Driving 3W and 5W LEDs with 18VAC

Thread Starter

bookem1964

Joined Apr 15, 2009
5
I am attempting to install two lights to illuminate my American Flag. I want to use LEDs instead of incandescent bulbs. The power source for my lights is the 18VAC transformer that powers my house number. It has two automotive style incandescent bulbs. I chose this because of its proximity to the flag. I put together the circuit shown as Circuit 1 and it seems to work fine. The problem is that it's kind of dim. So I have purchased some 3W and 5W LEDs from Vollong. Circuits 2 and 3 are my ideas for wiring these devices. My concern is the power dissipation required for the current limiting resistor.

I have also considered rectifying the AC and then using a constant current source like an LM317 but space is an issue and due to the voltage drop of the LM317 I probably couldn't drive the 5W device (with it's 7V Vf). So I am hoping that a simpler solution is possible.
 

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Thread Starter

bookem1964

Joined Apr 15, 2009
5
A little more information.....

The LEDs are being placed inside metal landscape light housings. The metal PCBs will be screwed to the cans with heatsink compound.
 

Thread Starter

bookem1964

Joined Apr 15, 2009
5
I realize now that my only option is a driver circuit. The power requirements for the resistors is just way too large. You can delete this post now.

Thanks,
Dan
 

Wendy

Joined Mar 24, 2008
21,929
Doesn't work that way, posts are forever.

There are many ways to drive LEDs, a buck boost is preferred due to the built in efficiencies, but it can be done economically with resistors.

You will not be able to use your house transformer, unless it can provide a lot of amps. A 5W LED requires 1.4 Amps, give or take.
 

DickCappels

Joined Aug 21, 2008
6,087
Circuits 1 and 3 will work, but circuit #2 could cause some damage to the LED because it allows a large reverse voltage across the LED. To find the

To calculate the resistor values, use average voltage (RMS x 0.9) and subtract the diode's forward drop, and divide that by the 2x the desired current. Now go back and check that the peak current (which is ((RMS x 1.41)-Vdiode))/R < Max Repetitive Peak Current for the LED.
 
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