How to match old-style 240v @ 13.7v AC tranny to match 12v DC LED light strip? - Page 1
Forum section: electronics / beginners

author: nzo
author_rank: Regular Contributor
timestamp: November 09, 2011, 07:25:31 am
content: Hi folks ~ another question if you don't mind...Rather than commit my mother's bedside light to a landfill, I want to see if I can hack it to power a nice bright LED strip (this one) which runs @ 12v DC.My tranny puts out 13.7v AC and I have one of those square-block rectifiers for DC but so far nothing else.The LED product info does not give the LED config (series/parallel) or the forward voltage and I'm stuck on how best to drop the volts to 12v DC. I realize a cheap switch-mode PS would be the most efficient but I hate to add to the landfill and prefer to work with what I've got.Is an LM317 the way to go, or will a simple series dropping resistor do?This LED strip seems more complicated than a simple LED and I don't know how to calculate the appropriate resistance/wattage for the array.Thanks for your input!

author: IanB
author_rank: Super Contributor
timestamp: November 09, 2011, 07:35:09 am
content: The LED strip looks like it simply contains 9 emitters wired in a 3p3s arrangement. There does not appear to be any electronic regulation.Your transformer should be fine as long as it can supply 1 A continuously without overheating. You will need to rectify it and then figure out a low value current limiting resistor. As a starting point I would suggest the following calculation, but you may need to adapt it depending on the results you get by experiment:Drop 2 V across the resistor at a current of 1 A.Resistor value therefore is 2 ohms.Power requirement of resistor is 2 W.Hence, try out a 2 ohm 5 W or 10 W power resistor in series to limit the current. If the LED strip is too bright or the current is too high (measure the voltage drop across the resistor), try a higher resistance like 3 or 4 ohms. Remember the resistor will get warm, so place it where air can circulate around it and make sure it is not touching anything that can be damaged by heat.

author: nzo
author_rank: Regular Contributor
timestamp: November 09, 2011, 07:57:23 am
content: Wow! That was fast IanB.Thank you

author: Psi
author_rank: Super Contributor
timestamp: November 09, 2011, 10:18:10 am
content: Be careful connecting the led strip to your transformer+rectifier+cap if it's powered at the time and the cap is large.With no load the transformer will charge the cap up to ~20V. (sqrt(2)*13.7)If you then connect the leds they will draw massive current for a millisecond or so until the voltage stabilizes which could damage them.You really want to power it up with the load already connected to avoid this.

author: IanB
author_rank: Super Contributor
timestamp: November 09, 2011, 11:07:36 am
content: No cap. Just rectifier.

author: Psi
author_rank: Super Contributor
timestamp: November 09, 2011, 11:26:41 am
content: Quote from: IanB on November 09, 2011, 11:07:36 amAt 50hz and no cap on the rectifier you will see flicker out of the corner of ya eye.But yeah, there's no peak voltage issue if a cap is left out of the circuit.

author: IanB
author_rank: Super Contributor
timestamp: November 09, 2011, 11:51:23 am
content: Quote from: Psi on November 09, 2011, 11:26:41 amFull wave rectified 50 Hz AC will produce 100 Hz DC. I doubt flicker will be a problem. Certainly no worse than a fluorescent tube.

author: nzo
author_rank: Regular Contributor
timestamp: November 09, 2011, 10:10:09 pm
content: You guys are a great help - much appreciation!

author: Psi
author_rank: Super Contributor
timestamp: November 10, 2011, 12:26:01 am
content: Quote from: IanB on November 09, 2011, 11:51:23 amTrue, 100hz should be fine.Try it and see i guess. No point adding a cap if it doesn't need one.