C
Clive Mitchell
- Jan 1, 1970
- 0
I've just noticed that the large LED annunciator panels recently
installed at Glasgow's Central Station are experiencing significant
failure of their white LEDs.
Fortunately the bulk of the information is on time-served yellow matrix
panels, but above each is a small panel populated with white LEDs that
says something like "Calling at". It's these small panels that look
like they may be made from discreet LEDs that are starting to look a bit
gappy.
There's also a large white LED digital clock on the same panel which
seems to be fully operational, but suffers badly from colour mismatch
between adjacent modules.
This early LED failure is very typical of the cheap bulk LEDs coming
from certain suppliers in China. The LEDs work initially, but seem to
have problems with the Gallium Nitride film, since they soon start
flickering occasionally and then get progressively dimmer until they go
out. I'm guessing that this is tiny holes in the film allowing the
direct passage of current briefly until the affected material is "blown
clear" before the state gradually occurs again.
It's sometimes possible to identify the prospective dodgy ones by
testing them on a 60uA supply (A PP3 and a 100K resistor). Good LEDs
will glow visibly while duffers will not light at all.
installed at Glasgow's Central Station are experiencing significant
failure of their white LEDs.
Fortunately the bulk of the information is on time-served yellow matrix
panels, but above each is a small panel populated with white LEDs that
says something like "Calling at". It's these small panels that look
like they may be made from discreet LEDs that are starting to look a bit
gappy.
There's also a large white LED digital clock on the same panel which
seems to be fully operational, but suffers badly from colour mismatch
between adjacent modules.
This early LED failure is very typical of the cheap bulk LEDs coming
from certain suppliers in China. The LEDs work initially, but seem to
have problems with the Gallium Nitride film, since they soon start
flickering occasionally and then get progressively dimmer until they go
out. I'm guessing that this is tiny holes in the film allowing the
direct passage of current briefly until the affected material is "blown
clear" before the state gradually occurs again.
It's sometimes possible to identify the prospective dodgy ones by
testing them on a 60uA supply (A PP3 and a 100K resistor). Good LEDs
will glow visibly while duffers will not light at all.
