power supply for leds ?

[mark krawczuk]

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

 

[Electronworks.co.uk]

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



"mark krawczuk" <krawczuk@adam.com.au> wrote in message
news:HOidndVj2YQNkSnVnZ2dnUVZ_uOdnZ2d@adnap.net.au...

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

 

[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

"Electronworks.co.uk" <bill.naylor@electronworks.co.uk> wrote in message
news:r6ydnQ5IYvVjzyHVnZ2dnUVZ8sbinZ2d@bt.com...

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



"mark krawczuk" <krawczuk@adam.com.au> wrote in message
news:HOidndVj2YQNkSnVnZ2dnUVZ_uOdnZ2d@adnap.net.au...
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