power supply for leds ?

hi , i have 19 rows of four , 10mm white leds. would it be better to power them straight from dc or a pwm supply ? thanks

mark k

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mark krawczuk
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Hi Mark

Do you have 19 LEDs per chain or 4 LEDs per chain? Just to clarify please...

Is this a 'one off' or a production circuit? The brightness of the LEDs is controlled by the current through them. However, the voltage drop across each LED will change depending on batch (from about 3.4V to 3.8V). As the voltage across each LED chain will differ, you need to generate a high enough voltage to bias on the 3.8V LEDs (worst case is if you have a whole chain of LEDs with a bias voltage of 3.8V) and live with the power loss of the lower voltage LEDs. This power can be absorbed in a current setting resistor on the low side of each LED chain.If this circuit is a one off, this is easily done. If is it for production, design your circuit to regulate on current, not voltage

rgds

Bill Electronworks.co.uk - electronic kits for education and fun

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Electronworks.co.uk

and in answer to your original question... (!), it does not matter if you use a dc or pwm power supply, assuming the PWM power supply switches at, say, >1khZ. The LED flicker will not be perceived by the naked eye

Bill

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Electronworks.co.uk

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