Uno pin output enough to power mini-Christmas tree light?

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DougSpindler
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Joined: Sat Feb 03, 2018 6:30 am

Uno pin output enough to power mini-Christmas tree light?

Post by DougSpindler » Sat Feb 03, 2018 6:50 am

I’m working on a project and want to use incandescent lights instead of LEDs. I thought I would use an incandescent lights from a string of mini-incasdent Christmas lights. I have not measured the current draw of one light but the voltage for a light is about 2.5 volts. I suspecting the current draw of an individual incandescent mini-light is about the same as a LED. My reasoning is the current draw for a string of LED Christmas lights is almost the same as a string of incandescent lights.

I’m wondering if the Uno has enough power per I/O pin to drive one Incandencent bulb?

Anyone given this a try?

Thanks

Valen
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Re: Uno pin output enough to power mini-Christmas tree light?

Post by Valen » Thu Feb 08, 2018 8:23 am

A single pin can supply as much as 40 ma. Beyond that it will risk damaging.You really need to start doing a measurements. Knowing voltage alone cannot solve this problem. Either measure the resistance of the light(s) and use Ohm's Law (resistance=voltage/current) to figure current based on applied pin voltage (which you never mentioned by the way!) Be aware though that when the incandescant light heats up the resistance changes. Or measure the current through it at a known voltage (but not supplied by the microcontroller pins). Ideally you would measure the current through (in series) and voltage (across, parallel) the lights with 2 meters simultaneously. As the resistor in the current-meter might alter the results.

Better be save than sorry and use an NPN transistor (with base resistor) to switch it.

darrellg
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Location: Southern California

Re: Uno pin output enough to power mini-Christmas tree light?

Post by darrellg » Fri Feb 09, 2018 3:28 pm

in addition to the single pin current limit, the AVR micros have total current sourcing limits. You can't draw 40mA from every pin at the same time. I don't know what it is off the top of my head, but it's in the '328 data sheet.

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