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October 2013 95
Advertising Index
Advantage Partnership Lawyers... 95
Altronics....................................... 17
element14...................................... 5
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Icom Australia................................ 7
Ask SILICON CHIP
. . . continued from page 94
(Samsung 13W chip) down-lights using Tenico UP-15W-D constant current
dimmable LED drivers. The dimmer
will be a Clipsal trailing-edge variety.
All of this is said to be fully compatible by my electrical wholesaler. I have
set it up as a test rig and all seems to
function correctly. What did surprise
me is the “blinding” brightness when
the dimmer was turned full on; I was
half expecting to see smoke!
I then studied the specs of the lamps
and the drivers and this is where I became confused. The lamp spec quotes:
voltage 26-30V DC; current 320mA;
power 13W; colour 3000K. The Driver
spec quotes: constant current dimmable LED driver; primary 200-240VAC
<at> 0.11A; secondary DC 23-38V 320mA
± 5%; dimmable with leading/trailing
edge or universal dimmer.
The variable DC requirement of the
lamp and output of the driver is what
confuses me. Is this to do with the constant current requirement of the LEDs?
If so, then how is the output voltage
controlled so that the current control
is correct for the lamp involved?
In my case it would appear that
for the lamp to generate its full 13W
output at 320mA, the voltage would
need to be 40.6V. So why does my
lamp quote a maximum rating of
30V <at>320mA which is only 9.6W?
Conversely, as my driver can supply
320mA <at> 38V, does this mean that I
can overdrive the lamps leading to
earlier failure? Then of course one can
bring the dimmer into the equation
and I am lost!
It would be really good to have an
article on this subject so that the mystique of LED lamps, drivers and dim96 Silicon Chip
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Notes & Errata
Nixie Clock, July 2007: the circuit
diagram (Fig.1) shows pin 14 of
IC1 connected to pin 12 of IC2.
The connection should in fact go
to pin 11 of IC2.
The PCB layout diagram on
page 73 of the August 2007 issue
is correct.
mers can be better understood. Failing
that I would appreciate any comments
you can make on my particular example. (M. F., Mount Eliza, Vic).
• Normally, LEDs are driven using a
current source and the voltage developed across the series string of LEDs
is dependent on the characteristics of
the LEDs. These have a higher voltage
across them with higher current, ie,
they have positive impedance. So the
LED lamp specifications of 26-30V at
320mA refers to the fact that this is the
range of possible voltage across the
lamp (of in-series LEDs) when driven
at 320mA.
The voltage depends on the individual LEDs themselves and can vary
from manufacturing batch to batch.
The 26-30V refers to the specified
limits that the LED voltage could be
at that 320mA current.
So long as it can deliver the required
voltage range of 26-30V at 320mA,
then the driver should be suitable and
Microchip Technology............... OBC
Mikroelektronika......................... IBC
Ocean Controls............................ 29
Premier Batteries......................... 55
Quest Electronics......................... 95
Radio, TV & Hobbies DVD............ 11
RF Modules.................................. 96
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Silicon Chip Binders..................... 95
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Silicon Chip Online Shop........ 88-89
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Tekmark Australia........................... 9
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Worldwide Elect. Components..... 96
xLogic............................................. 9
it can drive the LED lamp correctly.
The voltage across the LED lamp will
settle at the voltage that allows 320mA
current to flow. When dimmed, the
average current is lowered by reducing
the pulse width of the applied current
so that the lamp receives less power.
The power discrepancy may be that
13W is the total power of the LEDs
SC
plus the driver.
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