Re: [tlug] [OT] Any electricians / electrical engineers around?

[Godwin Stewart <gstewart@example.com>]

On Mon, 31 Dec 2007 19:46:36 +0900, Dave M G <dave@example.com> wrote:

> {massive snip}

About dimming LEDs...

It can be done, but in a roundabout way. What you want to build is what's
known as a Norton generator, which generates a regulated current rather
than a regulated voltage. Each LED is going to sink a nominal current of
20mA and wind up with about 2V across it, so the idea is going to be to put
as many of these things in series as possible in order to keep the number
of chains down to the minimum, bearing in mind the following:

1) If one LED burns out then the entire chain it's in will go out.
Therefore, the longer the chain, the greater the probability that it'll
blow.

2) The longer the chain, the less of them there'll be, thus simplifying the
whole kaboodle (less electronics).

3) The longer the chain, the higher the voltage across it and therefore the
higher the voltage your power supply must put out (but it won't have to
deliver much current).

The way I'd do this is as follows.

I'd arrange the 48 LEDs in 6 groups of 8. Each group of 8 is going to have
about 16V across it. Allowing 4V for transistor polarization and zener
breakdown voltage, you're going to have to give the circuit something like
20V-24V DC. Voltage regulation isn't critical here. As for current, you're
going to have to supply 6 times 20mA (120mA) plus a few tens of milliamps
through the rest of the circuitry. Let's say 200mA-250mA to be on the safe
side. If you can't find a power supply that'll deliver this then I can
provide schematics for you to build one.

Allow me a while to draw up schematics for the Norton generator, scan them
and put them online for you. The principle behind it will be a buffered,
regulated voltage that transistors are going to convert to regulated
current and suck through the chains of LEDs.

I'll try and do this today. If I can't then I'll do it on Wednesday.

-- 
G. Stewart - gstewart@example.com

Windows is a chicken-wire mosquito net.

		-- Morely Dotes in NANAE, 20-Jan-2006

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