I need to replace 2 blown LEDs in a keypad module. They are connected in series with a 240 ohm resistor to a 24 VAC supply. What LEDs should I choose? Something with a forward current of 100mA? Do I need to worry about the forward voltage? I've not seen LEDs connected like this before to AC.
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1\$\begingroup\$ Can you sketch the circuit? There are good ways and less good ways to solve this. Depending on how the existing circuit is made, some ways will be easier. \$\endgroup\$– JRECommented Jul 5, 2021 at 14:13
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2\$\begingroup\$ How many LEDs are there currently in the circuit? Just the two, or are there more? 100mA for the backlight of a keypad sounds just slightly excessive. \$\endgroup\$– JRECommented Jul 5, 2021 at 14:18
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1\$\begingroup\$ This circuit sounds dodgy. 2 LEDs in reverse parallel would make sense. But in series... not really. \$\endgroup\$– Criticizing Israel not allowedCommented Jul 5, 2021 at 14:24
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\$\begingroup\$ There are 2 LEDs plus a 240 ohm resistor in series. They are connected to 24 VAC. Resisitor looks a bit burnt and both LEDs are blown. Need to replace the LEDs but not sure what spec to choose. Was thinking 100mA, 24/240 = 0.1 A ? They illuminate the keypad area on a gate controller, not just a backlight. \$\endgroup\$– Andrew SmithCommented Jul 5, 2021 at 14:27
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\$\begingroup\$ I agree the circuit sounds dodgy, its inside a Videx keypad audio module. The whole module is very cheap and nasty but very expensive to buy! \$\endgroup\$– Andrew SmithCommented Jul 5, 2021 at 14:30
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2 Answers
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From your description, this is the existing circuit:
simulate this circuit – Schematic created using CircuitLab
That circuit leaves the LEDs open to failure. The LEDs are exposed to a high reverse voltage with every cycle of the AC. Along with that, the resistor allows a very high current to flow. 100mA sounds excessive.
This is the circuit you should use to replace what's already there:
With the LEDs in anti-parallel, they will protect each other from the high reverse voltage on each cycle of the AC.
Find an LED you like that seems like it would be bright enough. You'll probably want to get white ones.
Buy LEDs for which you can get a datasheet. There will be an entry in the datasheet that tells you the rated current - that's \$I_{forward}\$. There'll be another entry that tells you the expected voltage range for the rated current - pick the low end of that voltage. That's \$V_{forward}\$.
The peak to peak voltage of 24VAC is about 68 volts. That's \$V_{supply}\$.
The value of the resistor can be calculated like this:
$$ R = \frac {V_{supply}-V_{forward}}{I_{forward}}$$
Assuming 3V for \$V_{forward}\$ and \$I_{forward}\$ of 20 mA, you get a resistor value of about 3k. There are stupidly bright LEDs that will practically blind you on that lousy 20mA.
You also need to consider the power rating of the resistor. In my example, it is dropping 65V at 20mA. \$P = EI = 65 \times 0.02 = 1.3W\$ - you'll need at least a 2 watt rated resistor to limit the current.
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\$\begingroup\$ Listen closely to JRE! This is the root cause and the solution to your problem. \$\endgroup\$– winnyCommented Jul 5, 2021 at 15:55
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\$\begingroup\$ Thanks for the reply, will modify it with some new LEDs. I know the design is poor, probably deliberate as most people will call out installers and be charged hundreds for a new module. \$\endgroup\$ Commented Jul 5, 2021 at 17:51
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You can repair it with any LEDs and resistor but you must add a series blocking diode of any type or as suggested in the comments put the LEDs in antiparallel. The max reverse voltage is -5V for almost all LEDs. Some fail faster than others when exceeded.
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1\$\begingroup\$ Thanks for the reply, will definitely modify it. \$\endgroup\$ Commented Jul 5, 2021 at 17:52
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\$\begingroup\$ The old GaAs were more robust and in the 70's I replaced a doorbell button screw with a 5mm LED and resistor powered by a 24V doorbell transformer with switch arc and all. Great you could see it at night. I did not know about -5V then. The LED would short out with the switch and then flicker with the mechanical hammer back EMF when released. It got dimmer and dimmer after a few years until we moved. (circa 1976-79 ) \$\endgroup\$– D.A.S.Commented Jul 5, 2021 at 18:00