What resistor to use for a single LED

[Ralph Mowery]

In article <2f65767f-2974-4182-b25b-15888a6eca13@googlegroups.com>,
timothy42b@aol.com says...

ut I do
like the idea of putting two LEDs reversed polarity in there. Easy
enough to do...

Now I'll ask what is certain to be a dumb question. But maybe I'll learn something.

If we have two LEDs in parallel in reversed polarity, and we put 12 volts across them, aren't we exceeding both Vf and Vr? will they share the current, at some calculable ratio?

Sorry for my ignorance, that electrical circuits class was in the 80s.

LEDs do not really have a voltage rating as such. They are current
devices. You just have to limit the current to a safe level. That is
why there is almost always a resistor in series.

For many circuits you can forget the voltage of the led and just figuer
the resistance value dependin on the current your led will need for the
brightness you want. If very low voltage circuits, say under 12 volts,
substract the 1 to 3 volts the led is rated at from the supply voltage.

The LED will have a voltage across it of about 1 to 3 volts depending on
the type in the forward direction. That voltage is almost the same no
matter what the current is within reason.

 

[Tim R]

On Friday, April 13, 2018 at 1:06:30 PM UTC-4, Ralph Mowery wrote:

LEDs do not really have a voltage rating as such. They are current
devices. You just have to limit the current to a safe level.

Okay. 12 volt car battery, 1k resistor, the most current you can get is 12 mA, any LED should handle that. But that's not the fault condition I was wondering idly about.

The LED will have a voltage across it of about 1 to 3 volts depending on
the type in the forward direction. That voltage is almost the same no
matter what the current is within reason.

My first thought is if he used one LED and connected it backwards, he could think a circuit was dead that was really hot, and burn up his new red sports car. Or he'd always have to fuss with getting the right lead on. I didn't realize he had a ground clamp and probe, so I was probably just wrong on this one. I was thinking two probes.

My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.

 

On Friday, 13 April 2018 18:48:02 UTC+1, Tim R wrote:

On Friday, April 13, 2018 at 1:06:30 PM UTC-4, Ralph Mowery wrote:
LEDs do not really have a voltage rating as such. They are current
devices. You just have to limit the current to a safe level.

Okay. 12 volt car battery, 1k resistor, the most current you can get is 12 mA, any LED should handle that. But that's not the fault condition I was wondering idly about.



The LED will have a voltage across it of about 1 to 3 volts depending on
the type in the forward direction. That voltage is almost the same no
matter what the current is within reason.

My first thought is if he used one LED and connected it backwards, he could think a circuit was dead that was really hot, and burn up his new red sports car. Or he'd always have to fuss with getting the right lead on. I didn't realize he had a ground clamp and probe, so I was probably just wrong on this one. I was thinking two probes.

My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.

someone is not familiar with LEDs

 

[Ralph Mowery]

In article <6fc822f2-5067-4617-a903-107399f6912e@googlegroups.com>,
timothy42b@aol.com says...

My second thought was if he uses 2 LEDs in parallel, and they're both above 5 volts, then they're both conducting. How much current goes through the forward biased one? enough to give a bright light, or maybe dim enough he thinks the circuit is dead and he burns up his new red sports car.

You put them so one diodes anode is connected to the cathode of the
other. Only one will lite at a time on DC.

Best little handy dandy checker I ever had is one made by Fluke. I
think it is a T2. Has about 10 or 12 LEDs in it and 2 leads. You just
hook the leads to anything you want to check. If open, nothing hapens.
if low resistance it beeps and a led lights up. Anything over about 3
volts DC and it gives an indication of power and direction. Good to
about 200 or 250 volts dc. When used on ac, it starts somewhat higher
and goes to atleast 600 volts. The leds inbetween nothing and mix give
a rough indication of more common voltges such as 120,220 480 and 600
volts AC. It starts at 6, 12, 24 and I forget the others for DC.

 

[whit3rd]

On Thursday, April 12, 2018 at 8:19:07 AM UTC-7, Terry Schwartz wrote:

That can easily be addressed by using two LEDs in parallel, in opposite polarity.

At the risk of pointing out the obvious, an incandescent bulb doesn't care which end is positive. An LED on the other hand..................

One variant is to get a bicolor LED; these are usually red forward, green reverse biased.
It's better not to tease the R/G colorblind population, though, and
a white LED pair, behind + and - shaped windows, would be a better indicator.