Would a LED short itself?

  • Thread starter Thread starter Mr. Man-wai Chang
  • Start date Start date
Michael said:
But you don't need a 317 to make a current source. If we're talking
average LEDs, then one can use an FET (depending on the current,
sometimes a resistor isn't even needed) or a small signal bipolar with a
few resistors.

If I needed a higher current source, I'd go with the 317, but if I
needed a small current source, I'd be looking at other things, which I
could throw together with junk parts.

Michael
Click to expand...

I wrote that up in an original draft of my reply, but threw it
away (reply was getting too long). A zero bias JFET can make a
constant current source in the 10mA range, and I used such a
thing when I was younger (for some hobby project). But the JFETs
I'm used to, they don't span a very large range, whereas the LM317
covers 10mA to 1000mA at least (with a heatsink).

The constant current source can also help, in situations where
the input power is somewhat variable. You get a relatively
constant light output (with the light level perhaps
being affected by the LED getting too hot).

As a hobbyist now, you can find all sorts of info on building
small LED circuits. And some of those will include application
information, on why you'd want to do it a particular way and
so on.

Constant current sources can also be constructed with opamps,
so there are other families of components that could be considered.

The reason I like the LM317, is the thing has three legs, the math
is simple, and the results are acceptable. I have one set up right
now in my breadboard, for sorting LEDs by Vfb. In my last project,
I bought LEDs that were binned (sorted for similar characteristics),
and you can do a final sort on Vfb when building arrays, and get
a pretty good result (equal intensity over array, no hot spots).

Paul
 
Let me look up the price of LM317 in Hong Kong shops... :)
Click to expand...

I found LM317K, priced at HK$26; a 100-ohm resistor, HK$0.5!!!

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Mr. Man-wai Chang said:
I found LM317K, priced at HK$26; a 100-ohm resistor, HK$0.5!!!
Click to expand...

I get a 317 for about half your price.
And Digikey has TO-220 packaged parts (1.5A limit) for $0.60 USD,
which is one quarter of your price. The company I use in Canada
has them for $0.54 each (CDN). But then I'd have to pay shipping,
if I ordered them from there.

You should be able to get a better price than I ever could, because
you're closer to the source. There are lots of exotic components
you can get there, that I can't even get samples here. They're just
not available.

LM317 also come in smaller packages, with 0.1A upper limit
on current flow (and the package would make them harder
to cool with a heatsink). The packaging in that case, is plastic.
The best price would be about $0.12 each if you bought a reel of them
(too many), or $0.46 if you bought the devices one at a time.

I have a flashlight with an array of LEDs in it, and it has
one three-terminal device for current flow regulation (as near
as I can tell), and I doubt that is a 317. It's probably an
ordinary transistor of some sort, selected for the job. Anything
cheap and approximate, to do the job. Only problem with that
flashlight, is I can't use it, because it has quite a chemical
smell to it (repulsive).

And before you decide on a particular 100 ohm resistor, you'd
want to check the power dissipation expected of the thing, and
select an appropriate power handling level for it. If I got
a wire wound resistor with that resistance, I could end up paying
around $1 for it. Whereas a low-power resistor might be $0.05 to $0.10
or so.

Many components now, are in an inconvenient form, and make it
hard to use them in circuits. For example, I was shopping
for ceramic capacitors, and I found a real bargain. The only
problem with them, is they are surface mount capacitors, with
no leads on them. And I didn't have a PCB to mount them on
(I don't make my own PCBs any more). I ended up soldering wires
onto the sides of them, using an 80W soldering iron. Taking a
big chance on cooking the things in the process of making
the electrical connections. They survived somehow. At least
some of the forms of LM317 are such, that you can make
connections to them a bit easier. Some of the smaller packages,
you might need a magnifying glass to inspect your solder
connections.

Paul
 
problem with them, is they are surface mount capacitors, with
no leads on them. And I didn't have a PCB to mount them on
(I don't make my own PCBs any more). I ended up soldering wires
onto the sides of them, using an 80W soldering iron. Taking a
big chance on cooking the things in the process of making
the electrical connections. They survived somehow. At least
Click to expand...

If you built plastic models, you would like surface-mount LEDs.... :)

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