wall wart as power supply question

[Illum]

I've acquired some 9V @ 200ma wall warts previously used for recharging cordless handsets.

I was thinking, since I have some luxeon Is [Vin requirements shift from H to K] I can drive them at 3V [they function fine at 3V btw] a piece and connect 3 in series and epoxy them on a heat sink of sorts

I spliced the wall warts wires and measured it plugged in....
the meter shot to 14.5V open circuit voltage oo:

My question is, is this normal? and will this instaflash the luxeons instantly or does it operate at the rated 9V with load?

 

[Lynx_Arc]

Wall warts normally can run way in excess of rated voltage at amperage but under the recommended load they should settle down to be within that voltage. If you want to test it, put a variable resistor inline and crank it up and see. If it starts exceeding the current level then you got problems but if you can crank it wide open (no resistance) then you are set. If you want to add a resistor then crank it to what current level you desire, break the circuit and measure the resistance and use a suitable resistor for the load to drop excess.

 

[Illum]

ah, good idea with the variable resistor, didn't think of that before :thumbsup:

 

[enLIGHTenment]

Wall warts are not current limited supplies. You must have a current limiter--resistor, or something fancier--somewhere in the circuit or you will fry your LED(s).

 

[Norm]

Wall warts generally aren't regulated so as previously stated at there rated current they will be close to there rated voltage. If in doubt use a variable three terminal regulator LM317 or similar.
http://www.geocities.com/tomzi.geo/lm317/lm317.htm
All components to the left of the 100 nano farad capacitor are in your wall wart. With this circuit you could even dim the LEDs.
Norm

 

[enLIGHTenment]

Wall warts generally aren't regulated so as previously stated at there rated current they will be close to there rated voltage. If in doubt use a variable three terminal regulator LM317 or similar.
http://www.geocities.com/tomzi.geo/lm317/lm317.htm
All components to the left of the 100 nano farad capacitor are in your wall wart. With this circuit you could even dim the LEDs.

That's a constant voltage circuit. All it's going to do to an LED is let the smoke out.

Driving LEDs requires a constant current circuit, like this:

http://users.pandora.be/davshomepage/current-source.htm

 

[Norm]

I was just looking for something simple, obviously as the voltage is varied the current will be varied as well. Thanks for pointing out my error.
Norm

 

[Illum]

mmm.. :thinking:

enLIGHTenment, the wall warts rated at 9V 200ma and my circuit is composed of 3 luxeon Is in series capable of handling current up to 350ma as to lumiled specs, according to http://led.linear1.org/1led.wiz if I was going to add a current limiting resistor it would be 1 ohm at 1/8W for ~200ma and 1/4 watt for ~300 ma. I have no clue where I can find such resistor in single quantities

 

[enLIGHTenment]

the wall warts rated at 9V 200ma

The current rating is only a measure of how many milliamps the WW is guaranteed to provide without failing. It does not mean the WW will regulate at 200ma. Attempting to draw more current will either trip an overload protection device (e.g. a fuse) or cause permanent damage.

f I was going to add a current limiting resistor it would be 1 ohm at 1/8W for ~200ma and 1/4 watt for ~300 ma. I have no clue where I can find such resistor in single quantities

Digikey has 1 ohm 0.25W (overrating resistor wattage is no problem) resistors for under a dollar. They'll sell in single quantities but all orders under $25 USD are assessed a $5 handling charge.

 

[MrAl]

Hi there,

One other thing to be careful about is that many DC wall warts have a rather
large value built in capacitor. When the voltage of a 9v wall wart is up to
15 volts or so, this means the internal cap is charged up to 15v too.
When you connect a load that load gets 15 volts for a time period until
the cap discharges to the normal operating voltage of 9v. This could cause
a huge current surge through the device. It's better to make sure the
wall wart cap is discharged first, then hook up the circuit, then plug it in.
In this way the cap doesnt charge up to 15v but only up to the normal
operating voltage.
The only thing to remember then is to make sure the circuit is connected
*before* plugging the wall wart in.

 

[Mr_Light]

Do you think it would be safe to hookup one of the inexpensive DX or Kai regulator boards to an appropriate voltage wall wart to drive a Cree or SSC?

 

[eluminator]

Are we taking bets? I'm betting it will work. It really depends on the characteristics of the wall wart. If it is rated correctly, it seems to be a good match to your LED circuit.

The current will be limited by the wall wart. As the current rises, the voltage put out by the wall wart will drop. Much of the voltage drop is due to the resistance of the wire in the transformer.

I'm betting if something blows, it will be the wall wart. That assumes the Luxeon heatsink can dissipate the heat and the wall wart rating is close to reality.

 

[Mr_Light]

Here are the results of my 24 hour test. I hooked up a spare HP wall Wart 4.5vdc 500mA rating to the following DX regulator to a CREE attached to a large copper CPU heat sink. Wall wart gets warm (not hot), regulator board cool to the touch, CREE heat sink warm (seems about right for 700mA current).

http://www.dealextreme.com/details.dx/sku.4255

My conclusion is that these boards seem like a cheap way of adapting wall warts (that match there drive current/voltage) to drive CREE/SSC leds off wall current.

 

[Mash]

According to a knowledgable memeber here, 2Xtrinity, the modern cellphone chargers are very suitable to drive LEDs directly, since they up the voltage UNTIL they are supplying the maximum rated current. I have tested a Cree with a 5volt nokia charger, and didnt blow the LED.
Cant seem to search the forum now, but when u can, u will find a good explanation from 2Xtrinity.
Now I have a question too, regarding these boards:
If you have a board, suspposedly going to supply the LED at 800mAh, and you power it with a wall wart rated at eg 6v, 400mAh, does this mean the LED will get the full 800mAh? Ie if the mAh of the supply is less than what the board is meant to supply to the LED what happens?

 

[Illum]

well, I had the thing connected, then plugged it in...doesn't seem to pose a problem. I connected a 10 ohm 1/4 watt resistor just in case, after the first few minutes passed I bypassed it with slight increase in output and then remained there for the duration of keeping it on

I added a switch and...your right about the cap charging up, even with the resistor it was "BRIGHT" for a second or so..