Dimming 12V LED’s

Hey everyone, I have a question about dimming LED lights. backstory, a friend of mine bought a motorcycle in pieces, it's mostly brand new, with a few used parts here and there. he knows how to put bolts together, but not how to strip wires, so he's asked me for help with the electronics (simple stuff like lights) which i'm pretty confident on, for the most part, however there's a few things i'm not sure about. He has these two lights he wants to use as brake/turn/tail lights, inside are two small wedge base bulbs, look like 194's, they are different, but I can't read the number on the second one, the previous owner, or someone, had crimped the + wire for both of them together, so they were just a signal/brake light, with no tail function, my buddy wants to switch them over to LED, so I suggested the easiest way would be drop in LED replacement bulbs, the only thing is that most of the LED bulbs you find on ebay don't show the power consumption or output, so it's a guessing game as to what brightness they will have... My plan is to put two of these bulbs in the housing, and with the way the housing is designed the bulbs sit one behind the other, with glass incandescent bulbs, it's no big deal, but I'm hoping that if I put two identical lights in, the rear one will have most of it's light blocked and will be much dimmer, and be enough of a difference for a tail light, and leave the second bulb as turn/brake. this of course is a trial and error type thing, if it doesn't work, i'll need a way to dim the tail down, so it doesn't flood out the brake light, which brings me to my main question.. Can I use a POT to dim the bulb down to a tail brightness? I tried looking through the net for this answer, but I kept finding convoluted answers talking about how you need to put a 20 ohm resister in with the pot so you don't blow the LED, but these drop ins are designed to run off of 12v, i'm assuming they have resisters inside them... so yeah, can I use a pot to dim down an LED? I've heard something about how a resister in series won't dim it, you need one in parallel? thanks in advance for your help!no signature needed

You are correct... If they are 12V automotive rated LEDs (as opposed to 12V LEDs) they won't need any extra resistor for current limiting. (Obviously automotive bulb replacements should be designed for automotive 12V!) An external resistor should work but you'll have to experiment to get the correct brightness. I'd suggest a resistor for each LED (bulb) - you may as well dim both. Try any resistor up to ~1k (Ohm) to start with and be wary of hot resistors if under a few hundred Ohms. To function as a stop/tail is easy. Assuming +12V from the brake switch: Tail-light +12V to resistor. Other end of resistor to LED +ve and LED -ve to GND. Brake-switched +12V straight to LED +ve, hence bypassing the "dimming" resistor when braking. You'd probably want about 20 to 25% of the brake current for taillights - ie, 20-25% of brake brightness. Unfortunately it's tricky calculating the resistance required to achieve that. (To drop from 1/2 bright to 1/4 bright does NOT mean double the resistance.) If there are other lamps/LEDs connected (side clearance perhaps?) or things that monitor either circuit (eg alarms, checking systems, cruise controls, airconditioners or spa and sauna heaters, nuclear triggers) you might have to insert a diode or 2 to isolate tail power from stop power etc - ie, brake +12V may feed thru the resistor (try to) and light headlights & front parkers. If so, 1N400x diodes should be fine unless the bulbs (LEDs) consume more than 1A, but that is doubtful. And also if so, the tail diode should be added BEFORE trialling a suitable resistor. PS - putting a resistance or anything in PARALLEL has no effect on current thru the thing, it only adds to the total (current) load. (Exceptions - current limited supplies, and parallel short circuits.)

ok cool, that's what I thought, my dad was trying to convince me otherwise... the plan right now, if the bulb blocking method doesn't work, is to hook up a potentiometer to the tail bulb + lead, and turn the resistance up, to the point that the bulb is at a desirable brightness, takes guess work of the resister values out. I'll probably run one pot for both bulbs, just so I only have 1 adjustment, and they will be the same... last but not least, i'll make sure to get one much bigger (amps) than I need, so it won't burn out. Again, thanks oldspark!no signature needed

You'll probably find blocking doesn't work - shaded intensity is not necessarily the same as dimness. But if both LEDs are the same, I'd suggest the pot on one only to determine brightness - tho that assume my above "shading ain't dimness" holds true. But even if it doesn't, you'll probably find the dimness (resistor value) of each LED will differ. Or maybe only one LED needs to be dim(?). If you use one pot for BOTH LEDs, keep in mind that the resistance for each LED will be double that resistance. But note that I am saying one resistor per LED (bulb) for a constant dimness if one LED fails.