Sorry for the noobish question. I'm a computer scientist, so my electrical engineering knowledge is of the "don't put fingers in electrical outlet" level.

I'd like to know how to power a salvaged scanner light of the type Scanner Light Hack v2.0 from a Canon MP210 scanner/printer. It is a single RGB tube.

I've already discovered the terminals for Power and RGB. I can turn on the light with a 9V battery, no problem. But when I use a power adapter, set to 9V as well, it doesn't work. Actually, I can get the green light to work, but not the others.

I thought maybe it was noise from the adapter? So I added a capacitor, but didn't help. I also measure the voltage with a multimeter, it reads around 9.20. The adapter is a variable adapter, but set to 9V and max output 1700ma. I've tried resistors? Didn't do anything.

I'm not sure what to try next. Anyone have any ideas?

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    \$\begingroup\$ If you go back to the battery, do all the colours still work? That Instructable doesn't say anything about current limiting resistors... if the light doesn't have them built in, a little battery may be weak enough to limit the current, but the AC adaptor may simply blow the LEDs. So if you now only get green with the battery ...bad luck. \$\endgroup\$ – Brian Drummond Sep 18 '16 at 22:13
  • \$\begingroup\$ Alright, I went back to the battery. Ok yeah, only the green works now. Guess I blew the LEDs. But I don't get it, I also used 9V with the adapter. Shouldn't the LED only use what it needs? In future, how do I avoid this? \$\endgroup\$ – vega Sep 18 '16 at 22:53
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    \$\begingroup\$ The 9V battery has a high ESR, equivalent series resistance. It's a non perfect voltage source, and as the current draw increases, a large voltage drop occurs. This balances out and the leds are protected. A power supply, while still not perfect is more perfect than the battery, and it's ESR is very small in comparison. Imagine pushing water out of a water bottle, and a super soaker. Same amount of water, but one is much stronger. \$\endgroup\$ – Passerby Sep 18 '16 at 23:34
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    \$\begingroup\$ And "Instructables" strikes again! This is basic to LEDs; search for "what resistor to use with LEDs", you'll find it's asked almost daily and there are some really good answers. What you don't know is the current rating for this LED bar (unless you measure it in the scanner) - 20mA will be safe but maybe not very bright. \$\endgroup\$ – Brian Drummond Sep 19 '16 at 9:35
  • \$\begingroup\$ You may need to select 3 different resistor values when you do your next trial to get a acceptable white balance. There is no guarantee that the same voltage (if connected in parallel and not a likely design technique) or even the same resistors will form a neutral white. \$\endgroup\$ – KalleMP Jan 24 '17 at 21:32

From the ugly photos they appear to be about 1W which in my books makes the ESR ~1/1W= 1Ω above the threshold voltages. RGB are usually rated ~ Vf= 2.2V, 3.3V, 3.1V + tolerance . I suspect your power supply shut down as it might see a >6A load with the exception of Green which tend to have a higher ESR than others.

Meanwhile the 9V battery has an internal ESR that limits current to a few Amps to 1A depending on quality of Alkaline.

To make this work better you want a supply voltage between 1 and 2 x the LED voltage and compute using Ohms Law the difference voltage / R= 330 mA for a ballpark current level. If it burns your finger, you have 2 choices. make a better heatsink or increase R to reduce the current.

Thus with 5V >2A supply you can use for RGB

  • 5V minus {3.3,3.1,2.3V}/0.33A = 5.2Ω , 5.8Ω , 8.2 Ω in 1W ratings.
  • if unsure, get a variety of R's in this range.
  • if you get ambitious use 5V with three LM317 as a constant current source and compute I from 2.5V/R=Imax for each LED. Be prepared to heatsink the TO-220 cases or get hot.
  • one can also use 3.3V supply and smaller R's

but 9V is overloading the supply and your tried R's that were too big. If you did try 10 Ohms it would get hotter than the LEDs.

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So I think after reading more about this topic I've figured out the "best" way to hack one of these scanner lights. Obviously, as I experienced, you cannot connect the power directly to these and hope for the best. Old scanners have a nice driver attached to the light that controls the voltage and adds resistance, the newers RGB ones don't. I found a tutorial on instructables that conviently puts it all together.

You need something to manage the LED, such as a microcontroller, but it cannot power it directly since the current is too high for the Arduino.

The solution is to connect a power source to the 4 pins on the LED (VCC + 3 color grounds), via three transistors (such as 2222). The transistors will act as switches. You then connect 3 Arduino pins to three resistors (such as 330 ohm), and connect the resistors to the transistors. The Arduino activates the power to the RGB channels.

You can power it with a battery up to 9V, which is what I originally did. However, as the LED draws current the voltage drops since, as @Passerby mentioned in a comment, a 9V battery has high ESR. So if you power the LED with a plug, no more than a few volts, start with 3.3.

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