 

[Reloadron]

This is where more info would really help. What are you using for a power supply? What are the specs on the LEDs and how many? Maybe a drawing of the configuration? Yes, you may be coming up short for current but without seeing and knowing what you have it is a hard call.

Welcome to the forums too!

Ron

 

[audioguru]

LEDs are never connected in parallel unless they are matched by the manufacturer or matched by you.
LEDs with different colours have different forward voltages so they do not match. The LED with the lowest forward voltage (amber) will hog all the current and burn out or will be very bright and the LEDs with the highest forward voltage (white) will not light or will be very dim.

If you have enough supply voltage then connect LEDs in series and in series with a current-limiting resistor.

 

[joespaz7]

do you have a schematic of it? sounds to me that the design should be switched to the LEDs being in series, with a greater supply voltage.

 

[vlad777]

I agree with audioguru but of course if you need the leds to operate indipendently
you should give each led its one resistor and not connect them in series.
But of course with two resistors(+leds) in parallel you will waste more power then in series configuration.

 

[dmachado]

Never, never, use voltage as a reference to power leds... leds are operated by current, regulated current (x mA depending on the led's specs) is the way to go...

 

[Vizier87]

Voltages ARE important. The drop producd by LEDs differ, and the forward voltages add up to each other, which determines the minimum supply needed for a serial LED connection.

 

[PCBWING]

High-brightness LED (red, yellow, orange, blue color) to light only 1V ~ 2V, while the white LED light will be nearly around 3V.When the white LED in parallel with other colors, other turn earlier than the white color LED, resulting inVoltage drop, so that insufficient voltage white LED light show circuit will not come.

So when white and red LED in parallel, the red LED to add a larger series resistance, using sub-Compression theorem, so that white light is not substantially decrease the loop voltage.

 

[robertastle]

there seems to be a lot of complex sounding reply's here
I have found exactly the same problem some years ago while putting differing colour leds together in parallel
basically this occurs because the differing voltage drop across the leds cause ohms law to get messed up and some leds getting the correct current and others not
right as people have said series is the way to go however this is dependent on the voltage being used as a supply
what i do is do ohms law bit for the brightness you want 20mA 40mA etc
wire up one of the leds with resistor from ohms law then measure the voltage across it and make a note ie white 2.7V
do the same with the other colour led and note that too i.e. amber 1.2V
now is the clever bit work out how many you can put in series i.e. 12V supply could support 10 amber but then not leave any voltage left for calculations
so say 9 if you want all amber
if you want equal amounts of amber and white then (2.7)+(1.2)+(2.7)+(1.2)+(2.7)+(1.2)= 11.7
so there is enough volts 12V to supply 3 amber and 3 white
but the really clever bit is that because of the volt drop across the leds you can do the ohms law using the remaining volts
i.e.12 - 11.7 = 0.3 Volts
so V/I = 0.3/0.040 = just 7.5 ohms resistor this will result in a super efficient circuit
and should lite all the leds
the example voltage drops here are all guessed on my part so you will have to measure to get yours but give it a go
and let me know how you get on
and if I have just given an overcomplicated answer give us a shout and I will send you a circuit diagram
Good luck
Rob

 

[audioguru]

No visible LED has a forward voltage drop of only 1.2V. A few infrared LED are.
Look at the graph of the forward voltage of different colour LEDs.

 

[Reloadron]

No visible LED has a forward voltage drop of only 1.2V. A few infrared LED are.
Look at the graph of the forward voltage of different colour LEDs.

Way back when I made it clear my imaginary LEDs had a 1.2 volt forward voltage drop so I could easily use 5 of them in series and get 6 volts as I like simple round numbers in examples.

Ron

 

[audioguru]

I don't think anybody has seen an imaginary LED (only 1.2V). Please talk about the real forward voltages of LEDs.

 

[robertastle]

ah ha I have used a lot of IR LED's in the past I guess thats where I plucked that figure out of
I just measure what the drop is are prior to making the circuit then you know exactly what it is