IIci power LED - what voltage/amp rating?

[desertrout]

I want to replace the power LED on my IIci (it hasn't worked since I got the machine, tested bad, I have another one in there I grabbed from a parts bin but it's crazy bright). I'm shopping on Digikey but I don't know if I should be replacing it with a 5V LED or something else? Can we tell from the schematic (it's there under R111)?

[ymk]

Any LED that can tolerate 16mA will work fine there.  If you want to reduce brightness, you can replace R111 with a higher value.

Edited December 30, 2020 by ymk

[desertrout]

Brilliant @ymk - so it's more about the current rating than the voltage rating then... If you don't mind entertaining a n00b, how did you come to 16mA?

[cheesestraws]

15 hours ago, desertrout said:

I have another one in there I grabbed from a parts bin but it's crazy bright

 

As a footnote to the discussion above, modern LEDs give out a lot more light per milliamp than the older ones used in old Macs.  You will almost certainly need an extra series resistor to go inline if you want a modern LED to be about the right brightness (or, as @ymk suggested, the existing resistor increasing, but with my rather limited soldering skills I prefer to add a second one).  When I did this last I just tried a couple of resistors from the resistor box until I found one that looked about right.

[aeberbach]

Ohm’s law tells you current = voltage / resistance, so current = 5 / 180 = about 28mA. Normal for an old LED. Changing R111 to be a 470 or even 680 would probably be a good usable value for a modern power LED, the last thing you want is maximum brightness!

 

You can avoid desoldering anything on the main board by just adding a resistor in series on one leg of the LED. I don’t know how a IIci connects the LED but if by wire that is easiest and you can make it really neat with heat shrink.
 

(There’s a IIci I want but the seller is asking $399. Looks nice but they won’t say anything about working now, only that it worked when placed in storage “25 years ago”. Certainly looks good but I have my doubts. Still sitting on AU eBay months after it was first listed.)

[ymk]

11 hours ago, desertrout said:

If you don't mind entertaining a n00b, how did you come to 16mA?

 

The current through the LED and resistor is the same and the sum of the voltage across each add up to 5V.  Typical forward voltage through red and green LEDs is 2.1V.  This leaves 2.9V across the resistor.  2.9 / 180 = 0.0161 A.  Voltage across the LED will increase slightly with current, but this is negligible.  For blue/white LEDs with a forward voltage around 3.6V, the same resistor would pass less current.

Edited December 30, 2020 by ymk

[desertrout]

Thanks @aeberbach and @ymk. The LED is through-hole, 90deg mounted, like this one: https://www.digikey.ca/en/products/detail/5505607F/350-1660-ND/809058?itemSeq=349366796

 

I'm handy with a soldering iron, so I have no issues bumping up the resistance either by replacing R111 or adding another inline. I guess we'll see what this one looks like.

 

(I got this IIci for $75 and the seller threw in a CD 300e drive. The board and hi-res card were a bit of a corroded mess, but I'm happy with the repairs and it's been working flawlessly for a few months now. This LED is just the last thing to button it up)

Edited December 30, 2020 by desertrout

[desertrout]

In case anyone is ever wondering about this (like I once was), the replacement part I linked to above is a 1:1 fit, with the perfect shade of green and level of brightness with no need for adjustment. Chef's kiss.

[cheesestraws]

On 1/16/2021 at 4:33 PM, desertrout said:

In case anyone is ever wondering about this (like I once was), the replacement part I linked to above is a 1:1 fit, with the perfect shade of green and level of brightness with no need for adjustment. Chef's kiss.

 

Thankyou for following up on this, I'd like to get some LEDs the right colour for these power indicators