Re: [AH] Replacing LEDs in a Polymoog

From ChristianH
Sent Fri, May 3rd 2019, 10:26

I wouldn't expect much in terms of an original data sheet, at least not
online - that was 20 years before the internet took off.=20

And - LED usage wasn't as sophisticated as it is now. Gee, even other
colors than red weren't that common.=20
Rule of thumb for red was 1.2V, 20mA, which meant 180R for 5V TTL
operation. That was the same I did back then without thinking.=20
Oh, and flat side is negative, but that was about all you needed to know
abut LEDs.

BTW, I think forward voltage mainly is determined by physical properties
of LED color and hasn't changed since then. Current is a different thing,
20mA would be considered much by now for indicating purposes (in
contrast to illumination).

Chris



On Thu, 2 May 2019 20:11:18 -0700 Brian Willoughby
<xxxxxx@xxxxxxxxxxxx.xxx> wrote:

> If this were my Polymoog ( ;-) ), I would try to find the model number of=
 the original manufacturer=E2=80=99s LED and look for a data sheet. Hopeful=
ly, that data sheet would list the exact color wavelength of those LEDs in =
nanometers. I would personally want the =E2=80=9Csame=E2=80=9D color of red=
, not just any old red.
>=20
> Modern LEDs are available in various shades, usually with good documentat=
ion on the exact color wavelength. Of course, the really cheap LEDs just sa=
y =E2=80=9Cred=E2=80=9D but you can still afford LEDs with better quality c=
ontrol. Some have just a single wavelength, while others have a wide range,=
 but can still be =E2=80=9Cbinned=E2=80=9D by measured wavelength in much n=
arrower ranges. You could save a little money by buying the unmeasured LEDs=
, but the savings isn=E2=80=99t worth the tradeoff at your quantity. Instea=
d, I think it=E2=80=99s worth paying a little more to control the color.
>=20
> Assuming you can find the original specifications (or perhaps measure the=
m), then finding a modern match should be easy. Most modern data sheets hav=
e graphs showing brightness versus color, and you can match the brightness =
as well.
>=20
> As Murray pointed out, you=E2=80=99ll have to change the resistors, too. =
The 180 ohm resistor sets the current, based on the LED supply voltage and =
the LED forward voltage drop. If the modern LED has the same voltage drop, =
then you=E2=80=99ll surely have to change the resistor to get lower current.=
 There=E2=80=99s a random chance that you could find a modern LED with a hi=
gher forward voltage (that isn=E2=80=99t too high for the existing LED volt=
age) and this would lower the current without changing the resistor, but it=
 seems unlikely that it would work out precisely.
>=20
> One option that you should consider is buying modern LEDs with the same c=
urrent rather than lower current. The reason for doing this is that you wou=
ldn=E2=80=99t need to change the resistors. Of course, forward voltage and =
efficiency are factors.
>=20
> One thing to note is that LED brightness can be measured in lumens or can=
dles, so you might have to convert between the standards if you want to dia=
l in the same brightness. Also look out for the dispersion pattern - a dire=
ctional LED will only be bright directly in front, while a non-directional =
LED will spread out the brightness across a wider angle. The difference bet=
ween lumens and candles is related to dispersion in different ways.
>=20
> I suppose that I=E2=80=99ve made this sound like an incredibly difficult =
task, but it=E2=80=99s actually quite simple if you do the math. A good LED=
 data sheet will give all the information you need to calculate a match to =
the Moog original and perhaps even save you from changing the resistors.
>=20
> Good luck!
>=20
> Brian
>=20
>=20
> On May 2, 2019, at 1:51 PM, Dan Goldstein <xxxxxxxx@xxx.xxx> wrote:
> >   I=E2=80=99ve got a Polymoog I=E2=80=99m restoring, and I=E2=80=99d li=
ke to replace the old, original LEDs with new, modern ones. The originals w=
ere 5mm bulbs, =E2=80=9CSSL-22=E2=80=9D, each with a 180 ohm resistor hooke=
d up to it.
> > =20
> >    I=E2=80=99d like to replace them with new, modern red LEDs of equiva=
lent brightness and lower current use. There are so many different LED valu=
es these days, and I=E2=80=99m not sure how to know what would be the ideal=
 replacement LED. Can anyone offer any advice on specs for replacement LEDs=
 for a Polymoog?
> > =20
> > Thanks!
> > Dan
> >=20