"ian field" wrote in message
...

"mark krawczuk" wrote in message
...
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

The LEDs each have a more or less constant forward voltage so you have to
arrange for the current to be limited to the value stated in the data
sheet.

Traditionally this has been done with a series current limiting resistor,
but this wastes energy as heat in the resistor, the more LEDs you can
chain in series within the supply voltage the smaller the resistor and
energy waste - but this arrangement is not very tolerant of variations in
supply voltage.

Increasing numbers of wall-wart power packs are SMPSU, but that doesn't
necessarily mean good regulation - try a few off load and loaded at rated
power.

A SMPSU will at least minimise dissipation as compared to that in a linear
PSU.

A current regulated SMPSU isn't trivial, I'm currently experimenting with
a buck regulator to drive a single 1W TO220 white led from a 6V SLA
battery, early tests with a couple of ordinary LEDs drawing 20mA gave an
efficiency less than 30%, drawing higher current improved regulator
efficiency but its still only a small improvement on a series resistor.


"ian field" wrote in message
...

"mark krawczuk" wrote in message
...
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

The LEDs each have a more or less constant forward voltage so you have to
arrange for the current to be limited to the value stated in the data
sheet.

Traditionally this has been done with a series current limiting resistor,
but this wastes energy as heat in the resistor, the more LEDs you can
chain in series within the supply voltage the smaller the resistor and
energy waste - but this arrangement is not very tolerant of variations in
supply voltage.

Increasing numbers of wall-wart power packs are SMPSU, but that doesn't
necessarily mean good regulation - try a few off load and loaded at rated
power.

A SMPSU will at least minimise dissipation as compared to that in a linear
PSU.

A current regulated SMPSU isn't trivial, I'm currently experimenting with
a buck regulator to drive a single 1W TO220 white led from a 6V SLA
battery, early tests with a couple of ordinary LEDs drawing 20mA gave an
efficiency less than 30%, drawing higher current improved regulator
efficiency but its still only a small improvement on a series resistor.


Little wonder: the current sensor with a 1.25v feedback will eat up 35% of
the efficiency. You need to stick an amp in there to lower the drop. Or try
the arrangement shown on the third ('Hack') circuit he
http://www.quantsuff.com/LED2.htm.