I've got a couple of 6v leds lying around that I'd like to put in outside lanterns. It's only for a few hours per day and I don't particularly need a huge amount of light. I've got loads of step down transformers from 2-20v and I was wondering if it would be OK to run the leds directly from them or would it be necessary to use drivers? If yes, would it be preferable to use the lower power transformers? Thanks.
LEDs off transformers?
- Thread starter Gregers
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Illum
Flashaholic
I've ran 5mm LEDs, luxeon Is, K2s, etc on wallworts with no trouble...you need at most a current limiting resistor to go on the package and you should be all set
wall worts introduce ripple in the DC output but its mostly harmless if your thinking of direct drive, if your thinking of using wall worts to power circuits, the power may need filtering by caps.
my latest setup is running K2s mounted on heatsinks wired directly to a 4.5V @ 800ma wall wort as an aquarium light, no issues so far.
note: plug in the transformer only when everything is connected, due to internal capacitors the open circuit voltage is nearly twice the rated voltage and will burn something out if your not careful.
ones I know of...
Panasonic [9V @ 200ma] open circuit was ~13V:shakehead
Panasonic [12V @ 450ma] open circuit was ~28V
Motorola [4.1V @ 400ma] opencircuit was ~7V
wall worts introduce ripple in the DC output but its mostly harmless if your thinking of direct drive, if your thinking of using wall worts to power circuits, the power may need filtering by caps.
my latest setup is running K2s mounted on heatsinks wired directly to a 4.5V @ 800ma wall wort as an aquarium light, no issues so far.
note: plug in the transformer only when everything is connected, due to internal capacitors the open circuit voltage is nearly twice the rated voltage and will burn something out if your not careful.
ones I know of...
Panasonic [9V @ 200ma] open circuit was ~13V:shakehead
Panasonic [12V @ 450ma] open circuit was ~28V
Motorola [4.1V @ 400ma] opencircuit was ~7V
Mr Happy
Flashlight Enthusiast
You will want to rectify the transformer output with a full wave or bridge rectifier to get DC if it's not already rectified, and then put a current limiting resistor in series with the LED if required to prevent it being over driven.
You can drive an LED directly off AC without a rectifier, but you will only get half the brightness and there is a risk of exceeding the reverse breakdown voltage and harming the LED.
You can drive an LED directly off AC without a rectifier, but you will only get half the brightness and there is a risk of exceeding the reverse breakdown voltage and harming the LED.
UncleFester
Flashaholic*,
Keep in mind that most leds have a maximum reverse voltage of 5V. I wouldn't use them as their own recifier for that reason.
Thanks for that. How do you determine whether there is a rectifier present?
VidPro
Flashlight Enthusiast
I have done that many times, even used my computer speakers wall wart for keyboard light.
if it is a AC outputting transformer, use a bridge rectifyer and a cap to make dc (they usually say on the wall side what they are rated)
if its a DC transformer , usually you can just use it, add caps (as mentioned) to make any of them a bit cleaner.
bridge rectifyers are cheap, or one can be made from 4 diodes, but cheap bridges are often very high amp devices, and can be heat sinced insureing major longevity over some cheap barely spec diodes. bridges are often also well marked for ac input and its ouput polarity, making connection simplified.
measure with a voltmeter, on both DC setting and AC setting, if you get minimal AC showing with meter, and a good DC showing, then it is probably DC.
If any AC shows up on the DC side using a meter, then the pulsing/dirty DC is showing, and it would be a candidate for cleanup with capacitor. like a nice electrolytic cap in the correct polar direction.
as mentioned most of the partly or unregulated wall wart devices have a higher unloaded voltage, so the entire assembly will determine how it actually works, vrses being able to read the voltage unloaded, and determine how it will work.
so you can guess, then remove some resistance as needed, or use a driver thing that will control total current.
also, it is important to note that the speced MAX amperage output of the average wall wart, should not be used constantly. for example a 900ma wall wart might work great running 350ma-600ma of leds, not 900ma. over time the heat and the build quality and all it will fail, melt even.
the tools that would be usefull, a voltmeter that does amps too, and a variable wire wound 2-5W potentiometer, with just those items you can do a discovery, and tuning. without them, you could guess and hope.
if it is a AC outputting transformer, use a bridge rectifyer and a cap to make dc (they usually say on the wall side what they are rated)
if its a DC transformer , usually you can just use it, add caps (as mentioned) to make any of them a bit cleaner.
bridge rectifyers are cheap, or one can be made from 4 diodes, but cheap bridges are often very high amp devices, and can be heat sinced insureing major longevity over some cheap barely spec diodes. bridges are often also well marked for ac input and its ouput polarity, making connection simplified.
measure with a voltmeter, on both DC setting and AC setting, if you get minimal AC showing with meter, and a good DC showing, then it is probably DC.
If any AC shows up on the DC side using a meter, then the pulsing/dirty DC is showing, and it would be a candidate for cleanup with capacitor. like a nice electrolytic cap in the correct polar direction.
as mentioned most of the partly or unregulated wall wart devices have a higher unloaded voltage, so the entire assembly will determine how it actually works, vrses being able to read the voltage unloaded, and determine how it will work.
so you can guess, then remove some resistance as needed, or use a driver thing that will control total current.
also, it is important to note that the speced MAX amperage output of the average wall wart, should not be used constantly. for example a 900ma wall wart might work great running 350ma-600ma of leds, not 900ma. over time the heat and the build quality and all it will fail, melt even.
the tools that would be usefull, a voltmeter that does amps too, and a variable wire wound 2-5W potentiometer, with just those items you can do a discovery, and tuning. without them, you could guess and hope.
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