DIY LED Bulb -- converting a 240v circuit to 120v

S

starhawk

[H]F Junkie
Joined
Oct 4, 2004
Messages
8,908
OK, so sometime in the Precambrian era (by which I mean a few years ago) I tripped over a YouTube vid on building a homemade LED light bulb.

Today I found it again, with the help of what I'd scribbled down a few years ago when I saw it last. That video is here --> http://www.youtube.com/watch?v=TZYMc_IzBqk

It's not very clear so I've put together a quick schematic in CorelDraw...
http://i.imgur.com/bbjmkpC.jpg

Trouble is it's for 240V and that's entirely the wrong side of The Pond for me ;) I'm in the States where it's all 120v unless you're plugging in a dryer or some air conditioning units or heavy industrial equipment.

So how do I make this circuit good for 120V?
 
Do you already have a bunch of LED bulbs sitting around? The parts to make this will probably cost more than just buying one at a store.

For example, I've got most of my house lighted with this; in my area they're $25, in other states subsidies may make it significantly cheaper.
 
I'd rather inhale solder fumes than buy that ;) I don't like the look of that model; the ones available locally (WalMart) put out bugger all light (seriously, the glow is almost barely noticeable...), and as an added bonus, DIY means I know how well it's made.

On top of that, I'm planning on putting mine together in a long translucent tube if I can, sealed thoroughly at both ends -- the better to light the laundry alcove with ;)
 
Lowe's is the next town over and I don't drive ;) plus public transit is shit around here.

Advice on the circuitry...?
 
As an Amazon Associate, HardForum may earn from qualifying purchases.
starhawk, are you noticing a trend with your threads yet? :D

Yes, it can be done. But it's not worth it. I second Fenris_Ulf's suggestion
 
Insurance companies will always look for the slightest excuse not to cover you. That's how they make their money.

I don't think I want to live my life in fear of an insurance company tho. For now I'm interested in how to alter the circuit. Later I'll decide if I want to build it, having been fully informed of the risks that I take in doing so. (Just remember, please, it's MY risk, not yours!)
 
Over my workbench I use a G17974 LED panel from Goldmine Electronics (it's sold out now, Google for pictures). It's 24v, so I use a $3 boost converter from ebay and a 19v surplus laptop power supply. The parts I wired are all low voltage and the power supply is UL rated. Nice and safe.
 
Still not answering my question.

I've posted a comment on the YouTube page, maybe they'll be in the mood to help me out.

I really do want to modify the posted circuit and have it work, rather than anything else... while I appreciate the concern, it's my safety at hand rather than yours... I'll take the risks, with full knowledge of what I'm getting into. You've informed me there, now please help me do what I'm wanting to do ;)

FWIW I do not intend to open a CFL and reuse its housing -- I'll make my own, probably with a cord rather than a bulb base, thank you just the same!
 
Well, I can't help you out right now, YouTube and imgur are blocked by my work so I can't see the schematics.
 
Wire up the LED's like this
Two banks of 21 LED's in series
Then wire both banks in Parallel.
 
What I'm trying to figure out is how to alter the equation to find R when the voltage is 120v instead of 240v. Should I just halve the resistance? should I double it? or is it more complex than that?
 
That's easy. Each LED has a forward voltage drop. For white LEDs it's about 4-5 volts. Most 5mm LEDs like about 10ma of current. You can run them up to about 20ma each, but you won't get twice the light output. Above 20ma the lifespan drops quickly. The manufacturer should have a spec sheet that gives you pertinant data. Multiply the number of LEDs by the forward voltage drop (I'm assuming you're running them in series) and subtract from your DC supply voltage voltage. I'd recommend for safety's sake not exceeding 90% of the supply voltage. Take this remaining voltage and use V=I*R to calculate series resistance for the desired current. Let me give an example:

Let's say you wanted to wire up a string of cheap red LEDs to run off your computer power supply.
Supply voltage: 12V
LED forward drop: 1.8v
Current at that voltage: 10mA (.01 A)
We'll put 6 LEDs in series for a total of 6 x 1.8v = 10.8v drop
Subtract that from 12v and you have 1.2v remaining
Putting that into V=I*R you have
1.2= 0.01 * R
R = 120 ohms
 
Thank you tremendously -- that's EXACTLY what I wanted to know.

If I had a spare cookie right now... ;)
 
Just plug 120 into the formula on the schematic. It's just the standard V=IR formula rearranged to solve for R instead of V and accounting for the voltage dropped across the LED array.

If you just halved the resistance you most likely wouldn't have enough current left to power all of the LEDs. I did a quick example and at 240v the circuit required a 10k resistor, but dropping to 120v required a 2k resistor to maintain the requested current.
 
A quick recommendation - if using AC line power, please include a fuse as close to the socket as possible. I build tube amps as a hobby, so I'm quite familiar with working with high voltages and the safety aspects that need to be followed.
 
I can do that. I have a special loathing for fused cords (the fuses are so fiddly to replace -- if you can find the damn tiny things in the first place), but I'll see what I can come up with ;)

Tube amps? Cool! My late uncle, in his younger years, worked for RCA designing tube radios. He was a smart man.
 
Old christmas tree light strings are a great source of cords with fuses built in to the plug. If you do build this, I'd recommend a capacitor (a few uF in value with at least 200v rating) in parallel across the string. That will reduce 120Hz flicker, which may or may not be noticable. Also, realize that you will have 170v DC after the 120v AC is rectified, so do the math accordingly.
 
The extra 50V is due to the rectification of the AC. The rectifier essentially flips the negative portion of the the sine wave to positive, so the RMS value goes up.
 
RMS = root mean square. AC is constantly changing in a sine wave where the voltage is measured as the average and not the peak (it makes calculating power easier). Since it's a sine and not linear, the peak value is 1.414 times greater than the RMS average. Wiki "rectifier" for a good explanation of how it's measured
 
I think I get it well enough for these purposes. I'll look up that article another time.
 
Man, I leave for a week and the thread explodes.

@starhawk: easy solution to go from 240V to 120V: halve the number of LEDs in the string, and halve the resistance. Done. I still recommend the low-voltage option.
 
A simple 120v 10w supply and a 10w led will likely suffice for,this application... Comes in at 10$ on ebay too. 2700k, 3000k and 6500k to choose from.
 
You must log in or register to reply here.
Back
Top