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Help with connecting white LED's

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Junior Member level 2
Jul 21, 2007
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I have a issue with connecting my 2 blocks of white LED's for lighting. The problem is this way

there are 80 led's on the board and I want the action of 40 led's or 80 leds to be lit up as per the user requirement. So i got a 2 way switch which is ON-OFF-ON type. On the PCB, the LED's are 80 arranged in 2 blocks of 40 led's each with a DIODE IN4148 making or breaking the connection between the 2 blocks. If the 40 led option is switched on, the diode is in reverse bias to stop the supply to the other 40 led block, if the switch is turned the other way, the diode would be in forward bias condition to supply to first 40 led's as well as the other block of 40 leds making sure both the blocks are switched on.

However I see that one block of LED glows very dimly, if I remove the diode & connect a wire then all the led's glow properly. The diode is causing the issue. Can anybody suggest what could be going wrong and how it can be rectified. tried with normal diode of 1N4007, it has the same result.


Any schematics? :grin:
How the LEDs are connected? Series/parallel? How do you limit the LEDs current? By series resistor?

The schematic is added with this thread. The current limiting resistors are not shown for all led's but they are present.


It's not OK...
You need some small series resistors, or to use a second diode (schottky).


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Thanks for the extremly quick repsonse mister_rf , but could you tell me what is going wrong with my circuit and how does 2 back to back Schottky diodes be useful in my case.

You need to compensate for the 0.7V losses on series diode (1N4007). Using two diodes may apply equal voltages to each section.


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I connected as per your schematic but with the same result. One section of the LED's are dimly lit compared to the other block. It is a little better when 1N4007 diode is used but the difference in light is clearly obvious. 1n4148 has lesser current carrying capability and probably because of that the led block was even dimly lit.

Any suggestions in this condition

You need to reduce losses in diodes, better to use some power Schottky diodes, at least 3-5 Amps.
Anyway, what are the voltage and resistors values in your circuit?

My total current consumption is Approx 100mA with 7V full voltage full circuit. The 1N4007 is rated at 1A and it's more than necessary to sink in this current thro' it. The voltage drop across the diode is 0.8V. Resistor value is 33ohm

Use 4-5 diodes in parallel for all individual diodes in the circuit.

Use the circuit no 2, 3 in the above attachment but include an extra diode or in the loop for switch position one
so that the circuit will be balanced for both positions.

100mA for all 40 LEDs? 20 rows by two LEDs, equal 5mA for each LED, less then usual 20mA (standard white LED).
Anyway, I don’t know if you have only one resistor for all LEDs or each series of two LEDs has a separate resistor, you need to reduce the resistor values (up to obtaining the same current) and to use 3 diodes in order to have the same equivalent circuit for both situations.


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