Diode search

[dsavitsk]

Is anyone aware of a diode that will drop ~1.7V and that can pass 60mA (so probably no LEDs)?

[sawreyrw]

What do you really want? In other words, can you just put a silicon diode in series with a 17 ohm resistor? In other words, could you use two diodes in series?

[BWRX]

Why not just use 2 or 3 diodes in series? Or if you need a power diode that can pass a lot of current you can use a transistor and a diode. Connect the diode between the collector and base of an npn bjt and it will drop 0.7V and provide current to the base. The base emitter junction will drop another 0.6-0.7V. Most of the current should be handled by the transistor.

[Geek]

Parallel 4 LED's maybe?

[mrshow4u]

....There's a lot of high current LED's out there nowadays. I used to work at hp/Agilent and they (used) to have the market cornered on high power LED's. They sold off the LED business to Philips. It's called LumiLEDS Here's a link to some high power BRIGHT stuff. I didn't check Vf, but the power's there. <http://www.lumileds.com/pdfs/DS05.pdf>

[wrenchone]

Who needs a diode? Use a VBE multiplier made from an appropriate transistor.

[dsavitsk]

Originally posted by sawreyrw
What do you really want?

What I really want is to bias a constant current source up to about 6V and be able to tap the 2/3 point.

Originally posted by BWRX
Why not just use 2 or 3 diodes in series?

Originally posted by Geek
Parallel 4 LED's maybe?

The only problem with both of these is space, but I may just have to jam them in.

Thanks

[janneman]

Originally posted by Geek
Parallel 4 LED's maybe?

... err... parallelling LEDs (or diodes) does not change the conduction voltage (but it does increase the current handling capability)..

Jan Didden

[Geek]

Hi Jan,

Originally posted by janneman



... err... parallelling LEDs (or diodes) does not change the conduction voltage (but it does increase the current handling capability)..

Jan Didden

Reread his original post, he needs 1.7V @ 60mA. Paralleling 4 red LED's would be perfect if he had the space

[mrshow4u]

Geek responded to janneman:

Reread his original post, he needs 1.7V @ 60mA. Paralleling 4 red LED's would be perfect if he had the space

I think janneman is right. While 4 LED's/Diodes in parallel will establish a voltage drop, it would not be good practice to simply parallel the LED's. For a diodes V/I curve, the current rises steeply for voltages greater than Vf. Yes, the rise is not true vertical, the Vf does get larger as more current goes through the junction. One of the LED's in the set is likely to current hog. So, one LED may sink 25mA, while some of the others sink 5mA, etc.
In theory and maybe in practice, the Vf is more closely matched. I'd recommend using ballast/balancing resistors to equalise the current sharing.

This is probably a pointless statement for this issue, as there isn't enough room to do the Quad-LED approach, but I had had to chime about the 4 parallel LED's not being a perfect solution.