Newb + Truck Bed LED question

Community Projects
1

Howdy, I am new.

I have an LED battery powered light that operates on 4 AA batteries in series (6V). I want to ditch the batteries and use the truck 12V to power LED. I took the light apart it is pretty simple. 4 AA in series, through a switch, into a 10 Ohm resistor (did color code check), then into the 6 LEDs (they appear to be in series (kind of hard to see circuit board).

At the office we have a variable power supply. I set to 12 V and 5 Amp, then connected to the battery terminals. Did a quick on/off test and the LEDs came on. Then left it on for a second - everything ok. Decided to let it run for a bit - resistor started smoking.

I have done some research - Diodes (including LEDs) are unidirectional. They have a threshold voltage called forward voltage (get below that and no current flows). Since I had 6V of batteries, I assume the forward voltage is something less than 6 volt. I further found that once you meet the forward voltage, the diode acts basically like a short. That is the reason for the resistor (to provide a load and hence limit current).

Then did a little math V=I*R; V = 12v and R=10 Ohm. So current is 1.2A. Therefore power = about 14.2 watts.

If all my logic and math above is correct, I need a 10 Ohm resistor that can handle about 15 watts.

Is my logic, math, etc correct? It seems like a lot of power for 6 tiny lights. Further, a resistor that can handle 15 watts appears to be not terribly common - which makes me think I am doing something incorrect.

I have lots of options:

1 - Go to autozone/ebay/amazon and buy a strip LED

2 - Use an old cigarette adapter to step down the voltage

The real thing here is the education. Thanks in advance.

2

LEDs are NOT a short circuit when the threshold voltage is reached. Instead, like all diodes, they have a forward voltage drop (Vf). For white LEDs this is 3.2 volts to 3.5 volts. In this case, I suspect that the LEDs are in parallel, and not in series. LED current must be limited, or the LED will be destroyed. For non high-power LEDs, this is typically 20mA (0.020A). The current through your LEDs is supply voltage (6 volts) - Vf (3.2 volts) / resistor (10 ohms). This works out to 280mA (0.28A) which is too high. I therefore suspect that your resistor is 100 ohms, which would mean a max current of 28mA (0.028A), which would be reasonable. The power that the resistor needs to dissipate: P = I (0.028A) * V (2.8volts) = 0.078 watts. A 1/4 watt resistor would work fine here. However when 12 volts is used instead of 6, then the voltage across the resistor is now 8.8 volts, and the current through the 100 ohm resistor is 88mA (0.088A), which means the resistor has to dissipate 0.774 watts. No wonder the resistor smoked! You would need at least a 1 watt resistor. But 88mA would be too much for the LEDs, so the resistance would have to be raised to 430 ohms to limit the current to around 20mA.

Because the LEDs were supplied with too much current, they probably won’t last too long. I suggest you get a new light. The LED strip would be a good choice.