Re: [AFMUG] OT: 18V component LEDs buffer reqmnt...

[Forrest Christian (List Account)]

Really depends on the LED.

The purpose of having the LEDs opposite polarity of each other is that LEDs
have a finite reverse voltage limit.   By placing them back to back the
'on' led protects the 'off' led from getting too much voltage, since the
'on' led will be limiting the voltage to whatever it's forward voltage is
(usually a couple of volts).  Once the first LED failed, there is a good
chance that the second one failed due to the first one not protecting it
from reverse voltage.

See
http://www1.futureelectronics.com/doc/EVERLIGHT%C2%A0/334-15__T1C1-4WYA.pdf
as an example, totally random datasheet for a white LED (just the first one
on google).

For this particular LED, you shouldn't exceed 5V in the reverse direction,
so putting a second one in the forward direction would limit this to 3.6v
(I'm ignoring the fact that this particular diode seems to be protected
from reverse overvoltage).

According to the datasheet you can do 30mA on average, so you should limit
to this currrent.  As chuck said, 20mA is probably safe for most LEDS,
although some might need more to get the rated display power.   One note is
that you can probably do 60mA when driving it with AC, since it's an
average value.

18VAC RMS is equivalent power to 18VDC, so using that plugging into a
resistor calculator  (
http://www.ohmslawcalculator.com/led-resistor-calculator ), says that a 600
ohm resistor will get you around 25mA, which should be fine *for this
particular LED*.

A higher value one will be dimmer but the LED will likely last longer.

My method for doing this type of repair would be to get the brightest led
you can find at 20mA, then adjust the resistor until it is a reasonable
brightness.   This will reduce power consumption and make the led last
longer.   Be mindful that the resistor may need to be a fairly high power
one, depending on how much voltage you're dropping and how much current is
being drawn.   (Multiply the resistor value times the current squared
(W=(I^2)*R), being mindful of units.  In the case stated, you'd need around
a half-watt resistor, and I'd probably make sure I was using a 1W or 2W one
just for safety).

On Thu, Apr 12, 2018 at 8:02 PM, Bill Prince  wrote:

>
> We have an electric gate for our little neighborhood. The keypad uses a
> little circuit board with a pair of white LEDs mounted on a tiny circuit
> board to light the keypad. The little circuit board is connected to a 16VAC
> transformer through a 600 ohm resister.
>
> One LED is forward biased, and the other is backward biased. So I assume
> they alternate lighting up as they get juice from the transformer. Well,
> both LEDs are done, finny, kaput.
>
> What I'm wondering is if I can put a new pair of 12V LEDs in this circuit
> and whether the 600 ohm resister is enough to protect them from the ~~
> 18VAC that is coming through the transformer? Or should I put in a bigger
> resister? I have no idea how much forward current an LED can handle.
>
>
> --
>
> bp
> 
>
>


-- 
*Forrest Christian* *CEO**, PacketFlux Technologies, Inc.*
Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602
forre...@imach.com | http://www.packetflux.com
  

Re: [AFMUG] OT: 18V component LEDs buffer reqmnt...

[Chuck McCown]

20mA is a safe assumption

Sent from my iPhone

> On Apr 12, 2018, at 8:02 PM, Bill Prince  wrote:
> 
> 
> We have an electric gate for our little neighborhood. The keypad uses a 
> little circuit board with a pair of white LEDs mounted on a tiny circuit 
> board to light the keypad. The little circuit board is connected to a 16VAC 
> transformer through a 600 ohm resister.
> 
> One LED is forward biased, and the other is backward biased. So I assume they 
> alternate lighting up as they get juice from the transformer. Well, both LEDs 
> are done, finny, kaput.
> 
> What I'm wondering is if I can put a new pair of 12V LEDs in this circuit and 
> whether the 600 ohm resister is enough to protect them from the ~~ 18VAC that 
> is coming through the transformer? Or should I put in a bigger resister? I 
> have no idea how much forward current an LED can handle.
> 
> 
> -- 
> 
> bp
> 
> 

[AFMUG] OT: 18V component LEDs buffer reqmnt...

[Bill Prince]

We have an electric gate for our little neighborhood. The keypad uses a 
little circuit board with a pair of white LEDs mounted on a tiny circuit 
board to light the keypad. The little circuit board is connected to a 
16VAC transformer through a 600 ohm resister.


One LED is forward biased, and the other is backward biased. So I assume 
they alternate lighting up as they get juice from the transformer. Well, 
both LEDs are done, finny, kaput.


What I'm wondering is if I can put a new pair of 12V LEDs in this 
circuit and whether the 600 ohm resister is enough to protect them from 
the ~~ 18VAC that is coming through the transformer? Or should I put in 
a bigger resister? I have no idea how much forward current an LED can 
handle.



--

bp