How to supply a 40V LED driver from mains?

D

dent

Guest
Hi all,

I want to install some microcontroller based mood lighting using high-
flux Cree LEDS. Most of the interesting led drivers which can be
coupled to a microcontroller for this sort of application (e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the easiest way
to get a suitable 40VDC input for these from the UK mains? I imagine
that there must be some standard way of doing this, based upon how
many such drivers there are, but my background is low-voltage embedded
systems so I'm not familiar with this sort of application. Any tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for mains
application, but all but the most expensive seem to use potentiometers
or phase-dimming.
 
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains? I
imagine
that there must be some standard way of doing this, based upon
how
many such drivers there are, but my background is low-voltage
embedded
systems so I'm not familiar with this sort of application. Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.
Click to expand...

What current level? A single string of 20mA LEDs? The easiest is
probably a 15-0-15V transformer used as a 30V source. This gives
42.4V peak and about 40Vdc at light load with a bridge rectifier
and capacitor filter. If it's easier to get one with a single 15V
winding, a voltage doubler will do the job. Some transformers are
wound to supply somewhat more than the rated voltage with no
load.
 
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains? I
imagine
that there must be some standard way of doing this, based upon
how
many such drivers there are, but my background is low-voltage
embedded
systems so I'm not familiar with this sort of application. Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The easiest is
probably a 15-0-15V transformer used as a 30V source. This gives
42.4V peak and about 40Vdc at light load with a bridge rectifier
and capacitor filter. If it's easier to get one with a single 15V
winding, a voltage doubler will do the job. Some transformers are
wound to supply somewhat more than the rated voltage with no
load.
Click to expand...
It will be 3-6 LEDs per string, at a current of 500-1000ma (I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.
 
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains? I
imagine
that there must be some standard way of doing this, based
upon
how
many such drivers there are, but my background is low-voltage
embedded
systems so I'm not familiar with this sort of application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The easiest
is
probably a 15-0-15V transformer used as a 30V source. This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a single
15V
winding, a voltage doubler will do the job. Some transformers
are
wound to supply somewhat more than the rated voltage with no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma (I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.
Click to expand...
A string of even 6 LEDs in series needs only about 20V. Add a few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.
 
dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which
can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains?
I
imagine
that there must be some standard way of doing this, based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source. This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers accept
an
input range up to 40V, and require 2-3V more input than the LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?
Click to expand...
It depends on what you can get. Technically, there's no reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the proper
level. However, operating it from an unnecessarily high voltage
wastes that much more power, especially if the current is set by
a simple resistor. That calls for a larger, costlier and heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably regulated,
should be easily available.

(It's already past 2:00 am here and I have to go to bed. Heavy
day tomorrow. Anything else I can help with will have to wait for
some hours).
 
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains? I
imagine
that there must be some standard way of doing this, based
upon
how
many such drivers there are, but my background is low-voltage
embedded
systems so I'm not familiar with this sort of application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The easiest
is
probably a 15-0-15V transformer used as a 30V source. This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a single
15V
winding, a voltage doubler will do the job. Some transformers
are
wound to supply somewhat more than the rated voltage with no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma (I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add a few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.
Click to expand...
The 40V figure comes the catalogue pages (Farnell, LedRise, ledTech,
etc.). Looking more closely I see now that such drivers accept an
input range up to 40V, and require 2-3V more input than the LED string
itself requires. Is the problem of supplying a lower voltage (e.g.,
20V) any easier?

thanks,

dan.
 
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom" <Pimpom@invalid.net>
wrote:

dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which
can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains?
I
imagine
that there must be some standard way of doing this, based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source. This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers accept
an
input range up to 40V, and require 2-3V more input than the LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the proper
level. However, operating it from an unnecessarily high voltage
wastes that much more power, especially if the current is set by
a simple resistor. That calls for a larger, costlier and heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably regulated,
should be easily available.
Click to expand...
---
The Cree XP-G is an LED with a typical forward voltage drop of 3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf


The XP Power LDU series of power supplies are wide-range DC input,
constant-current DC output switchers with PWM-able outputs which
require an input voltage of at least 2 volts higher than the DC output
to operate properly.

Once that criterion has been met, the LDU acts pretty much like a
power out = power in kind of device, so if the output load is 3 XP-G
LEDs, that's 3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will need to be
about 12V, and in order to put 10 watts into the load it'll take about
830mA from the 12V bulk supply in order to do that, neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about 930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its input
voltage is raised, then the input current required to supply a given
output power will fall, so there's no advantage to be gained by using
a transformer in the bulk supply which requires larger diameter wire
in the secondary.

---
JF
 
On Mar 14, 10:55 pm, John Fields <jfie...@austininstruments.com>
wrote:
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom" <Pim...@invalid.net
wrote:



dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which
can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains?
I
imagine
that there must be some standard way of doing this, based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source. This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers accept
an
input range up to 40V, and require 2-3V more input than the LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the proper
level. However, operating it from an unnecessarily high voltage
wastes that much more power, especially if the current is set by
a simple resistor. That calls for a larger, costlier and heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably regulated,
should be easily available.

---
The Cree XP-G is an LED with a typical forward voltage drop of 3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf

The XP Power LDU series of power supplies are wide-range DC input,
constant-current DC output switchers with PWM-able outputs which
require an input voltage of at least 2 volts higher than the DC output
to operate properly.

Once that criterion has been met, the LDU acts pretty much like a
power out = power in kind of device, so if the output load is 3 XP-G
LEDs, that's  3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will need to be
about 12V, and in order to put 10 watts into the load it'll take about
830mA from the 12V bulk supply in order to do that, neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about 930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its input
voltage is raised, then the input current required to supply a given
output power will fall, so there's no advantage to be gained by using
a transformer in the bulk supply which requires larger diameter wire
in the secondary.    

---
JF
Click to expand...
Would a laptop power supply be suitable then? I see that these 19V
supplies are cheap to come by. I assume that I could use that to drive
several of the LDUs in parallel? If I did so, would I need to match
the power of the PSU closely to the required power? That is, if I had
three LDUs each using 10W, if I used a 65W laptop power supply, would
that be more wasteful/inefficient than using something like a 40W
power supply?

Thanks for you help everyone.

dan.
 
dent wrote:
I want to install some microcontroller based mood lighting using high-
flux Cree LEDS. Most of the interesting led drivers which can be
coupled to a microcontroller for this sort of application (e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the easiest way
to get a suitable 40VDC input for these from the UK mains?
Click to expand...
Probably a "power supply." ;-)

Good Luck!
Rich
 
dent wrote:
On Mar 14, 10:55 pm, John Fields
jfie...@austininstruments.com
wrote:
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom"
Pim...@invalid.net
wrote:



dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood
lighting
using high-
flux Cree LEDS. Most of the interesting led drivers
which
can
be
coupled to a microcontroller for this sort of
application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is
the
easiest way
to get a suitable 40VDC input for these from the UK
mains?
I
imagine
that there must be some standard way of doing this,
based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of
application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source.
This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with
no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add
a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers
accept
an
input range up to 40V, and require 2-3V more input than the
LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no
reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the
proper
level. However, operating it from an unnecessarily high
voltage
wastes that much more power, especially if the current is set
by
a simple resistor. That calls for a larger, costlier and
heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which
should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably
regulated,
should be easily available.

---
The Cree XP-G is an LED with a typical forward voltage drop of
3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf

The XP Power LDU series of power supplies are wide-range DC
input,
constant-current DC output switchers with PWM-able outputs
which
require an input voltage of at least 2 volts higher than the
DC
output to operate properly.

Once that criterion has been met, the LDU acts pretty much
like a
power out = power in kind of device, so if the output load is
3 XP-G
LEDs, that's 3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will
need to
be about 12V, and in order to put 10 watts into the load it'll
take
about 830mA from the 12V bulk supply in order to do that,
neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about
930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its
input
voltage is raised, then the input current required to supply a
given
output power will fall, so there's no advantage to be gained
by using
a transformer in the bulk supply which requires larger
diameter wire
in the secondary.

---
JF

Would a laptop power supply be suitable then? I see that these
19V
supplies are cheap to come by. I assume that I could use that
to drive
several of the LDUs in parallel? If I did so, would I need to
match
the power of the PSU closely to the required power? That is, if
I had
three LDUs each using 10W, if I used a 65W laptop power supply,
would
that be more wasteful/inefficient than using something like a
40W
power supply?
Click to expand...
The idea of using a laptop PSU actually crossed my mind earlier,
but I wasn't sure what you'd be able to get there. A 19V supply
will let you use up to 4 or 5 white LEDs in series and yes, you
can wire more than one string in parallel as long as you don't
exceed the PSU's rating. As a matter of practical consideration,
I wouldn't load it right up to the max rating. Over here in
India, we get cheap Chinese multi-output laptop supplies. They
come with a variety of DC jacks and the output can be switched
for different voltages. Some models can go up to 24V

Regarding the power rating: There's no practical reason not to
use a 65W unit. In theory, the higher-powered model *may* be
slightly less efficient at light load, but that shouldn't matter
for your application.
 
On Mar 16, 7:21 am, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 10:55 pm, John Fields
jfie...@austininstruments.com
wrote:
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom"
Pim...@invalid.net
wrote:

dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood
lighting
using high-
flux Cree LEDS. Most of the interesting led drivers
which
can
be
coupled to a microcontroller for this sort of
application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is
the
easiest way
to get a suitable 40VDC input for these from the UK
mains?
I
imagine
that there must be some standard way of doing this,
based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of
application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source.
This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with
no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add
a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers
accept
an
input range up to 40V, and require 2-3V more input than the
LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no
reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the
proper
level. However, operating it from an unnecessarily high
voltage
wastes that much more power, especially if the current is set
by
a simple resistor. That calls for a larger, costlier and
heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which
should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably
regulated,
should be easily available.

---
The Cree XP-G is an LED with a typical forward voltage drop of
3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf

The XP Power LDU series of power supplies are wide-range DC
input,
constant-current DC output switchers with PWM-able outputs
which
require an input voltage of at least 2 volts higher than the
DC
output to operate properly.

Once that criterion has been met, the LDU acts pretty much
like a
power out = power in kind of device, so if the output load is
3 XP-G
LEDs, that's 3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will
need to
be about 12V, and in order to put 10 watts into the load it'll
take
about 830mA from the 12V bulk supply in order to do that,
neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about
930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its
input
voltage is raised, then the input current required to supply a
given
output power will fall, so there's no advantage to be gained
by using
a transformer in the bulk supply which requires larger
diameter wire
in the secondary.

---
JF

Would a laptop power supply be suitable then? I see that these
19V
supplies are cheap to come by. I assume that I could use that
to drive
several of the LDUs in parallel? If I did so, would I need to
match
the power of the PSU closely to the required power? That is, if
I had
three LDUs each using 10W, if I used a 65W laptop power supply,
would
that be more wasteful/inefficient than using something like a
40W
power supply?

The idea of using a laptop PSU actually crossed my mind earlier,
but I wasn't sure what you'd be able to get there. A 19V supply
will let you use up to 4 or 5 white LEDs in series and yes, you
can wire more than one string in parallel as long as you don't
exceed the PSU's rating. As a matter of practical consideration,
I wouldn't load it right up to the max rating. Over here in
India, we get cheap Chinese multi-output laptop supplies. They
come with a variety of DC jacks and the output can be switched
for different voltages. Some models can go up to 24V

Regarding the power rating: There's no practical reason not to
use a 65W unit. In theory, the higher-powered model *may* be
slightly less efficient at light load, but that shouldn't matter
for your application.
Click to expand...
Okay, I'll give that a go.

Mostly as a matter of curiosity, my (limited) understanding is that
both the LED driver and the laptop PSU would be using some sort of
power management electronics to try to match the required input to the
required output. Does this result in funny interaction effects between
the two (e.g., if the LED driver tries to pull more power, the laptop
PSU supplies more, which then changes how the LED driver is pulling
the power, which changes the laptop PSU again, and so on), or are they
quite happy to play nicely together?

Thanks for your helpful and informative answers.

regards,

dan.

of the laptop PSU is that it would be switched-mode, and try to both
the LED driver and the laptop PSU would be trying to take their input
and change it to the output in some way.
 
On Tue, 15 Mar 2011 15:32:08 -0700 (PDT), dent
<dwtowner@googlemail.com> wrote:

On Mar 14, 10:55 pm, John Fields <jfie...@austininstruments.com
wrote:
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom" <Pim...@invalid.net
wrote:



dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood lighting
using high-
flux Cree LEDS. Most of the interesting led drivers which
can
be
coupled to a microcontroller for this sort of application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is the
easiest way
to get a suitable 40VDC input for these from the UK mains?
I
imagine
that there must be some standard way of doing this, based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source. This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers accept
an
input range up to 40V, and require 2-3V more input than the LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the proper
level. However, operating it from an unnecessarily high voltage
wastes that much more power, especially if the current is set by
a simple resistor. That calls for a larger, costlier and heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably regulated,
should be easily available.

---
The Cree XP-G is an LED with a typical forward voltage drop of 3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf

The XP Power LDU series of power supplies are wide-range DC input,
constant-current DC output switchers with PWM-able outputs which
require an input voltage of at least 2 volts higher than the DC output
to operate properly.

Once that criterion has been met, the LDU acts pretty much like a
power out = power in kind of device, so if the output load is 3 XP-G
LEDs, that's  3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will need to be
about 12V, and in order to put 10 watts into the load it'll take about
830mA from the 12V bulk supply in order to do that, neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about 930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its input
voltage is raised, then the input current required to supply a given
output power will fall, so there's no advantage to be gained by using
a transformer in the bulk supply which requires larger diameter wire
in the secondary.    

---
JF

Would a laptop power supply be suitable then?
Click to expand...
---
Yes.
---

I see that these 19V
supplies are cheap to come by. I assume that I could use that to drive
several of the LDUs in parallel?
Click to expand...
---
Yes.
---

If I did so, would I need to match
the power of the PSU closely to the required power?
Click to expand...
---
No, you'd just have to make sure to stay within the PSU's output
range.

For instance, if you had three LDUs each dissipating 10 watts, then
your PSU would have to be capable of supplying 30 watts at its rated
output voltage.

Since the PSU is a constant voltage supply, its 19V output would have
to supply

P 30W
I = --- = ----- ~ 1.6A into the 3 paralleled LCUs.
E 19V

Each individual LCU would then drop its 19V input to whatever voltage
was required to drive its LED string at the desired current.

Assuming three LEDs in each series string, with 1A in each string and
each LED dropping 3.3V at 1A, then the outputs of the LDU's would each
be at 9.9V and, since their inputs need to be at least 2V above their
outputs, the 19V inputs from the PSU would be fine.
---

That is, if I had
three LDUs each using 10W, if I used a 65W laptop power supply, would
that be more wasteful/inefficient than using something like a 40W
power supply?
Click to expand...
---
There's no way of telling without knowing the efficiencies of the
supplies involved, but my guess would be that, using decent supplies,
there'd be little difference.
---

Thanks for you help everyone.

dan.
---
Click to expand...
:)

---
JF
 
On Mar 16, 7:21 am, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 10:55 pm, John Fields
jfie...@austininstruments.com
wrote:
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom"
Pim...@invalid.net
wrote:

dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood
lighting
using high-
flux Cree LEDS. Most of the interesting led drivers
which
can
be
coupled to a microcontroller for this sort of
application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is
the
easiest way
to get a suitable 40VDC input for these from the UK
mains?
I
imagine
that there must be some standard way of doing this,
based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of
application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers for
mains
application, but all but the most expensive seem to use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source.
This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage with
no
load.

It will be 3-6 LEDs per string, at a current of 500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V. Add
a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell, LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers
accept
an
input range up to 40V, and require 2-3V more input than the
LED
string
itself requires. Is the problem of supplying a lower voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no
reason
why it can't be run from 40V as long as a resistor or a more
sophisticated control circuit sets the LED current to the
proper
level. However, operating it from an unnecessarily high
voltage
wastes that much more power, especially if the current is set
by
a simple resistor. That calls for a larger, costlier and
heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which
should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably
regulated,
should be easily available.

---
The Cree XP-G is an LED with a typical forward voltage drop of
3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf

The XP Power LDU series of power supplies are wide-range DC
input,
constant-current DC output switchers with PWM-able outputs
which
require an input voltage of at least 2 volts higher than the
DC
output to operate properly.

Once that criterion has been met, the LDU acts pretty much
like a
power out = power in kind of device, so if the output load is
3 XP-G
LEDs, that's 3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will
need to
be about 12V, and in order to put 10 watts into the load it'll
take
about 830mA from the 12V bulk supply in order to do that,
neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about
930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its
input
voltage is raised, then the input current required to supply a
given
output power will fall, so there's no advantage to be gained
by using
a transformer in the bulk supply which requires larger
diameter wire
in the secondary.

---
JF

Would a laptop power supply be suitable then? I see that these
19V
supplies are cheap to come by. I assume that I could use that
to drive
several of the LDUs in parallel? If I did so, would I need to
match
the power of the PSU closely to the required power? That is, if
I had
three LDUs each using 10W, if I used a 65W laptop power supply,
would
that be more wasteful/inefficient than using something like a
40W
power supply?

The idea of using a laptop PSU actually crossed my mind earlier,
but I wasn't sure what you'd be able to get there. A 19V supply
will let you use up to 4 or 5 white LEDs in series and yes, you
can wire more than one string in parallel as long as you don't
exceed the PSU's rating. As a matter of practical consideration,
I wouldn't load it right up to the max rating. Over here in
India, we get cheap Chinese multi-output laptop supplies. They
come with a variety of DC jacks and the output can be switched
for different voltages. Some models can go up to 24V

Regarding the power rating: There's no practical reason not to
use a 65W unit. In theory, the higher-powered model *may* be
slightly less efficient at light load, but that shouldn't matter
for your application.
Click to expand...
Okay, I'll give that a go.

Mostly as a matter of curiosity, my (limited) understanding is that
both the LED driver and the laptop PSU would be using some sort of
power management electronics to try to match the required input to the
required output. Does this result in funny interaction effects between
the two (e.g., if the LED driver tries to pull more power, the laptop
PSU supplies more, which then changes how the LED driver is pulling
the power, which changes the laptop PSU again, and so on), or are they
quite happy to play nicely together?

Thanks for your helpful and informative answers.

regards,

dan.
 
dent wrote:
On Mar 16, 7:21 am, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 10:55 pm, John Fields
jfie...@austininstruments.com
wrote:
On Tue, 15 Mar 2011 02:13:06 +0530, "Pimpom"
Pim...@invalid.net
wrote:

dent wrote:
On Mar 14, 4:28 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
On Mar 14, 2:02 pm, "Pimpom" <Pim...@invalid.net> wrote:
dent wrote:
Hi all,

I want to install some microcontroller based mood
lighting
using high-
flux Cree LEDS. Most of the interesting led drivers
which
can
be
coupled to a microcontroller for this sort of
application
(e.g., XP
Power LDU series) are DC-DC with ~40V input*. What is
the
easiest way
to get a suitable 40VDC input for these from the UK
mains?
I
imagine
that there must be some standard way of doing this,
based
upon
how
many such drivers there are, but my background is
low-voltage
embedded
systems so I'm not familiar with this sort of
application.
Any
tips
would be appreciated.

thanks,

dan.

* I know that there are various dimmable LED drivers
for
mains
application, but all but the most expensive seem to
use
potentiometers
or phase-dimming.

What current level? A single string of 20mA LEDs? The
easiest
is
probably a 15-0-15V transformer used as a 30V source.
This
gives
42.4V peak and about 40Vdc at light load with a bridge
rectifier
and capacitor filter. If it's easier to get one with a
single
15V
winding, a voltage doubler will do the job. Some
transformers
are
wound to supply somewhat more than the rated voltage
with
no
load.

It will be 3-6 LEDs per string, at a current of
500-1000ma
(I'm
thinking of Cree XPG or similar, which are 3 watt).

dan.

A string of even 6 LEDs in series needs only about 20V.
Add
a
few
volts as overhead for the current setting circuitry. You
shouldn't need 40V.

The 40V figure comes the catalogue pages (Farnell,
LedRise,
ledTech,
etc.). Looking more closely I see now that such drivers
accept
an
input range up to 40V, and require 2-3V more input than
the
LED
string
itself requires. Is the problem of supplying a lower
voltage
(e.g.,
20V) any easier?

It depends on what you can get. Technically, there's no
reason
why it can't be run from 40V as long as a resistor or a
more
sophisticated control circuit sets the LED current to the
proper
level. However, operating it from an unnecessarily high
voltage
wastes that much more power, especially if the current is
set
by
a simple resistor. That calls for a larger, costlier and
heavier
transformer (if it's to be based on a mains-frequency
transformer). And that wasted power turns into heat which
should
also be taken into consideration. A 24V supply will provide
enough headroom and readymade 24V supplies, preferably
regulated,
should be easily available.

---
The Cree XP-G is an LED with a typical forward voltage drop
of
3.3V
with 1A through it.

http://www.cree.com/products/pdf/XLampXP-G.pdf

The XP Power LDU series of power supplies are wide-range DC
input,
constant-current DC output switchers with PWM-able outputs
which
require an input voltage of at least 2 volts higher than the
DC
output to operate properly.

Once that criterion has been met, the LDU acts pretty much
like a
power out = power in kind of device, so if the output load
is
3 XP-G
LEDs, that's 3 * 3.3V * 1A = 9.9W.

Since the LDU needs at least 2V of headroom, the input will
need to
be about 12V, and in order to put 10 watts into the load
it'll
take
about 830mA from the 12V bulk supply in order to do that,
neglecting
efficiency.

Assuming an efficiency of 90% raises that current to about
930mA.

Now, since The LDU is a switcher where Pout ~ Pin, if its
input
voltage is raised, then the input current required to supply
a
given
output power will fall, so there's no advantage to be gained
by using
a transformer in the bulk supply which requires larger
diameter wire
in the secondary.

---
JF

Would a laptop power supply be suitable then? I see that
these
19V
supplies are cheap to come by. I assume that I could use that
to drive
several of the LDUs in parallel? If I did so, would I need to
match
the power of the PSU closely to the required power? That is,
if
I had
three LDUs each using 10W, if I used a 65W laptop power
supply,
would
that be more wasteful/inefficient than using something like a
40W
power supply?

The idea of using a laptop PSU actually crossed my mind
earlier,
but I wasn't sure what you'd be able to get there. A 19V
supply
will let you use up to 4 or 5 white LEDs in series and yes,
you
can wire more than one string in parallel as long as you don't
exceed the PSU's rating. As a matter of practical
consideration,
I wouldn't load it right up to the max rating. Over here in
India, we get cheap Chinese multi-output laptop supplies. They
come with a variety of DC jacks and the output can be switched
for different voltages. Some models can go up to 24V

Regarding the power rating: There's no practical reason not to
use a 65W unit. In theory, the higher-powered model *may* be
slightly less efficient at light load, but that shouldn't
matter
for your application.

Okay, I'll give that a go.

Mostly as a matter of curiosity, my (limited) understanding is
that
both the LED driver and the laptop PSU would be using some sort
of
power management electronics to try to match the required input
to the
required output. Does this result in funny interaction effects
between
the two (e.g., if the LED driver tries to pull more power, the
laptop
PSU supplies more, which then changes how the LED driver is
pulling
the power, which changes the laptop PSU again, and so on), or
are they
quite happy to play nicely together?
Click to expand...
Generally speaking, yes. The main job of most power supplies
(with exceptions like a constant current supply) is to make a
certain voltage available and the load will draw as much current
as it needs. A regulated supply like a laptop PSU has control
circuitry that will try to maintain a constant level of output
voltage despite changes in load current or mains voltage. Most
have protective mechanisms that, if you exceed a certain load
current, will pull down the output voltage. Until you reach that
limit, all that the PSU does is to make a constant voltage source
available. "Funny interactions" will happen only in unusual
circumstances.

In a way though, interactions do happen. The output voltage of a
simple unregulated power supply will drop as you increase the
load current, and will also change with mains fluctuations. In
that sense, the control electronics of a regulated supply does
interact with the load as it continuously compensates for those
changes. But under normal conditions, such interactions are
transparent to the user and are not the "funny" kind you
envisaged.

Thanks for your helpful and informative answers.
Click to expand...
You're welcome.
 
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