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I wanted to put some light to my 3D printer. The power supply outputs 12V DC and I have this old car workshop LED torch that had it's batteries leaked. I disassembled it and cleaned the salt from the leak. I'd now want to remove the battery connectors and replace them with a cable that goes to a socket where I can plug in 12V DC from the power supply of the 3D printer.

The circuit board of the thing is quite simple. It's basically just the switch and a resistor between the (clean) battery connector and the red cable.

The LED strip has 10 white LEDs that I don't know the exact type, connected in parallel. I just know that they been running on 4x 1.5V AAA batteries (=6V).

Can I just run this on 12V or would I better replace the resistor so the LEDs don't heat up too much and lose life? Which size should the resistor be to run the strip from 12V DC?

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Two options. Replace the resistor, using ohms law to calculate it, or bring the 12v down to 5 or 6v. 12v to 5v adapters are everywhere. A dollar store car usb adapter does that nicely, and it's easier to get than an appropriate resistor.

To replace the resistor, measure the voltage across it with fresh batteries. Then using ohms law, I = V / R, you have all three numbers you need, the voltage dropped by the resistor ohmage and the current through it. You'll want to keep the current the same, so adjust for the voltage difference and increase r until you get the same current, R = V / I. Make sure the resistor wattage is correct using P = V*I

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  • \$\begingroup\$ I think the 12 to 5V adaptor is a way to go since I don't have the equipment to measure the voltage. \$\endgroup\$
    – user6329530
    Commented Feb 16, 2019 at 11:41
  • \$\begingroup\$ Buy a cheap multimeter for these things \$\endgroup\$
    – Passerby
    Commented Feb 16, 2019 at 20:43
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If you have an iPhone, iPad, or Android device, you can download the piece of software "Adafruit circuit playground". What you need is an estimate of Vf for your LEDs.

For a single white LED, with a nominal Vf if 3.5V and 20ma draw, and a 12V power supply, iPhone screenshotthe LED circuit calculator tells me 430Ω. Not sure how to extend this to parallel LEDs.

In looking for information about Vf, I found what might be useful: https://www.maximintegrated.com/en/app-notes/index.mvp/id/3070

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  • \$\begingroup\$ I found this calculator what seems pretty suitable for my case, or? ledcalc.com \$\endgroup\$
    – user6329530
    Commented Feb 17, 2019 at 13:29
  • \$\begingroup\$ Yeah that works too. Led calculators are all using the same formula. \$\endgroup\$
    – Passerby
    Commented Feb 18, 2019 at 20:56

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