Non-destructive way to determine LED operating current?

[Bart Z. Lederman]

This is a variation on the "testing LED's" subject.

I bought some Blue LEDs to experiment with from
American Science & Surplus. This is a fairly neat
company that gets all sorts of surplus items in
(including some interesting lighting items from
time to time).

The problem is, they don't always know the specifications
for the items they have.

I've done a quick test on the units I got, and they appear
to light (but not very bright) at around 40 to 60 mA. I
think they were designed to go higher: my question is,
is there a non-destructive way to determine the safe operating
current for an LED?

Thanks.

FYI, they're at http://updates.sciplus.com/ , and I have
no financial interests with the company: just a satisfied
customer.

 

[Bart Z. Lederman]

Basically, there is no real information that came with
the units. They're in standard 5mm cylindrical with
rounded top packages, clear (as opposed to frosted) blue
plastic, with no visible markings (though I'll try to
remember to look again this evening).

I agree that the current ratings on some of the other LEDs
sold by this outlet seem high, but they probably got them
from the supplier.

I suppose if I want long life I should stick to 20mA max.
I was just hoping someone with LED experience would know of
a more sophisticated test.

Bart.

 

[Don Klipstein]

Basically, there is no real information that came with
the units. They're in standard 5mm cylindrical with
rounded top packages, clear (as opposed to frosted) blue
plastic, with no visible markings (though I'll try to
remember to look again this evening).

I agree that the current ratings on some of the other LEDs
sold by this outlet seem high, but they probably got them
from the supplier.

I suppose if I want long life I should stick to 20mA max.
I was just hoping someone with LED experience would know of
a more sophisticated test.

Most LEDs 3, 5, 8, or 10 mm diameter with wire leads have 20 mA
characterizing current and 30 mA rated maximum current. Although I
believe life expectancy projection is at 20 mA (and in 25 C ambient
temperature) like other performance data, and not at 30 mA.
It is somewhat common to get away with a little more.

- Don Klipstein ([email protected])

 

[Bart Z. Lederman]

Blue plastic bit sounds a bit of a giveaway, not liable to be ultra
bright which are usually clear resin package, but could be old but
interesting Silicon Carbide Blue LEDs.

This weekend I can try measuring Vf at 20mA and see if that helps
at all.

I bought these because they were inexpensive and I wanted to
experiment: but I actually have a potential application for them
where they don't have to be very bright at all, so I think I'll
get my money's worth out of them.

Thanks for the comments.

Bart.

 

[Clive Mitchell]

Theres plenty of LED vendors on Ebay for superbright LEDs just take
some of the candela ratings are as another Chinese vendor put it `lying
like dogs in the sun` ;-)

So what are you saying here. Are you suggesting that my 50,000mcd
(50cd) at 20mA 5mm whites aren't actually putting out 50cd?

How disappointing. Do you think they'll give me a refund?

 

[Clive Mitchell]

ooops, Just correct that ,of course SiC require a higher Vf and the
best effect for lighting up at really low currents is with Indium
Gallium Nitride Green Leds.

Yeah, I was showing someone how bright a 5mm green was at 75uA (that's
MICRO amps!) and was astonished to see the beam shining clearly on their
face.