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I have an HVAC which I would like to add some status indicators to. I have some tri color LEDs, but could use suggestions as to how to utilize those LEDs.

What I am figuring is that I need a diode to protect the LED, and supply it DC, and then a current limiting resistor to limit the current from the rectified 24 VAC.

I have a bag of a thousand or so 1N4001 diodes, which I figured I might use.

Is there a better way?

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  • \$\begingroup\$ A half wave rectifier will give you 24/pi = ~7.5VDC which is quite useful for driving tri-color (RGB) LEDs with low currents (e.g. 5-10mA for indicators). \$\endgroup\$ Commented Jul 17, 2017 at 6:24
  • \$\begingroup\$ I think you mean 24 * pi, which is considerably more than 7.5V :) \$\endgroup\$
    – DiBosco
    Commented Jul 17, 2017 at 12:15

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Better still is a simple series diode plus current-limit resistor.

schematic

simulate this circuit – Schematic created using CircuitLab

There can be advantages to putting a reverse-polarity diode in parallel with a LED when that LED is being fed from an AC source but there are usually specific reasons for doing so.

The problem with a single series resistor feeding a LED with a reverse-polarity diode in parallel is that the resistor is dissipating power for most of the entire AC half-cycle.

If you use a series resistor plus a diode also in series, the resistor is dissipating power for only half of the time. This reduces the power by half and either lets you use a smaller resistor OR the resistor runs cooler.

You can add a resistor in parallel with the LED if you want to trim the turn-on voltage of the LED. This can help stop the LED from illuminating with small leakage currents.

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A single diode in parallel with the LED, but reversed so it conducts in the opposite direction, will protect the LED from large reverse voltages.

schematic

simulate this circuit – Schematic created using CircuitLab

But the result can flicker noticeably when run on AC.

An alternative, if you have plenty of diodes, is just to make a bridge rectifier, and rectify the AC into DC. Without a capacitor, the LED will still flicker to some extent, but it will be a lot better.

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  • \$\begingroup\$ Flicker is not an issue on an indicator light. But at 30Hz it would be more like a strobe. \$\endgroup\$
    – mongo
    Commented Jul 17, 2017 at 18:31

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