Powering 12V LEDs in buttons via USB

Question

I got some buttons and switches that are for automotive usage, so the LEDs inside are for 12V supply.

Is there a safe and efficient way to power these from an USB connector? I am planning to use an Arduino clone as a controller for these buttons and have the computer recognize it as an USB keyboard. Though, I would like to see the buttons LED on too, without adding an extra wire from an AC adapter just for the lights.

Would that be dangerous for the USB port on the motherboard? I do recall that the max current on USB is 500mA, although I have no info about how much power is needed to power the LED in the buttons (10 of them).

Is this possible and safe, or should I avoid to bump up the voltage of the USB port to power the LEDs?

Answer 1

If you power the LEDs directly from the USB power they will draw less current, since the resistor inside is designed for an higher voltage. So you don't risk overloading the USB (those LEDs won't draw nowhere near 500mA, even at 12V).

The problem you will probably face is that at 5V they could be quite dim, or could even be off. But you could try and see if they light up sufficiently for your taste.

Disclaimer: I never took one of those apart, so I assume they have a limiting resistor inside (it's probably the cheapest option), but they could also have some sort of integrated constant current regulator chip instead (unlikely but possible). In other words, the LED itself could be one of those designed to be powered directly from 12V.

Anyway the best thing to do to test them is to hook them to a bench power supply set for 5V output and measure the current they draws using a multimeter.

If you don't have a bench power supply, an easy alternative is to put 3 new alkaline AA or AAA batteries (they are very common everywhere) in series: they have a nominal voltage of 1.5V, but if they are new they could source something more like 1.6V: you will end up with 3x1.6V=4.8V with which you could test your LEDs quite safely. Note that USB standard requires that the USB power lines are 5V±0.25V, so testing at 4.8V is quite reasonable.

BTW, what do you really mean with "... to bump up the voltage of the USB port..."?

If you plan to increase the voltage on the power lines of the USB bus with some additional circuitry, then DON'T DO THAT. That will damage your motherboard.

Answer 2

You have three options, if you don't add an external supply. First, power wise. 10 leds at 20mA is 200mA. That's 1 Watt. Well within the "standard" 2.5W/500mA of USB. But most USB ports can provide more (not that you need them).

1. Run them at 5V. The existing internal resistor will be (12 Vs - 3.5 Vf) / 0.02A = 425 ohms (or so). At 5V source and 425 ohms, I figure the Led would actually take around 5mA instead. This is still quite visible.

2. Build a voltage doubler circuit. You would need a PWM signal for the circuit to produce the boost.

3. Use an off the shelf/ebay 5V to 12V boost circuit. Simple, prebuilt. At 12V and 20mA and 10 Leds, that's 2.4W, plus the efficiency loss, more like under 3 Watts, well within what a 500mA USB port can provide. You will need transistors/fets to turn on/off the leds since you can't power them directly any more.

For 3, you can make it boost to 9V or 10V instead of 12V. This would bring down the current (and brightness) and lowers the power you need. An Led at 15mA is frankly just as bright as one at 20mA for most purposes.