Just wanted to check if these leds would be a good swap from green on a KBT Pure.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320999089508&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en.
or the diffused white if those would look better.
Also, is it possible to go from red to purple without changing the diodes on a QFR for the caps lock, win lock, and scroll lock?
I've never calculated any voltages for my LEDs, I've just done swaps. I haven't had issues so far. But check out this thread for some actual information and numbers.
In my own experience, I haven't had trouble with just putting LEDs into a board. Doesn't mean that I won't hit a problem in the future though. I think your LEDs will be fine and I think your QFR swap will be fine.
Instead of necro'ing that thread, maybe someone here can give me some info. My understanding is that the current through the LED determines its brightness. The LEDs I have are rated for up to 20mA (I think), but what is the minimum current that will still make it light up a reasonable amount.
Assuming the following is true for my leds
I = (5.05 - 3.2) / resistor
I currently have 1.13k resistors, so this would give me...
I = (5.05 - 3.2) / 1131 => I = 1.64mA
Is that even close to enough current to make the LEDs light up a reasonable amount?
well.
It depends on what you mean by "reasonable".
Taking an LED with similar characteristics (but with a datasheet)
http://www.mouser.com/catalog/specsheets/cree.C503B%20BAS%20BAN%20BCS%20BCN%20GAS%20GAN%20GCS%20GCN%201094%5B1%5D.pdfyou see that brightness is pretty close to linear with current.
This graph (for a different brand) shows the same thing, but I think the double log scale is easier to read:
http://www.mouser.com/ds/2/315/LNJ998CKCDA_E-255675.pdf (perhaps not a good comparison, they are listed as 400 mcd, which is pretty dim)
So 1.64 is 8% of 20, so you'll get 8% of the light out from it. If it's rated for 10,000 mcd (at 20 mA), then you'll get 800 mcd out (at 1.64 mA).
To compare, some LEDs are only "supposed" to put out about 400 mcd normally, so they'll certainly be visible. Weather or not that's a reasonable level is up to you. Looking at the numbers, I think it'll be fine, though you'll have more current with a lower Vf (red or amber) or lower resistance.
Another thing to keep in mind: if you are flowing 1.64 mA through each LED, you can only have a certain amount of LEDs on the board before there's too much current for the USB to support. When I was doing LED resistance calculations for sprit, I remember that 1-2 mA (certainly less than 5) was about all you could get before you ran into the USB power requirement.
Of course, Looking at it yourself is the only way to know for sure, but I think it'll be more than adequate.
Hope this helps.