Light emitting whats?

Thread Starter

Cackles

Joined Feb 7, 2008
7
So ... I don't even know how to word this because I get a feeling the word 'noob' might be used so I'll just explain

I bought 1.8V LED colored switches. I haven't built anything in a while so I daisy chained 5 together with a 9V supply and none came on, tested and the supply end was 9.15v with 1.85v running across each switch's LED, not my math that's just what it said both of the meters.

I used an Ethos and a cheap generic Maplin multimeter on them separately and I get nothing from diode mode.

I dug out a step down I have and they came on in parallel at 1.8V. Only I haven't used my step down in 10 years and then I remembered I don't own a step down, I own a buck converter and there's no decimal point between 1 and 8. So they ran at 18V for several minutes, about 5.

I then went and connected one to a pot and my multimeter. It came on at 1.7/1.8V, but never really kicked in until about 2.2V. But it turns out my pots are junk it would seem so now I have to either put an Arduino on the case when I get time or wait for all my new gear that's 3 weeks in the post.

Apart from 1.8V for the lamp and the switches ratings there are no other clues. I then asked the seller for the range but all they could tell me was '1.8V'.

I don't know what I bought. I thought it would have been all clear LED's inside, has anyone got any ideas? Maybe I'm missing something, my last post on here was 12 years ago and that's when I stopped. My stuff is all ATMel PIC chips, home made JTAGs and several still sealed Arduinos and an ATMega 1280.

If this is something obvious I forgot, don't laugh too much :D Though I am sort of hoping it's because I am behind the times at the moment.

Edit: I forgot, my old posts prior to some point were deleted, so yea, 2 posts before I fell off the Earth.
 

Boggart

Joined Jan 31, 2022
67
So ... I don't even know how to word this because I get a feeling the word 'noob' might be used so I'll just explain

I bought 1.8V LED colored switches. I haven't built anything in a while so I daisy chained 5 together with a 9V supply and none came on, tested and the supply end was 9.15v with 1.85v running across each switch's LED, not my math that's just what it said both of the meters.

I used an Ethos and a cheap generic Maplin multimeter on them separately and I get nothing from diode mode.

I dug out a step down I have and they came on in parallel at 1.8V. Only I haven't used my step down in 10 years and then I remembered I don't own a step down, I own a buck converter and there's no decimal point between 1 and 8. So they ran at 18V for several minutes, about 5.

I then went and connected one to a pot and my multimeter. It came on at 1.7/1.8V, but never really kicked in until about 2.2V. But it turns out my pots are junk it would seem so now I have to either put an Arduino on the case when I get time or wait for all my new gear that's 3 weeks in the post.

Apart from 1.8V for the lamp and the switches ratings there are no other clues. I then asked the seller for the range but all they could tell me was '1.8V'.

I don't know what I bought. I thought it would have been all clear LED's inside, has anyone got any ideas? Maybe I'm missing something, my last post on here was 12 years ago and that's when I stopped. My stuff is all ATMel PIC chips, home made JTAGs and several still sealed Arduinos and an ATMega 1280.

If this is something obvious I forgot, don't laugh too much :D Though I am sort of hoping it's because I am behind the times at the moment.

Edit: I forgot, my old posts prior to some point were deleted, so yea, 2 posts before I fell off the Earth.
They are just simple LEDs inside the switch, they won't have any internal current limiting, so the 1.8V is the nominal forward voltage at rated current, it will vary a bit. You drive them just like a normal small LED of the same colour.

LEDs are current driven, you limit their current and they set their own forward voltage, for small LEDs like this, a resistor is simplest. There's a good LED resistor calculator at ledcalc dot com
 

crutschow

Joined Mar 14, 2008
31,128
LEDs are current operated devices (look like a forward-biased diode) which drop about 2V or so when operating, depending upon their color.
They thus need to have a current limiting resistor in series with them they don't get zapped.
Small LEDs usually have a maximum operating current of 20mA, so a good starting design current would be about 10mA.
The resistor required would be the supply voltage minus the LED voltage divided by 10mA.
For example with a 9V supply and one LED, the resistor value would be (9V-1.8V) / 10mA = 720Ω

When you connect 5 together, the total voltage drop is 5 x 1.8V = 9V so not much is left to power the LEDs with a 9V battery.
 

BobTPH

Joined Jun 5, 2013
6,080
If they did not light at all with 5 in series and 9V, you most likely had the polarity backwards on one or more of the LEDs. If any one was backwards, none would light.

Bob
 

Thread Starter

Cackles

Joined Feb 7, 2008
7
If they did not light at all with 5 in series and 9V, you most likely had the polarity backwards on one or more of the LEDs. If any one was backwards, none would light.

Bob
That's a good point, they might have got them wrong way around in the factory. I didn't think of that and now you reminded me there is a warning for such things on some listings.

The other three things I was thinking is:

They aren't really LED's, tempted to break one open, the tabs might survive it.

Another is they were made for my order, they advertised 1.8V and messaged to see if 3V was OK and then messaged back to say 'forget the last message' so they might be mixed value or 3V.

Lastly the ranges on a lot of products are 1.8V LED's = 1.8-6V, 3V = 3-6, 5V = 3-6, 6V = 6-12, 12 = 6-12, 24 - 12-24.

There's no specific 1.8V or 3.2V or anything else, which is why I keep referring to a flat 3V and they ran for a good 5 minutes at 18V.

Anyone any idea how long it takes to burn an LED out? I haven't tried it but might put one of these on at 3.2V and another at 5V and see if they survive.
 

Thread Starter

Cackles

Joined Feb 7, 2008
7
They are just simple LEDs inside the switch, they won't have any internal current limiting, so the 1.8V is the nominal forward voltage at rated current, it will vary a bit. You drive them just like a normal small LED of the same colour.

LEDs are current driven, you limit their current and they set their own forward voltage, for small LEDs like this, a resistor is simplest. There's a good LED resistor calculator at ledcalc dot com
LEDs are current operated devices (look like a forward-biased diode) which drop about 2V or so when operating, depending upon their color.
They thus need to have a current limiting resistor in series with them they don't get zapped.
Small LEDs usually have a maximum operating current of 20mA, so a good starting design current would be about 10mA.
The resistor required would be the supply voltage minus the LED voltage divided by 10mA.
For example with a 9V supply and one LED, the resistor value would be (9V-1.8V) / 10mA = 720Ω

When you connect 5 together, the total voltage drop is 5 x 1.8V = 9V so not much is left to power the LEDs with a 9V battery.
Thanks, but not really looking to do that at the moment.
 

Jon Chandler

Joined Jun 12, 2008
433
"Thanks, but not really looking to do that at the moment."

The LEDs really don't care about what you want to do. As several have explained, the current through an LED must be controlled – the simplest way is with a series resistor. If you just connect a voltage source with no current limiting that's higher than the LED's forward voltage (Vf), it will die a horrible death rather quickly.
 

Wolframore

Joined Jan 21, 2019
2,579
Most LED's that are on switches have built in resistors. They usually set the voltage that it's meant to run at. A 1.8V wouldn't last a second at 18V without some help.
 
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