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LEDs

R

Roger Dewhurst

Jan 1, 1970
0
I have been given a large number of LEDs soldered into a board. Since the
pins have been cut t the same length what is the best way of determining
their polarity when removed from the board?

R
 
J

John Popelish

Jan 1, 1970
0
Roger said:
I have been given a large number of LEDs soldered into a board. Since the
pins have been cut t the same length what is the best way of determining
their polarity when removed from the board?

If these are the cylindrical body with dome end type, they
probably have a small flat at the base of the cylinder
(usually indicating cathode (more negative lead). But I
have seen exceptions. But which ever version they are, they
should be consistent. You can also look through the epoxy
and see which lead is the reflector base the die is mounted
on. That is also usually the cathode. The anode lead
generally has a wire bond to the top of the die.
 
R

Roger Dewhurst

Jan 1, 1970
0
John Popelish said:
If these are the cylindrical body with dome end type, they
probably have a small flat at the base of the cylinder
(usually indicating cathode (more negative lead). But I
have seen exceptions. But which ever version they are, they
should be consistent. You can also look through the epoxy
and see which lead is the reflector base the die is mounted
on. That is also usually the cathode. The anode lead
generally has a wire bond to the top of the die.

Thank you. I needed a magnifying lens to see the flat! Looking through the
side I see two small triangles with the hypotenuses facing each other. The
upper one is associated with the flat on the body.

Roger
 
J

John Popelish

Jan 1, 1970
0
Roger said:
Thank you. I needed a magnifying lens to see the flat! Looking through the
side I see two small triangles with the hypotenuses facing each other. The
upper one is associated with the flat on the body.

I suggest you pull one LED and find out by test, whether the
flat is the cathode or the anode.
 
M

Michael Black

Jan 1, 1970
0
Roger Dewhurst" ([email protected]) said:
I have been given a large number of LEDs soldered into a board. Since the
pins have been cut t the same length what is the best way of determining
their polarity when removed from the board?
For decades, I've always had a 9v battery lying around with about a 1K
resistor soldered to one terminal, and sometimes a piece of wire soldered
to the other. Since you know the polarity of the battery, when the LED
lights up when you connect it, you know which terminal of the LED is which.
If it doesn't light up, you reverse the LED. If it doesn't light up then,
the LED is a dud (or your battery has finally died.

This not only gives you polarity when it may not be clear, but if the flat
of the package isn't visible (like when you've installed it with one of
those LED holders) or whatever reasons, you can easily check before soldering.
I got into the habit of checking LEDs this way before soldering because when
I first started buying them, LEDs were relatively new and it was easy to
get some that were not particularly bright, or for that matter I think
some that were cheap because they had the terminals reversed.

Michael
 
B

Ban

Jan 1, 1970
0
Michael said:
For decades, I've always had a 9v battery lying around with about a 1K
resistor soldered to one terminal, and sometimes a piece of wire
soldered
to the other. Since you know the polarity of the battery, when the
LED lights up when you connect it, you know which terminal of the LED
is which. If it doesn't light up, you reverse the LED. If it doesn't
light up then, the LED is a dud (or your battery has finally died.

This not only gives you polarity when it may not be clear, but if the
flat
of the package isn't visible (like when you've installed it with one
of
those LED holders) or whatever reasons, you can easily check before
soldering. I got into the habit of checking LEDs this way before
soldering because when I first started buying them, LEDs were
relatively new and it was easy to
get some that were not particularly bright, or for that matter I think
some that were cheap because they had the terminals reversed.
Most LEDs have a max. reverse voltage rating of less than 7V and might be
damaged or degraded by the full 9V battery, so a 4.7V Zener across the O/P
should limit the reverse voltage to safe values. But beware of the
continuous current draw.
 
J

Jonathan Kirwan

Jan 1, 1970
0
<snip>
Most LEDs have a max. reverse voltage rating of less than 7V and might be
damaged or degraded by the full 9V battery, so a 4.7V Zener across the O/P
should limit the reverse voltage to safe values. But beware of the
continuous current draw.

6V screw-terminal carbon battery, then?

As a side-bar, I've noted this site:
http://www.m3electronix.com/sa.html

Which appears to offer a nicely featured tester. I haven't purchased
it (and I am definitely not affiliated in any way), but I was recently
considering the idea of buying one.

Jon
 
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