How to tell positive on printed circuit

[joaks]

Trying to replace light car radio with led. How to tell which end is pos without energzkng circuit? Sorry for englisb typing on phone.

 

[RiceCake]

Usually you can follow the traces and find which one is common with everything else (other lights, etc), that's almost always ground.

 

[joaks]

Correct ricecake. Thank you!

 

[HerrStig]

Got an ohm meter, follow the negative wire which, unless you have a rare positive ground car, is connected to the body . You won't hurt most leds by connecting them backwards, btw, they just won't light, but you will need about 20 thousand ohms in series with the led to make it work on "12" volts without burning out.

 

[Kiwi_ME]

More like 600 ohms for 20mA load providing a drop from 14 V to 2 V.

 

[djb]

Most modern small LEDs operate on 1-2mA for best efficiency, are at full brightness at 5mA and will overheat on 20mA.

 

[RiceCake]

Almost every 5mm LED on earth operates on 20mA...I have no idea what you're referring to. Let alone the fact the OP never asked about any of this so its funny why its even considered relevant.

Kiwi_ME is correct though. For 14.4 volts on a 20mA (3 or 5mm) LED, with a forward voltage of 3.3 volts (most green, blue, or white LED's), the optimal resistance is 560 ohms, so 600 ohms is perfect.

 

[Xlr8r1]

Also if you have not removed the bad one you can usually look inside the LED. Most of them have what looks like a little flag on the inside of them. The is the Negative side of the LED.

Follow this link:

http://www.makingthings.com/documentation/how-to/connect-an-led-to-a-digital-output/LED2.jpg

 

[joaks]

Got it finished! The old bulb was an incandescent so was no help. I think you can now see my radio from space. Might be a little too much.

 

[RiceCake]

Easy hack is colour the bulb with a Sharpie a little to cut the light.

But yeah getting LED's of similar brightness to the OEM incandescent is a really difficult job most of the time.