20 Oct
2005
20 Oct
'05
1:45 p.m.
Dear Classic Computers,
I regret that my EE knowledge was never deep and is
a little rusty now. I am refurbishing some old PC
boards and wanted to replace a few of the LED which
had become damaged or broken off. Here is my lame
question, how do I correctly pick replacement LEDs?
I know colors, and size, but how do I make certain
that I have picked correct wattage, etc.?
If some one with more experience could provide a
few sentences of guidance I would be grateful. I
can even put a Tektronix scope on it and take
measurements if necessary.
Bradley Slavik
20 Oct
20 Oct
2:27 p.m.
Dear Classic Computers,
I regret that my EE knowledge was never deep and is
a little rusty now. I am refurbishing some old PC
boards and wanted to replace a few of the LED which
had become damaged or broken off. Here is my lame
question, how do I correctly pick replacement LEDs?
I know colors, and size, but how do I make certain
that I have picked correct wattage, etc.?
Unless they were doing something 'clever' like using the LED as a
poor-quality voltage reference [1], then it's not going to be that
critical. I assume thsse LEDs were used simply as indicators.
Pick the same physical size, same colour (the forward voltage drop does
depend on the colour), and for old stuff you don't want the modern 'high
efficiency' or 'low current' LEDs. If you have more choice, pick something
where the specified forward current (may be called If) is around 10-20mA.
That's the value for most, if not, all, general purpose LEDs.
[1] At least one Zenith MDA monitor used that trick....
-tony
2:37 p.m.
On 10/20/2005 at 3:45 PM Bradley Slavik wrote:
I regret that my EE knowledge was never deep and is
a little rusty now. I am refurbishing some old PC
boards and wanted to replace a few of the LED which
had become damaged or broken off. Here is my lame
question, how do I correctly pick replacement LEDs?
I know colors, and size, but how do I make certain
that I have picked correct wattage, etc.?
If you're talking about vintage garden-variety red or green LEDs, just pick
replacements with the same size and color. In general, LED's today are
more efficient than the old ones and offer a wider choice of illumination
angles but newer ones should work just fine as replacement for the
antiques. Most red LED's still exhibit a forward voltage drop of about 1.6
volts and the practice is to insert a current-limiting resistor in series
with the diode, so your risk is negligible.
Cheers,
Chuck
2:49 p.m.
>>>> "Chuck" == Chuck Guzis
<cclist at sydex.com> writes:
Chuck> On 10/20/2005 at 3:45 PM Bradley Slavik wrote:
> I regret that my EE knowledge was never deep and
is a little rusty
> now. I am refurbishing some old PC boards and wanted to replace a
> few of the LED which had become damaged or broken off. Here is my
> lame question, how do I correctly pick replacement LEDs? I know
> colors, and size, but how do I make certain that I have picked
> correct wattage, etc.?
Chuck> If you're talking about vintage garden-variety red or green
Chuck> LEDs, just pick replacements with the same size and color.
That should work for yellow and orange too. But (unlikely case in
vintage gear) it may give you trouble for blue, since there are at
least two very different semiconductors in blue LEDs: silicon carbide,
and some III/V compound (GaP? GaN?) which almost certainly have
different forward drops.
paul
7004
days inactive
7004
days old
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