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From: email@example.com (Byron A Jeff)
Subject: Re: Best way to power array of LEDs?
Date: 6 Oct 2002 21:39:55 -0400
Organization: College of Computing, Georgia Tech
In article ,
-On 4 Oct 2002 12:48:17 -0400, firstname.lastname@example.org (Byron A Jeff) Gave
->In article ,
->-"John Woodgate" wrote in message
->-> I read in sci.electronics.design that Uwe Zimmermann
->-> wrote (in <3D9D5939.CD795C11@kth.se>) about 'Best way to power array of
->-> LEDs?', on Fri, 4 Oct 2002:
->-> >You then connect the drain of the transistor to the positive supply and
->-> >the gate/source to the anode of the first LED in each chain. The cathode
->-> >of the last LED is connected to the negative supply.
->-> This is an *extremely* unreliable circuit, because it has no protection
->-> against the large transients, of both polarities, that occur on vehicle
->-> electrical supplies.
->That's an apples and gorillas discussion. Transient and spike surppression
->is a completely different issue from powering the LEDs. I didn't even attempt
->to address it.
->-> Don't bother with adding transistors just to get rid of a few resistors
->-> (or maybe add a few more, and MOVs and capacitors and fuses ...). KISS!
->But it isn't simple if the array isn't bright enough to do the job. It's
->quite likely that the LEDs will need to be pulsed with higher current in
->order to give brighter perceptual intensity. Once you start down that road
->you're going to have to have transistors to modulate/switch power to the
->And frankly an extra 2N2222 and base resistor per string isn't exactly rocket
- Yeah... they're all of what 10 cents a piece? Hehehe...
- DC/DC converter from surplus store for five bucks, drop to five
-volts, clean instead of 12, dirty.
It really doesn't matter too very much since it'll be the same amount of
current regardless of the voltage...
- Two copper rails.
- A series resistor in each leg of each LED that gets BUSSED in parallel onto
- the copper rails.
Again why when you can string multiple LEDs per string this saving both
components (resistors) and power loss (due to the lower voltage drop.
- Ten bucks total... maybe. On a bad day.
I never heard price being an issue.
Any you still haven't dealt with the most important issue, brightness! Here
are the simple facts:
* The LEDs at continuous max current probably won't be bright enough.
* More current can be delivered only at a lower than 100 percent duty cycle.
* This requires that the power to the LEDs to be switched.
* Ergo you need a transistor. The current limiting is simply a bonus.
LEDs are not about voltage. They don't care about voltage. I've run them at
2.5V. I've run them at 120V AC. LEDs are current devices and have a brighter
response to more current. So any LED supply is about current management.
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