# Electronics geeks question **URL:** \ **Category:** Factual Questions\ **Created:** [November 19, 2002, 12:14pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502 "2002-11-19T12:14:03Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\ **Post date:** [November 19, 2002, 12:14pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/1 "2002-11-19T12:14:03Z")
I have a “light” with a 5-led array powered by 2 AA batteries (3V). I want to run it off vehicle current (12V). How do I figure out what I need to do that? Do I need a choke? A resistor? How do I calculate how much resistance to add? ---
**Author:** ![andy\_fl](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@andy\_fl](https://boards.straightdope.com/u/andy_fl)\ **Post date:** [November 19, 2002, 12:27pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/2 "2002-11-19T12:27:51Z")
Simple Solution - Use a 150-180 Ohm 1 watt (2-3 watt resistance preferable) in series. Assuming each LED consumes 10mA, 180 ohm will give you 50mA \* 180ohm = 9 V voltage drop. Or buy a DC-DC converter from Walmart used to run CD players. ---
**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\ **Post date:** [November 19, 2002, 1:29pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/3 "2002-11-19T13:29:05Z")
**andy\_fl** Thanks, guy. Just what I needed. (The DC converter isn’t practicable for what I’m doing) ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [November 19, 2002, 2:08pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/4 "2002-11-19T14:08:09Z")
**andy\_fl** is correct, assuming (of course) the five LEDs are connected in parallel. Here’s a first-order analysis: Definitions V[sub]S[/sub] = Maximum battery voltage (V) R = value of resistor (ohms) V[sub]R[/sub] = Maximum voltage across resistor (V) I[sub]R[/sub] = Maximum current through resistor (A) P[sub]R[/sub] = Maximum power dissipated by resistor (W) I[sub]LED[/sub] = Desired current through each LED (A) V[sub]LED[/sub] = Voltage across each LED @ I[sub]LED[/sub] (V) N = number of LED’s connected in parallel Given V[sub]S[/sub] = 13.5 V I[sub]LED[/sub] = 10 mA = 0.01 A V[sub]LED[/sub] @ 10 mA = 3 V (you can get this from the LED’s data sheet) N = 5 Calculations I[sub]R[/sub] = N \* I[sub]LED[/sub] = 5 \* 0.01 = 0.05 A V[sub]R[/sub] = V[sub]S[/sub] - V[sub]LED[/sub] = 13.5 – 3 = 10.5 V R = V[sub]R[/sub] / I[sub]R[/sub] = 10.5/0.05 = **210 ohms** P[sub]R[/sub] = I[sub]R[/sub] \* V[sub]R[/sub] = 0.05 \* 10.5 = **0.53 W** I would use a 220 ohm, 1 to 3 watt resistor. Of course, this assumes all five LEDs are connected in parallel. A more efficient arrangement is to connect LEDs in **series**. With LEDs that have a 3V voltage drop, and assuming a supply voltage of around 13V, it is most efficient to connect four LEDs in series; this would constitute one “string.” Multiple strings may be connected in parallel for greater light output. Pick the right LEDs, and you may not even have to use a dropping resistor in each string… ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [November 19, 2002, 3:14pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/5 "2002-11-19T15:14:51Z")
Would a voltage regulator (like a LM317) be impractical for this situation? Brian I think I got the part # right ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 19, 2002, 3:20pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/6 "2002-11-19T15:20:22Z")
I’d use a voltage regulator. A voltage regulator is a 3-legged component that looks like a big transistor. It shouldn’t be hard to find a 3-volt regulator that can handle 13 volt input. You connect one terminal to the 13 volt supply and one to ground, and you get a constant 3 volt output from the third terminal. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [November 19, 2002, 4:25pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/7 "2002-11-19T16:25:19Z")
There are advantages and disadvantages to using a voltage regulator vs. a resistor. A resistor is definitely more reliable. And cheaper. (The latter is only important for mass-produced items.) The disadvantage to using a resistor is that the brightness of each LED may vary a little depending on supply voltage. In other words, they may be slightly dimmer when the car is off (V = 12.6) verses on (V = 13.2 to 13.5). This won’t happen with a voltage regulator. And in case you’re wondering, there is no efficiency difference between the two. No matter what you do, I would highly suggest sticking a silicon diode in the circuit somewhere. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [November 19, 2002, 5:25pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/8 "2002-11-19T17:25:48Z")
connecting any kind of diodes in parallel is a bad idea. Better use independent limiting resistors for each LED or connect them in series. ---
**Author:** ![kryptonite2](https://avatars.discourse-cdn.com/v4/letter/k/85e7bf/32.png) [@kryptonite2](https://boards.straightdope.com/u/kryptonite2)\ **Post date:** [November 19, 2002, 5:29pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/9 "2002-11-19T17:29:17Z")
switched mode power supply for max efficiency. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [November 19, 2002, 6:11pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/10 "2002-11-19T18:11:56Z")
If you’re using white, blue or green LED’s, be careful not to reverse the polarity. They are awfully sensitive to reverse voltage. Also, most modern LEDs are rated for 20mA ea and 30mA is common. Here’s an auction (all I could find quickly) with some common LED specs. These are the specs for the individual diodes, not the flashlight being sold. [http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1790188687](http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1790188687) Quote: “Directivity(viewing angle); 20 degrees DC Forward Voltage; Typical: 3.6 Max: 4.0 volts DC Forward Current; Typical: 20 Max: 30 milliamps Pulse Forward Current ( Pulse width 10ms); 100 milliamps” Another helpful page: [http://www.plasma-ireland.com/lp/applications.html](http://www.plasma-ireland.com/lp/applications.html) If you have many questions, check out : > **[Candle Power Forums](https://www.candlepowerforums.com)** > > Come join out flashlight community. Flashlight reviews, and deals posted to our forum for free! My second favorite forum page 😉 All about portable lighting and full on bona fide experts. Personally, I would use te LM317 regulator. Constant output from 5-25 volts in. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [November 19, 2002, 6:36pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/11 "2002-11-19T18:36:28Z")
Another helpful site. [http://www.geocities.com/SiliconValley/Bay/4397/led/led.htm](http://www.geocities.com/SiliconValley/Bay/4397/led/led.htm) One question I haven’t resovled is the series or parallel concern. I would have thought that parallel would be the way, but this assumes a regulated circuit and same-chemistry diodes. It seems to me that eitherway, you would need some regulation for constant, high output… hmmmm. ---
**Author:** ![NutMagnet](https://avatars.discourse-cdn.com/v4/letter/n/ea5d25/32.png) [@NutMagnet](https://boards.straightdope.com/u/NutMagnet)\ **Post date:** [December 3, 2002, 1:35pm UTC](https://boards.straightdope.com/t/electronics-geeks-question/137502/12 "2002-12-03T13:35:28Z")
Sorry, haven’t been checking in lately. Thanks for all the good links and info.