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Power supply for LED array

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.
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[2] LED power, e.g.
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[3] Switched pot, e.g.
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Reply to
Dave B
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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.

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

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dimmer-circuit.gif

Mind the word wrap. HTH

Reply to
Randy Day

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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
Reply to
Tim Wescott

Hi,

Apologies for my slow reply - especially after you gave such a quick resp> >

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

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

Cheers, Dave.

Reply to
Dave B

Hi,

Apologies aga> >

Would the more efficient approaches be active current limiting, like

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? 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.

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

Thanks! Dave.

Reply to
Dave B

possibly they are current rtegulated instead of voltage regulated?

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

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:

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replace the motor with your led array and use the same 12V to run both.

--
?? 100% natural
Reply to
Jasen Betts

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