issues finding a replacement voltage supply

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I have a device that comes in two flavors.

1) 5 leds ( if-red ) on the right in series. With a 22 ohm resistor. and 5 leds ( if-red ) on the left in series. With a 22 ohm resistor.

2) 3 leds ( if-red ) on the right in series. With a 47 ohm resistor. and 3 leds ( if-red ) on the left in series. With a 47 ohm resistor.

They both use a near 7.5 voltage source. These devices are connected to a video game console ( Nintendo wii for those that are wondering ) via a proprietary connection. I simply want to convert this to a DC power adapter. Easy enough right?

Well here in lies the problem. When I find a suitable power supply and connect it up all works great for 3 days at which point the first Led or last depending on how you see it, burns up. I also want to point out that the resistor remains very hot. So hot that its not friendly to the finger. So one would think its simply drawing too much current. Though why?

I dont own one of these consoles so I dont know the current being drawn but I think its in the 80 milli volt range ( just a guess ). Calculating it I get around 300 milli amps. Both versions seem to do the something. From what I read the current is constant fom the console and does not shut of for any amount of time.

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Are you sure the output of the Wii is a constant voltage, and not internally current-limited? When you say "suitable power supply", what do you mean, exactly? Is it regulated? Because a nominally 7.5V unregulated wallwart will put out a significantly higher voltage without a substantial load in it.

Where are you getting the 80 millivolts (did you mean milliamps?) and 300mA figures? Have you measured the Vf of the LEDs?

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What is (if-red)? If we assume the average red led has a forward voltage of 1.2V, 5 of these give us 6V, so the current using a 7.5v supply and a 22ohm resistor will be 68mA. At 12v it will be 272mA. At 7.5V the resistor is dissipating 0.1W, at 12V 1.6W. I can then understand if you're feeding the leds 12V, that the resistor is going to get real hot and the leds failing. Therefore, if you want to run your leds on say 12V, you need another resistor to limit the current. R = V/I = (12-7.5V)/0.068 = 66ohms. The closest E12 value is either 56 or 68 ohms. The resistor will want to be rated about 1W.

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Kartman wrote:
What is (if-red)?

Took me a second, but since the OP is talking about the Wii's sensor bar, presumable it means infrared. (The Wii's controllers rely on two infrared sources a certain distance apart to track their position and angle relative to the TV.)

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Are you sure the output of the Wii is a constant voltage, and not internally current-limited? I do not, and dont have one to to play with.

When you say "suitable power supply", what do you mean, exactly? Just a good match for what the wii is doing. Granted there are a lot of unknowns here.

Where are you getting the 80 millivolts (did you mean milliamps?) I did mean mA, I have been working on this for a few hours...

and 300mA figures? Have you measured the Vf of the LEDs? I knew that the LEDs had to be figure in but that LEDs are again, unknows. I could not figure out a way to get the Vf from one LED. Is it as simple as measuring the voltage before and after?

Kartman, that is basically what I was hopping to learn here. I dont know the Vf of the infrared LEDS. In the case of my power source I think it would be best to target a USB as from what I know its fairly constant and clean. At least I think that a better option then hunting down a PS that will give me X.

Hmm, looks like I found some interesting talk on here.
https://www.avrfreaks.net/index.p...

Last Edited: Mon. Jul 16, 2012 - 12:00 PM
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You have three voltages - across the leds, across the resistor and the supply voltage. Measure any two of these and you can infer the third. Once you know the voltage across the string of leds, then divide by 5 and you get you Vf for a given led. You'll only get 5 V from USB, so that won't power your 5 leds. Basically, with a led once you hit it's Vf, it will start to pull current. If the voltage increases, so will the current exponentially until it smokes. That why you have a resistor to limit the current.

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thx Kartman I think I will have better luck now. USB is more like 4.7 and dropping to 3 LEDS and given the right resistor it "may" work, no?