# I have an LED with no specs, but it does have a resistor. I want to double the voltage

It's one of those "stick anywhere" lights that look like a household switch. It's powered by 4 AAA batteries and has two-2 inch LED strips connected in series with a 3 ohm resistor(looks like a 1 watt). I want to cannibalize the parts and reassemble into a ceiling fixture I bought at the thrift store. That way I can use it in my 12 volt system I'm installing in my house. I'm having a heck of a time figuring out what resistance to use so I can up the voltage to 12 volts. I'm probably just looking at it wrong, I normally have the specs on the LEDs. Thanks in advance for any help with this.

• If you have a pair of the "stick anywhere" light, you can simply put 2 sets in series as an option. – K H Aug 6 '18 at 20:13

1. Get a multimeter.
2. Switch it to 2000 mA range. Connect leads to mA and COM socket.
3. Wire the meter in series with the (good set of) batteries. Record the current 'i' converting it from mA to amps (A).
4. Your additional resistor needs to drop 6 V when passing i. We can calculate the required resistor value from Ohms law.

$$R = \frac {V}{I} = \frac {6}{i}$$.

Pick the nearest standard value.

You might want to check the power rating required.

$$P = VI = 6i$$

Chose a power rating greater or equal to that value.

As an alternative, buy a second lamp and just wire the two 6 V units in series in your new fitting. You'll have twice as much light for the same current draw on your battery.

• If it's inconvenient to put your multimeter in series or you've burned out the ammeter fuse, you can also find the current by measuring the voltage drop over the existing resistor while it's running. – K H Aug 6 '18 at 20:13
• Very good! I like using math to figure out what I need. I'm going to copy and print your answer and keep it in my parts box. I'm almost thinking about buying another light and placing it in series, I could always use more light. Thank You very much! – Gary Stenzel Aug 6 '18 at 20:24
• Much more efficient to have two in series than drop 6 more volts but cheaper to buy stripLED’s – Tony Stewart Sunnyskyguy EE75 Aug 6 '18 at 21:16
• Thanks Tony, that's what I'm going to do. My local dollar store has them (2 strips) for $3.35. The cheapest I have found the strips online is$2.45 ea. that's why I bought them at the dollar store. LOL – Gary Stenzel Aug 9 '18 at 18:29