Wednesday, March 13, 2019

LED minimum forward voltage about 2.75VDC

A while back I picked your excellent brains about LEDs.  I was assuming that as long as the drive motor had 1.7V, that was enough.  No.  It isn't keeping up with the sky at 2.62V, so I want an LED with a forward voltage of 2.75-2.9V.

Also, to simplify testing, what is the name of the connector that you solder wires into one end, and then the male end of it plugs into the female end of the connector on the other side.  I am sure there  is a specific name for it.

This amusingly named website indicates that the best that I can do is the InGaN green (3-3.5V) or the blue LED (2.8-3.5V).  The green one is too high; the blue one is pretty close.  Those are ranges; I suspect that the manufacturer data sheets will give me more specific ranges.  MSC Direct has one that is $6.95.  But no evidence of who the LED manufacturer is.

Looking for manufacturer datasheets, I find plenty whose typical forward voltage is 3.9V.  And there is the worrisomely named Chinese maker whose URL is sparkfun,com.

Mouser has a number of 2.8V forward voltage LEDs.  Price per unit is trivial; but I am sure ordering one would be very expensive.   Actually ordering five of the AEMD-CB3L-ST002 blue LEDs and five of the 150 ohm resistors would only be $14.69.  (Need five of each, in case I let the smoke out of one or two of them.)   Let me make sure that I have this right: 3V/20 mA=150 ohms.


  1. I can't get to their website right now, but I'm pretty sure is based in the US. Of course most of their components are sourced in the PRC, but that's not their fault. They're a goto company for me, along with Adafruit which is more systems oriented, and of course Mouser.

  2. I can't tell what you mean about the connector.

    Regarding the forward voltage, take a look at the graph in the data sheet of that part - when I Googled the part number, the link downloaded the PDF data sheet. Page seven, upper right graph, shows forward current vs. forward voltage, so across the range where forward current is measurable, the LED will glow with brightness increasing with the current. Note that either green or blue of that series has the same typical forward voltage. Also note that their definition of forward voltage is: the voltage when the forward current is 20mA. The LED will glow with a lot less than 20mA. The graph suggests it will start glowing around 2.25 - 2.4 volts. That will vary some (up to .5V) with temperature (bottom right graph).

  3. Connector: one of those white plastic plugs that do not require you solder wires together.

  4. I still can't answer your connector question. There are a myriad of connectors that match the description - literally hundreds of thousands of varieties if you look in DigiKey. You might be talking about a socket, where you plug the LED into it for testing, and have the socket soldered to something that provides power - sockets fall under "connectors" in many parts listings. I just don't know.

    BTW... best source for parts is Mouser or, for a crazy huge selection, DigiKey. Nice maker-space companies that have interesting stuff are those mentioned by Karhu above - SparkFun and Adafruit.

    I use all of those. DigiKey may require a commercial account, I don't remember, but I just used my LLC name. Mouser sort of straddles the line between hobby and professional - it has a lot more selection than the hobby sites like SparkFun and AdaFruit. You can think of SparkFun and AdaFruit collections as curated for hobbyists.

  5. "SparkFun Electronics (sometimes known by its abbreviation, SFE) is an electronics retailer in Niwot, Colorado, United States"

    They sell a lot of Chinese stuff, of course, since there are no alternatives for that market.

    But they're not a Chinese company.

    (I'm not sure if I've ever bought from them, but their name is real familiar.)

    I can't find the LED post you refer to - what problem are you trying to solve here? Might be a simpler alternative you're "too close to the problem" to see?

  6. Personally, I'm not a huge fan of using the LED as a regulator. I'm assuming you need a fairly accurate voltage to keep the motor turning at the right rate, and LEDs are notoriously temperature sensitive and have a *very* high manufacturing variability. And since you're talking ~ $15, there are much better solutions for that kind of price.

    What you want is a real voltage regulator. I'm not sure the exact application, but I'm assuming you've got a fairly low current draw on something you want to power from a battery given that you want this for your telescope. The easiest way to do this fairly efficiently is a buck converter. There are also boost converters and whatnot, but let's try and keep this simple since boost converters aren't as battery friendly.

    A buck converter requires that you have a higher source voltage than the one you're regulating down to. There are innumerable buck converters out there and you can buy a converter, slap on some external elements and create almost any voltage you want. But why make life hard? If you don't mind 3.3V output, do something like a packaged R-783.3-0.5 module for $7 at DigiKey that will put out 3.3V with pretty high efficiency from a supply of 4.5V or more. A 4-stack of D cells will put out about 6V and have about 15 amp-hours of capacity and run you about $3 for the holder. That module will consume about 5 mA of current, but you'll get 80-90% efficiency of battery power for the power you deliver to the motor. And it doesn't get any easier than that module with 3 wires: voltage in, voltage out, and ground. Take a 4 D-cell tray, wire in that module, and you can supply 3.3V for quite some time.

    If for some reason you're really set on putting out 2.75V you can do it. You'll just need a controller (Maxim, Linear, Rohm, TI and many others make them), and a circuit with a couple of resistors, a couple more capacitors, and an inductor to make it work; and there's a decent chance the custom solution might be $1-2 cheaper. But honestly, if you can take 3.3V it's much easier to use a pre-built regulator module where some poor engineer did all the math and wrapped those elements around a controller for you.

    You can also find a ton of regulator modules for standard voltages on Amazon, if you care. But when you go there you'll have to understand the tradeoffs between buck, boost, buck/boost, linear and LDO regulators.

  7. Perhaps you mean MOLEX connectors? There are other alternatives, Anderson Power Poles being one.

    Or you can just use spade terminal connectors.... a search on any of these will be illustrative.

  8. PhaseMargin: not to regulate the voltage, but tell me if I am above 2.75V.