Re: [AH] Replacing LEDs in a Polymoog

From Brian Willoughby
Sent Fri, May 3rd 2019, 03:24

(sorry to follow up my own post=E2=80=A6)

Aiming for =E2=80=9Clower current use=E2=80=9D may create more of a =
problem for you than it solves. I=E2=80=99d look at all of the variables =
before deciding on the requirements.

If you can=E2=80=99t find a data sheet, you could still use a meter to =
measure the actual Forward Voltage drop of the original Moog LEDs. If =
you can probe the resistors, you might be able to measure their =
resistance, and from their voltage drop you can calculate the actual =
Current. Those few measurements will allow you to select a modern LED by =
looking at data sheets. The only thing that might be difficult is =
measuring the color wavelength if you can=E2=80=99t find a =E2=80=9CSSL-22=
=E2=80=9D data sheet with the nm spec.

Unfortunately, there are many LED models that are almost completely =
undocumented. Those that have a data sheet sometimes have very important =
data missing. That said, there are still many brands and models to =
choose from that have sufficient documentation.

As I mentioned, a lot of it comes down to math, and you can probably get =
very close. Once you have the data, you might actually be able to aim =
for lower current use without things getting out of hand. Simple =
equations like V=3DIR and the graphs of brightness versus current and =
forward voltage versus current can help you predict the brightness.

Brian


On May 2, 2019, at 8:11 PM, 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. =
Hopefully, 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 =
documentation on the exact color wavelength. Of course, the really cheap =
LEDs just say =E2=80=9Cred=E2=80=9D but you can still afford LEDs with =
better quality control. 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 narrower 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. Instead, 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 =
them), then finding a modern match should be easy. Most modern data =
sheets have 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 higher forward voltage (that isn=E2=80=99t too high =
for the existing LED voltage) 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 current rather than lower current. The reason for doing this is =
that you wouldn=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 =
candles, so you might have to convert between the standards if you want =
to dial in the same brightness. Also look out for the dispersion pattern =
- a directional LED will only be bright directly in front, while a =
non-directional LED will spread out the brightness across a wider angle. =
The difference between 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 =
like to replace the old, original LEDs with new, modern ones. The =
originals were 5mm bulbs, =E2=80=9CSSL-22=E2=80=9D, each with a 180 ohm =
resistor hooked up to it.
>>=20
>>   I=E2=80=99d like to replace them with new, modern red LEDs of =
equivalent brightness and lower current use. There are so many different =
LED values 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
>=20