LED'S what are they and how are they used?

P

playmofire

Registered
23 Oct 2010
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The triangle with the "Bar" is the universal electrical symbol for a Diode... Light Emitting Diode.

The 2 "free" wires go to power, does not matter which is plus or minus... or plus and earth..

electrical current flows like water, has to come from somewhere and go to somewhere, otherwise known as a complete circuit

Here's one with power drawn in...

Yes, it does help, thank you. Symbols by themselves are fine so long as people know what the symbols mean. But no one has yet explained how achieve inverse polarity.
 
ntpntpntp

ntpntpntp

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24 Oct 2009
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Yes, it does help, thank you. Symbols by themselves are fine so long as people know what the symbols mean. But no one has yet explained how achieve inverse polarity.
In the diagram, the two LEDs are arranged in parallel but the opposite way round to each other, that's the "inverse parallel" bit. So, when fed with AC one LED lights for half the cycle then they swap. All the while one LED is conducting, the voltage across it is only 2-3 volts (the rest is dropped by the resistor). That same couple of volts is all that is across the other LED, and that is a safe reverse voltage that will not blow the other LED.
 
P

playmofire

Registered
23 Oct 2010
6,640
118
North Yorks
In the diagram, the two LEDs are arranged in parallel but the opposite way round to each other, that's the "inverse parallel" bit. So, when fed with AC one LED lights for half the cycle then they swap. All the while one LED is conducting, the voltage across it is only 2-3 volts (the rest is dropped by the resistor). That same couple of volts is all that is across the other LED, and that is a safe reverse voltage that will not blow the other LED.
And when fed with DC?
 
korm kormsen

korm kormsen

Registered
24 Oct 2009
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The triangle with the "Bar" is the universal electrical symbol for a Diode...
question:
a (not light emitting) diode is a black cylinder with a white or silver belt around one end.

does the end of the triangle symbol, with the point and bar symbolize the "belted" end of the diode?
 
S

Sarah Winfield

Registered
20 Jun 2016
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I have my 9V battery and my LED by my side.

The battery has a + and - sign presumably indicating positive and negative.

The LED has 2 wires coming from it. One plain and the other with a small resistor. Which wire do I connect to the + please?

Thanks.

SW

You have probably indicated which in one of your diagrams but I would like to know from my set-up please.
 
P

playmofire

Registered
23 Oct 2010
6,640
118
North Yorks
I have my 9V battery and my LED by my side.

The battery has a + and - sign presumably indicating positive and negative.

The LED has 2 wires coming from it. One plain and the other with a small resistor. Which wire do I connect to the + please?

Thanks.

SW

You have probably indicated which in one of your diagrams but I would like to know from my set-up please.
Looking at the diagram and using what little knowledge I have on this, Sarah, the wire with the resistor is attached to the + side. To continue the water analogy, the battery is the water tank and the current flows from ths into the LED, the resistor acts as a valve to limit the flow of the current to the LED.
 
M

Michael

Registered
26 Jan 2010
78
15
With a 9 volt battery and a resistor in series with the LED there is virtually no chance of causing any damage.

Try it both ways round. One way it will light, the other way it won't. I been testing LEDs like this for over 50 years:)

Michael
 
Neil Robinson

Neil Robinson

Registered
24 Oct 2009
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N W Leicestershire
Try it both ways round. One way it will light, the other way it won't. I been testing LEDs like this for over 50 years:)

Michael
Agreed. There is no rule regarding which side of an led the resistor should be fitted.
There is a convention regarding physically identifying led polarity but some cheap leds (including one I fitted yesterday) get it wrong!

105609_018c744c39cf09d7c9f15f249f58f4c9.jpg
 
Last edited:
Rhinochugger

Rhinochugger

Retired Oik
27 Oct 2009
25,249
548
North West Norfolk
Slightly off-piste, but another cheap source of LEDs (which I was taught by a Maplins employee when they were still in business) is to buy a cheap LED torch and dismantle it - usually cheaper than buying the same number of LEDs.

There is a slight down side :eek: the LEDs are obviously on a PC board with virtually no tails - however, a fair few of them now grace my loco headlights, and I haven't run out yet :cool::cool::cool:
 
PhilP

PhilP

G Scale, 7/8th's, Electronics
5 Jun 2013
23,469
488
Tamworth, Staffs.
The LED has 2 wires coming from it. One plain and the other with a small resistor. Which wire do I connect to the + please?
Even from China, this is a little poor.. - The wires should be different colours, or some obvious way, or telling which lead is which..

Look for the 'flat' on the diode, for a start..

Hopefully, once you know whether the lead with the resistor is positive, or negative, they will all have been wired the same. :think::nod:
<edit> Once you know, write it on a bit of paper, and stick it in the bag with the LED's, for next time! :)
 
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Rhinochugger

Rhinochugger

Retired Oik
27 Oct 2009
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<edit> Once you know, write it on a bit of paper, and stick it in the bag with the LED's, for next time! :)
Aaaaaaaaaagh - how can anyone be that organised ? :emo::emo::emo::emo::emo::emo:
 
Rhinochugger

Rhinochugger

Retired Oik
27 Oct 2009
25,249
548
North West Norfolk
Sarah is a lady..

They HAVE to be organised to know where we men have put things! :rolleyes::nod:
Men to the left, as Women are always right :mask::mask::mask::mask::mask::mask:
 
G-force1

G-force1

Prevarication Rules!
4 Aug 2015
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North Middle Earth
Even from China, this is a little poor.. - The wires should be different colours, or some obvious way, or telling which lead is which..

Look for the 'flat' on the diode, for a start..
The ones I bought from China last week don't have colours or a flat on them, just one wire longer than the other. They were cheap though! :)
 
PhilP

PhilP

G Scale, 7/8th's, Electronics
5 Jun 2013
23,469
488
Tamworth, Staffs.
Now that's one up-manship :nerd::nerd::nerd::nerd:
But it does show the 'conventional' arrangement of the longer lead, and the correct way round for the LED symbol. - Lower-right..
Bought it back in 2012, and it still has the same PP3 I fitted then.

The two 10mA outputs are really useful, when you want to match two, or more, LED's for output..
You can also see the effect of under-running your LED's for current. - Really useful for getting that jaundiced pale-yellow glow.. :)
 
Rhinochugger

Rhinochugger

Retired Oik
27 Oct 2009
25,249
548
North West Norfolk
But it does show the 'conventional' arrangement of the longer lead, and the correct way round for the LED symbol. - Lower-right..
That little bit is worth more than the ability to test them - I don't think I've seen that all in one place before (carefully goes there because I haven't looked at Greg;'s website :mask::mask: )