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Electronics question about LEDs

Started by theZapper Jan 26, 2006 at 10:41 AM 11 replies 1.1k views
Original Post
theZapper
theZapper
Hi I've just taken my Digitec RP5 guitar effects pedal apart to swap out a cracked LED. However, upon inspection of the LEDs in the unit I noticed that none of them have the flat side to mark the -ve (cathode?) leg. Very strange I thought, so I decided to power up the unit and poke a new LED onto the contacts of one of the lit LEDs. I did this and tried touching the contacts with the new LED in both directions, but it never lit up. Now being even more confused I tried my multimeter (why I didn't do this first i don't know). Anyway, the multimeter was reading less than a volt from the power terminals, and nothing across the lit LED contacts. So I looked at the power adapter and it says AC to AC converter! So I switch the multimeter to AC and if reads 10 volts across the power connectors and about 1.3 across the LEDs. So my question is, are these LEDs special AC ones? (that would explain the lack of a cathode), and what do I have to do to fix the broken one? Cheers
---When I'm in command, every mission's a suicide mission!
theZapper
theZapper
I am 100% certain they are 3mm LEDs, they look like LEDs, are solid, I know this because of the cracked/chiped one, and they have the same internal spade shaped innards. Also, there is no filament like you'd get with a bulb.
---When I'm in command, every mission's a suicide mission!
Monder
Monder
Could you post a few pictures of the busted LED, the circuit it came from, one of the working LEDs etc?
Mastaba
Mastaba
There are tri-color LED's that accept AC and DC in either direction. Typically, they are green when current flows one way, red in the other, and so appear yellow when an AC current is supplied.
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theZapper
theZapper
Are these ok?

Zoomed Out

Zoomed In

edit: Put the pics in the post
---When I'm in command, every mission's a suicide mission!
theZapper
theZapper
Yeah I know, and they only have two legs so are not Bi or tri coloured, they look like bog-standard 3mm normal brightness LEDs. So why are they running on AC?!

You can see from D15 too that they have no flat side.
---When I'm in command, every mission's a suicide mission!
Mastaba
Mastaba
It is perfectly fine to drive an LED with AC if the electrical specifications of the device are not exceeded. The only other requirement to make that practical is the AC frequency must be high enough to fool the typical person's persistence of vision.

Also, not all LED's have a flat side to differentiate the cathode from the anode, some simply use a longer lead on the anode.
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theZapper
theZapper
Thanks Mastaba, I did think that they might be flashing at 60hz and I simply couldn't see it.
Also, I just remeasured the voltage across one of the lit LEDs and it was registering around 1 volt, so would that be to low to light one of my LEDs? or should it just be dim? I think they have a minimum operating voltage of about 3v, i.e. they won't light if I connect them to a single AA battery.
---When I'm in command, every mission's a suicide mission!
haegarr
haegarr
3 volts will make a LED normally being a flash light :)

LEDs are usually supplied with 1.2 to 1.4 volts depending on the color of the LED. Some kinds of LEDs may differ from that, but the "good old" 3mm round LEDs types are in that range.


EDIT: Okay, to be precise: A LED has a voltage drop of 1.x volts in direction of passing the current, if driven by its nominal current. The intensity depends on the current, which is chosen to around 10 to 20 mA. All these values depend on the concrete type of the LED, of course.
Extrarius
Extrarius
I've seen LEDs similar to the 'multicolor' LEDs that were actually just 2 identical(same color, ratings, etc) LEDs packaged together with the diodes 'facing' opposite ways, such that they would work reguardless of which way you hooked them up. I suppose such a thing might be used where the only power supply available is AC, as it should appear brighter than a normal LED hooked up to the same power since it takes advantage of the full cycle instead of only half.
"Walk not the trodden path, for it has borne it's burden." -John, Flying Monk

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