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From: email@example.com (Ben Bradley)
Subject: Re: How to make/solder a circuit fast?
References: <firstname.lastname@example.org> <email@example.com>
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NNTP-Posting-Date: Sat, 14 Sep 2002 14:21:53 EDT
Date: Sat, 14 Sep 2002 18:37:16 GMT
In sci.electronics.design, Ian Stirling
>The little lost angel wrote:
>> The reason for having L2 separate is I need something like 16mA to get
>> a brightness that's not too much diff from 20mA. If I put three
>> together, two blows and the one remaining won't have much chance of
>> surviving 48mA rite?
>I'd say you'r going way too complex, if the device is in a PC, it's
>You have a regulated 12V supply available, there is absolutely no
>point in putting in another regulator.
Good idea, excellent point...
>I'd go with something really, really simple, like simply connecting
>a resistor in series with each LED socket to +12V, and the other side
>of the sockets to 0V.
>The resistors should be around 12V/.02A, or 600 ohms, maybe a bit less.
>If they are 600 ohms, then when the LED is shorted they'll dissapate
>under a quarter of a watt each.
The simplest solution (and most power-efficient, even though the
absolute power wasted is quite small regardless) is to put all three
LED's in series and have one series resistor scaled appropriately for
the 12V source. If one LED needs less current than others, put an
appropriate resistor in parallel with it. This way the total current
pulled is only that needed for the highest-current LED. Three LED's
and one to three resistors and you're done.
Layout the board in expresspcb.com's program, you can probably fit
16 of these small circuits (I stand up thru-hole resistors, putting
the leads 0.1" apart) on their standard miniboard size, arranged as
8X2 so you can cut them apart with straight cuts with a bandsaw,
hacksaw or coping saw. With expresspcb's 3 boards you get 48 circuits.
While waiting for the PCB's and parts from Digi-key to be delivered,
wire the other two by hand (buy parts from the local Radio Shack if
neccesary) on two little protoboards with all the 0.1" holes in it.
Presuming everything is ordered Friday before 2PM, you could spend
Tuesday night stuffing the other 48 boards. Don't forget to buy solder
if you're running low.
>This has the slight problem that it may blow some sensitive LEDs with the
>12V reverse bias if the LEDs are plugged in backwards.
>Connecting a 4V zener across each diode is one way.
With my circuit, a single 1N4148 across the three series LED's,
conducting in the opposite direction, should do fine for
reverse-polarity protection. Actually, this only protects if the whole
circuit is plugged in backwards. If the LED's are installed backwards,
sue the assembly house for their mistake. ;)
>Connecting a small signal diode to a common zener is another.
>I think that connecting signal diodes to the 5V line is probably a bad
>idea, in some (rare) circumstances that could cause problems.
If you put three LED's in series you probably need more than 5V to
power them anyway.
>http://inquisitor.i.am/ | mailto:firstname.lastname@example.org | Ian Stirling.
>What a wonderfull world it is that has girls in it! -- Robert A Heinlein.
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