Hello everyone! Ignoramus back again with probably a basic question that I can’t seem to find the answer for online. Web searching has led me down a pretty deep rabbit hole that I don’t understand.
I ordered Cree LED’s on a triple MCPCB and I am trying to wire them to my daughter’s ride-on toy as headlights. I am obviously going to wire two of them in parallel to the 12v drill battery, but I had a couple questions regarding resistors and “thermal runaway.”
Long story short, if I were to wire two of these MCPCB’s with 6x total LED’s in parallel, then wire to a toggle switch and to the 12v battery, would I actually need a driver? Would the lights just get brighter and brighter because if I’m understanding electrical currents correctly, wiring the lights in parallel would only draw 3v max each so I should use a resistor, or is there a default voltage they set to without a driver?
Last, in this formula R = (V1 - V2) / I to figure out resistance needed, V1 would be 12v, V2 I believe would be typically 2.85-3v since they’re parallel? What would I be? I couldn’t find any info for a “default” but looking at the Cree datasheet would it be 1A?
Presumably, all the LEDs on the board would be in parallel, so would want ~3V(ish). Connect them to as 12V battery, and they’d instantly go blue then go pouf.
You’d definitely need a buck driver to run ’em. And if they’re not securely connected to a heatsink, you’d have to limit the current to something not very bright at all, triples or not.
You can grab a pair of cheepcheepcheep 9LED craplights, pull the boards, and wire them in series to suck down maybe 6V or so. Then maybe 200mA total out of the remaining 6V, R=E/I or 6V/0.2A or 30Ω. P=EI or 1.2W, so find a 5W 30Ω resistor (standard value). Done.
Or buy a strip of 12v LEDs.
Chop off however many you want to make the toy look totally cool.
Front, back, sides, bottom?
Wire them up directly to 12v.
They come with sticky tape on the back, so even mounting them should be no big deal.
Add a switch to turn them off when the toy isn’t being used.
No Muss, No Fuss, And away you go.
All the Best,
Jeff
Oops, just noticed you already ordered the LEDs.
As LB says, you will need a buck circuit and heatsinks.
It might be easier to get a cheap adjustable buck circuit from someplace instead of trying to put a flashlight driver in the works.
All the Best,
Jeff
Thanks for your reply! As far as a heatsink goes, how big would one have to be? I don’t want it to be really bright, probably somewhere around medium on the fw3a default settings. I do have some copper rod that I use to make spacers for convoy s2 triple builds; are we talking to same size as one of those?
Also, would a buck driver like This one possibly work? Would I need one per triple, or one for both?
And if I’m understanding this correctly, I could set the output voltage to something like 3v on the buck driver to keep a constant brightness?
No idea how to set the current on a buck-driver, as they’re all different. Might be able to change a sense-resistor.
The heatsink should be pretty big, because at least in a flashlight, your own hand wicks away excess heat. Leave a light tailstanding undisturbed, and it’ll roast. Might fashion some sheetmetal as a heat-spreader.
It looks like the one you linked to is an adjustable voltage supply. With a 5A rating.
Usually with these sorts of things the current capacity is way overrated.
If you read the fine print they list 1.5a max.
But since you are getting a 5 pack, one for each light would work (I think).
The way to limit current is to reduce the output voltage.
So I believe (notice the word believe) you could put the 12v on the input side.
Then hook a voltmeter to the output. Before adding the LEDs.
Crank the voltage down to about 2v. Then add the LED - With Heat Sinks attached.
Slowly increase the voltage, while watching the LED and the output volts.
Stop when they get bright enough. And don’t exceed the LEDs max voltage.
Keep a close watch on the big chip on the Buck supply.
If it gets too hot to touch, better turn that sucker down.
This:
Might be a better choice. Higher current rating and has current control. But you are still going to have to come up with some heatsinks.
A hunk of metal (Aluminum if possible) about the thickness of a quarter and about 3 inches across should do it.
A little heat sink compound between the LED board and the heatsink and a couple of screws to fix the LEDs to the heatsink.
Thanks so much for all the info!!! I just ordered the bigger buck supply. No problem making a heatsink, I’ve got plenty of material for that, haha! Maybe I’ll try to fabricate a “cold air intake” to cool off the lights. Thanks for the link to those LED’s. I might have to check those out if this project ends up in flames, or burnt LED’s. I’m treating this more like a learning experience for myself and I’m learning way more than I ever thought I’d need to know about electricity!
Thanks for the link to those LED’s. I might have to check those out if this project ends up in flames, or burnt LED’s. I’m treating this more like a learning experience for myself and I’m learning way more than I ever thought I’d need to know about electricity!