Convert 110vac LED bulbs to 24vdc?

Re: Convert 110vac LED bulbs to 24vdc?

Remember that CFL and LED lamps need special driver circuits called "ballasts".

A CFL is actually a form or "arc lamp"... It takes (relatively high) voltage to start the "arc", then the resistance drops a lot--And the ballast is to keep constant current going to the lamp. If you just feed DC current to the bulb, it will basically short circuit once the arc starts and something will blow (wiring, internal components of bulb, etc.). CFL's may also require AC power for long life... Many things when feed DC will "plate" materials from one electrode to the other--And quickly blacken/ruin one of the electrodes. AC current does moves a tiny amount of metal ions back and forth--So do not erode electrodes over time (guess on my part).

LED's also require a ballast. The IV (current voltage) curve of an LED is non-linear and current is very sensitive to voltage (as well as temperature dependent). Put 6.0 volts on an LED--works OK. put 6.2 volts on an LED, excessive current, over heats, dies. Similar 70F at 6.0 volts, current OK. LED die heats to 120 volts, "resistance drops, more current, runs hotter, thermal runaway, dies.

LEDs either use a resistor to "ballast" (not very efficient), or a capacitor (such as Christmas tree lights on 120 VAC @ 60 Hz), or an actual switch mode DC power supply that will output a specific level of current to drive the LED (sometimes with thermal feedback to limit heat/current).

For DYI projects, you can go here and find lots of information and even suppliers of ballast and ballast kits/circuits for your projects:

http://www.candlepowerforums.com/vb/forum.php

-Bill

Re: Convert 110vac LED bulbs to 24vdc?

You can put a single ballast in series with multiple LEDs--Assuming you meet the specifications for current/voltage/power dissipation. LEDs are, mostly, driven based on specified current. Voltage is allowed to "float" behind the current ballast.

Of course, you need to make sure that your LED die are well heat sinked. The "old fashion" Txx cased white LEDs were typically overdriven to get lots of (cheaper) light from them. The only heat dissappation was via the lead frames (and that was not much). Inside of 100-500 hours, most of them turned black (phosphors+plastic overheated by LED).

The high quality LEDs that are larger than a fraction of a watt have heavy metal frames that are mounted to good sized heat sinks. Heat is the enemy of LEDs (and electronics in general).

-Bill

Re: Convert 110vac LED bulbs to 24vdc?

vtmaps wrote: »

But what if you have two 12 volt LEDs lightbulbs each with its own ballast... can they be put in series on a 24 volt circuit? --vtMaps

It depends...

If they are resistor or (capacitor if 24 VAC) ballasts--should be fine.

If they are active (electronic ballasts)--Probably not. Active ballasts are usually "constant current" output devices... And will probably exhibit "negative resistance" on the input that is seen with constant power devices (p=v*i, if v falls, i increases--exactly what you see with AC inverters on their DC input). In a constant output current device (that is a power converter), if input voltage falls, to maintain constant current, the input current will increase causing the input voltage to fall even more). So you have an unstable feed back loop (positive feed back). You might get blinking lights or even an over voltage on the ballast input (if two 12 volt ballasts are in series with their LEDs... One ballasts collapses its voltage, the other sees over rated input voltage).

-Bill