I bought 2 CREE XM-L T6 (1800 lumens) Bike lights on ebay. First off I think these are really good and they have been working very reliably so far.
I’ve been using these as spot lights on a mirror ball for some house parties and they work very well (in a smaller room). But, of course the batteries run out at some point.
Now I want to get a simple AC adapter to plug these into, so I can power them that way. When I use the 8.4V AC charger that came supplied with the lights the only put out about half of the brightness as they do when I use the 8.4v battery pack.
Can someone tell me what kind of AC adapter will make these run at full brightness. Is it a constant voltage thing?
Sorry, I really don’t know much about this.
Thanks for any help!
You typical low voltage walwart is unregulated, so there is a pretty clear relatinship between current draw and supply voltage. My guess is your walwart
is probably rated about 1 amp, so the output voltage falls rapidly when you exceed 1 amp. As a result the 8.4 volts drops to something on the order of 3.3 volts. In effect the AC adapter is acting as a series resistor to limit the current.
The box you are in is that unless there is some sort of regulation, using a larger AC adapter could simply fry the LED. Once the LED lights up, the slope on the VI curve changes dramatically. So if the ultimate output of the ac adapter is over 4 amps and if the voltage is materially above Vf, the LED will burn up in very short order.
A better solution is to invest in a driver that will limit the current to the LED that the AC adapter connects to (assuming you invest in a larger AC adapter). Generally when you get above about 1.5 amps, the adapter usually is a switching type power suppy. Below that level, it is usually not much more than a transformer, bridge rectifier and filter capacitor.
What you want isn’t constant voltage, you want constant current so the slope of the VI curve ceases to matter.
The light has a driver built in, the plug only connects batt power from the pack, the pack doesnt do any regulation or anything, the light handles all that.
What you would need is to find either a 8.4V power supply with at least several amp’s or get an adjustible one. You can figure out how much current it needs by measuring draw from the pack. as long as the power supply output’s 8.4V at atleast the current the light draws from the pack it will work at full brightness.
Thanks for your responses. I will measure the draw from the battery pack and try an AC adapter with at least that capacity.