Power supply for LED array

[Dave B]

Hi,

I hope I'm in the right group to ask these two questions, so here
goes...

I'm building a lighting feature with 60 LEDs[0], planning to power it
with a standard switched mode DC adapter [1], and incorporate a
combination on/off dimmer switch of some sort.

Q1: I recently encountered specialised LED power supplies [2] and
wondered what the difference from switched mode is. E.g. maybe they
offer better efficiency or overload protection, but not anything
essential compared to a good switched PSU?

Q2: With 12V x 0.42A for all 60 LEDs giving a total of 5W (maybe 5.2W
without rounding), what can I use for on/off and dimmer? My initial
idea was a switched potentiometer [3] - until I realised that they
won't cope with 5W, so now I'm a bit lost.

I've got this far on GCSE science and web pages, but figured it was
time to ask a human for their expertise

Cheers,
Dave.


[0] 60 LEDs in 12 parallel groups of 3 serial LEDs, and 12 parallel
groups of 2 serial LED, each group has their own resistor. Total
current 0.42A at 12V
[1] Switched power, e.g.
http://www.rapidonline.com/sku/Electrical-Power/Power-Supplies/Plug-In-PSUs/5W-ndash-12W-Mini-plugtop-switch-mode-power-supply/80857/85-3071
[2] LED power, e.g. http://www.excelsys.com/products/shelf_power.html
[3] Switched pot, e.g. http://www.maplin.co.uk/standard-potentiometers-with-switch-2208

 

[Randy Day]

In article <216cfeeb-f683-4013-be25-
9defa7ac19a4@w36g2000vbi.googlegroups.com>,
david.a.balch@gmail.com says...

Hi,

I hope I'm in the right group to ask these two questions, so here
goes...

I'm building a lighting feature with 60 LEDs[0], planning to power it
with a standard switched mode DC adapter [1], and incorporate a
combination on/off dimmer switch of some sort.

Q1: I recently encountered specialised LED power supplies [2] and
wondered what the difference from switched mode is. E.g. maybe they
offer better efficiency or overload protection, but not anything
essential compared to a good switched PSU?

They're used for traffic signals and outdoor
displays. The contstant current feature can
probably be set for various brands of high-
brightness leds. Also, a wide operating
temperature for outdoor use.

Q2: With 12V x 0.42A for all 60 LEDs giving a total of 5W (maybe 5.2W
without rounding), what can I use for on/off and dimmer? My initial
idea was a switched potentiometer [3] - until I realised that they
won't cope with 5W, so now I'm a bit lost.

I've got this far on GCSE science and web pages, but figured it was
time to ask a human for their expertise

PWM is the standard way to dim leds. Here's a
simple 555 timer circuit doing just what
you're asking about:

http://www.rcexplorer.se/page14/LED/files/ne555-pwm-led-
dimmer-circuit.gif

Mind the word wrap. HTH

 

[Tim Wescott]

On 02/01/2011 02:17 PM, Dave B wrote:

Hi,

I hope I'm in the right group to ask these two questions, so here
goes...

I'm building a lighting feature with 60 LEDs[0], planning to power it
with a standard switched mode DC adapter [1], and incorporate a
combination on/off dimmer switch of some sort.

Q1: I recently encountered specialised LED power supplies [2] and
wondered what the difference from switched mode is. E.g. maybe they
offer better efficiency or overload protection, but not anything
essential compared to a good switched PSU?

Q2: With 12V x 0.42A for all 60 LEDs giving a total of 5W (maybe 5.2W
without rounding), what can I use for on/off and dimmer? My initial
idea was a switched potentiometer [3] - until I realised that they
won't cope with 5W, so now I'm a bit lost.

I've got this far on GCSE science and web pages, but figured it was
time to ask a human for their expertise

Cheers,
Dave.


[0] 60 LEDs in 12 parallel groups of 3 serial LEDs, and 12 parallel
groups of 2 serial LED, each group has their own resistor. Total
current 0.42A at 12V
[1] Switched power, e.g.
http://www.rapidonline.com/sku/Electrical-Power/Power-Supplies/Plug-In-PSUs/5W-ndash-12W-Mini-plugtop-switch-mode-power-supply/80857/85-3071
[2] LED power, e.g. http://www.excelsys.com/products/shelf_power.html
[3] Switched pot, e.g. http://www.maplin.co.uk/standard-potentiometers-with-switch-2208

You're going to be burning up a lot of power in your loading resistors

-- there are probably more efficient ways of limiting current, although
yours certainly is aces for simplicity.

You want to control the power to the LEDs with the pot. Either PWM the
current to the LED at 100Hz or so using a power transistor driven by a
timer, or rearrange your LED drive and control the voltage or current
directly.

The simplest way to do this would be to find a logic-level FET that can
handle half an amp, and PWM it from a 555 or similar timer, controlled
by the pot.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Dave B]

Hi,

Apologies for my slow reply - especially after you gave such a quick
response! I've quit one of my bands, which has freed up some time for
this project

On Feb 2, 12:03 am, Randy Day <randy....@sasktel.netx> wrote:

In article <216cfeeb-f683-4013-be25-
9defa7ac1...@w36g2000vbi.googlegroups.com>,
david.a.ba...@gmail.com says...

Q1: I recently encountered specialised LED power supplies [2] and
wondered what the difference from switched mode is. E.g. maybe they
offer better efficiency or overload protection, but not anything
essential compared to a good switched PSU?

They're used for traffic signals and outdoor
displays. The contstant current feature can
probably be set for various brands of high-
brightness leds. Also, a wide operating
temperature for outdoor use.

Ah, thanks. I guess they're worthwhile in heavy use and harsh
conditions.

Q2: With 12V x 0.42A for all 60 LEDs giving a total of 5W (maybe 5.2W
without rounding), what can I use for on/off and dimmer? My initial
idea was a switched potentiometer [3] - until I realised that they
won't cope with 5W, so now I'm a bit lost.

PWM is the standard way to dim leds. Here's a
simple 555 timer circuit doing just what
you're asking about:

http://www.rcexplorer.se/page14/LED/files/ne555-pwm-led-
dimmer-circuit.gif

Mind the word wrap.  HTH

That's just the job, thanks. Now I need to do some studying!

Cheers,
Dave.

 

[Dave B]

Hi,

Apologies again for my slow reply - your response was really quick as
well

On Feb 2, 12:36 am, Tim Wescott <t...@seemywebsite.com> wrote:

On 02/01/2011 02:17 PM, Dave B wrote:

Q2: With 12V x 0.42A for all 60 LEDs giving a total of 5W (maybe 5.2W
without rounding), what can I use for on/off and dimmer? My initial
idea was a switched potentiometer [3] - until I realised that they
won't cope with 5W, so now I'm a bit lost.

You're going to be burning up a lot of power in your loading resistors
-- there are probably more efficient ways of limiting current, although
yours certainly is aces for simplicity.

Would the more efficient approaches be active current limiting, like
http://www.vidisonic.com/2008/07/10/current-limiting-circuit/ ?
That'll need some study as well, and some calculations to see how much
power it would save - to balance against the time/cost to add active
limiting circuits.

You want to control the power to the LEDs with the pot.  Either PWM the
current to the LED at 100Hz or so using a power transistor driven by a
timer, or rearrange your LED drive and control the voltage or current
directly.

The simplest way to do this would be to find a logic-level FET that can
handle half an amp, and PWM it from a 555 or similar timer, controlled
by the pot.

Cool, it's good to have two responses recommending the same
approach

Thanks!
Dave.

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details athttp://www.wescottdesign.com/actfes/actfes.html

 

[Jasen Betts]

On 2011-02-01, Dave B <david.a.balch@gmail.com> wrote:

Hi,

I hope I'm in the right group to ask these two questions, so here
goes...

I'm building a lighting feature with 60 LEDs[0], planning to power it
with a standard switched mode DC adapter [1], and incorporate a
combination on/off dimmer switch of some sort.

Q1: I recently encountered specialised LED power supplies [2] and
wondered what the difference from switched mode is. E.g. maybe they
offer better efficiency or overload protection, but not anything
essential compared to a good switched PSU?

possibly they are current rtegulated instead of voltage regulated?

Q2: With 12V x 0.42A for all 60 LEDs giving a total of 5W (maybe 5.2W
without rounding), what can I use for on/off and dimmer? My initial
idea was a switched potentiometer [3] - until I realised that they
won't cope with 5W, so now I'm a bit lost.

power rated pots can be had, (eg in an audio L-PAD)

60 LEDs in 12 parallel groups of 3 serial LEDs, and 12 parallel
groups of 2 serial LED, each group has their own resistor. Total
current 0.42A at 12V

PWM is your best bet, using voltage or current limiting on that lot
will lead to lower series voltage strings being disproportinayely
bright at low powers. the 555 + MOSFET approach is probaby the
simplest

eg: http://www.dprg.org/tutorials/2005-11a/index.html
replace the motor with your led array and use the
same 12V to run both.


--
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