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