Halloween Forum banner

1 - 17 of 17 Posts

·
Registered
Joined
·
438 Posts
Discussion Starter #1
I had some old LED flashlights laying around that I no longer use and I found some $1 ones at a Harbor Freight tent sale (that's 50 cents each). http://www.harborfreight.com/led-flashlight-mini-2-piece-9-led-3-12-69065.html I've cut them in half & soldered wires to the + and - so I can power them with a wall wart. They all take 3 1.5V batteries, but the problem is finding 4.5V transformers that can handle more than one or two lights - I would like to have a little distribution center.

I have loads of 12VDC & 5VDC sources (like computer power supplies) that can handle large loads like 20A. I measured the draw of the HF flashlights at .48A at 3VDC, but (I think) it was also .13A at 5VDC. Without additional resistors, some of the flashlights flash on & off with 5V (actually about 5.2V), like they're getting too much juice and I don't want to burn them out.

I've used the LED resistor calculators for plain resistors, but these are multi-LED lights which are fine (resistor less) at 4.5VDC, but what would I need to hook them to 5VDC and 12VDC? Or, any other thoughts? Thanks!!!
 

·
Human Candy Shovel
Joined
·
1,107 Posts
At 50 cents each, I would sacrifice one for destruction to examine what electronic components are in the flashlight. There very well may be resistors integrated already that are dropping the voltage from 4.5 to anywhere from 3.2 to 3.6, as well as finding any other unexpected surprises along the way.
 

·
Registered
Joined
·
237 Posts
how many batteries and what size does a flashlight take? thinking they take AA i think those are 1.5 vdc then add up the batteries voltage, should get you what it takes to run.
 

·
Registered
Joined
·
237 Posts
volts stay the same it's the amp draw that increases the more lights you have so a 12v wall wart will still send 12v to each light not lower the voltage.
 

·
Registered
Joined
·
63 Posts
Batteries have a small amount of resitance which is why they work being directly connected, but you should open one up to check. I am surprised by your measurements of the current at 480mA at 5V, I would have expected something around 25mA @ 5V

These multi-led lights are hooked up in parallel so the Vforward is the same, about 2.1V but the current is tripled, about [email protected] So use a 5V supply and a 47ohm resistor for each light. Your numbers indicate three LEDs but if there are four then use a 36 ohm resistor per light.

You could also just cheat and see if your PS has a 3.3V supply and use that directly. The lights will get hot since all the power drop will occur accros the LED but they should last for some time. The resitor is your best bet.

Also the lights should be hooked in parallel so go from the + on the PS to a resistor to the + on each LED and then connect the - lines. EACH LED needs it own resistor.

You could also try the 12V line but you need to hook three lights in series so PS+ to LED+ and LED- to next LED+ and so forth, this setup seems like a hassle to me.

-Tim

You don't need to use the 12V line as 5 volts is enough, you just need to worry about the current. If you PS has 10A on the 5V then you can power all the LED lights you want from this supply.
 

·
Human Candy Shovel
Joined
·
1,107 Posts
Batteries have a small amount of resitance which is why they work being directly connected, but you should open one up to check. I am surprised by your measurements of the current at 480mA at 5V, I would have expected something around 25mA @ 5V
25mA would be closer to the draw for each LED on these 9-LED flashlights, rather than the full load. Obviously, between the 4.5v current of the three batteries and the substantial mA drain, these flashlights are wired in parallel. Still, 480 mA for 9 LED is rather high, in excess of 50 mA per LED.

Anyway, I think the solution here might be to get some sort of step down transformer. I have a few small ones from ebay that can have the output voltage dialed down as low as 1.5 volts, while the source voltage can be anywhere from 4.5 to 36 volts. They're small, so they top out at a 4 or 5 amp load. They're cheap, running as low as $4 per unit if you search hard enough.
 

·
Registered
Joined
·
1,329 Posts
I agree with tvromero... 480mA is crazy for an LED light, regardless of how many LEDs are in there. If you ran that from batteries, I doubt it would last an hour!
I would check that measurement again... and the difference between 4.5 volts and 5 volts is so small I would be very surprised if you couldn't run them from a 5 volt wall wart.
 

·
Registered
Joined
·
237 Posts
I agree with tvromero... 480mA is crazy for an LED light, regardless of how many LEDs are in there. If you ran that from batteries, I doubt it would last an hour!
I would check that measurement again... and the difference between 4.5 volts and 5 volts is so small I would be very surprised if you couldn't run them from a 5 volt wall wart.
and odds are the 5 v wall wart is only putting out 4.5 v
 

·
Registered
Joined
·
438 Posts
Discussion Starter #11
Thanks, folks. Each flashlight has 9 LEDs in it and all of the electronics are contained in the half inch at the top, along with the LEDs, so it's *very* compact with nothing recognizable.

tvromero, I hooked up my Fluke Multimeter according to the manual and the digital readout said 0.48...which I read as 480 mA at 3V. Seems wrong, yes, and I've tried it since and cannot replicate it...I've also seen it flash 0.12. I must be doing something wrong and/or just going nuts. I have plenty of resistors, but I've had some trouble with making them work at times. I thought I was fairly electrically inclined. Ha. Here's what I tried today:

I hooked one LED flashlight to a 5V wall wart that, unloaded, showed 5.15V and loaded, showed 4.9. After 5 minutes, the LED went out. I disconnected it and tried it again later and, it came on, but 3 of the 9 LEDs were out. My ATX PSU is now giving me grief even with a small fan(load) on it (the power is pulsating by 3V + and - about twice per second), so I can't test the 3V to see if it will run the LED without an added resistor. That would be my preferred method at this point...no resistor with the LED flashlight running straight from the PSU.

I had no trouble last year using 12VDC through a resistor (470 Ohm, I think) to 3 bare LEDs that I soldered...so I don't know why this is so different. I'm just replacing the 4.5VDC alkaline batteries with a near equivalent, right?
 

·
Registered
Joined
·
63 Posts
I hooked one LED flashlight to a 5V wall wart that, unloaded, showed 5.15V and loaded, showed 4.9. After 5 minutes, the LED went out. I disconnected it and tried it again later and, it came on, but 3 of the 9 LEDs were out.
Those three LEDs are gone, they are now a stuck open, the other ones will break as well if you continue to expose them to 5V without a resistor.

9 LEDs on a 5V supply would need a resistor of 15 or 16 ohms.
6 LEDs on a 5V supply would need a resistor of 24 ohms
 

·
Registered
Joined
·
438 Posts
Discussion Starter #13
Thank you, tvromero. If I can ever get the 3V line working again, could I use that without a resistor? And, what size resistor would I need for 12V? Or, is there a good resistor calculator that I could use, instead of bothering you? I've tried several and wasn't sure what values to put in there, because of the unknown array. And, I certainly didn't get any of the values that you posted.
 

·
Registered
Joined
·
1,329 Posts
You always need a resistor of some kind. And its value depends entirely on how the LEDs are wired together, what their forward voltage drop is, and what your supply voltage is.
 

·
Registered
Joined
·
63 Posts
there a good resistor calculator that I could use, instead of bothering you?

No bother, you seemed to have found the right calculators but you just don't have the information needed to feed it.

The basics for one LED light are:
- a LED has a forward voltage drop of about 2.1V, but this is a just a guideline, each color and manufactory will be different but 2.1 is a good estimation.
- a LED needs about 20mA of current to operate, to much more will blow the LED, less will dim it, this has the same color and manufactory differences.
- To figure out the resister needed for one LED you do the following. Resistor = (PS_Voltage - forwad_voltage)/(Current)
Ex: (12V-2.1)/002 = 495 ohms
- There are two ways of hooking up LEDs, one is serial and other is parallel.
- Serial means that the plus in each LED is hooked to the minus in the next and so forth
- in serial you need to make for the forward_voltage times the numbers of LEDs but the current is the same
EX: 3 LEDS - (12V-(3*2.1))/002 = 285 ohms
- Parallel means that each LED plus is hooked to each LED plus and the negatives are connected together (This is the most common)
- in parallel the current is multiplied by the number of LEDs but the forward voltage is the same
EX: 3 LEDs - (12V-2.1)/(3*0.02) = 165 ohms

All of this is based off of estimations, these should not blow out your LEDs but to get them as max brightness you need to specific LED parameters.

Good Luck
-Tim
 

·
Registered
Joined
·
438 Posts
Discussion Starter #16
Thanks, tvromero! That really helped. I did find this online calculator which looks like it translates your equations: http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

I was confused, so it may be helpful for other folks to know this when using online calculators:
"Forward Voltage" (specification from the LED manufacturer) = "Voltage Drop Across LED" (on the online calculators)

At least, I hope that's correct. :)

Also, I figured that since these LED flashlights run off of 4.5VDC, that I wouldn't need a resistor if supply voltage was equal or less than this. CreepyCreations, I see your "always need a resistor" comment, so is that a bad assumption on my part?

Well, Harbor Freight is having their annual tent sale this weekend and I picked up several more 9 LED flashlights. They're $2 for two this year, not $1 like last year, but it's still a decent deal...so I'm off to the races to keep getting my whole setup running with these little flashlights. I probably should do a full tutorial, but basically, I do this:
-Cut the flashlight in half (I use an angle grinder with metal cutoff wheel)
-Solder wire to the center spring (goes to + on PS)
-Drill small hole in aluminum side, run small screw, attach another wire to screw, tighten, solder a little if possible (goes to - on PS)
-Stand LED flash light on table so opening is pointing up, put small amount of glue inside (~ 1/4") to cover all exposed circuitry, allow to cool
-bend up one end of half of a coat hanger so it will fit into flashlight and allow for grip by hot glue, set inside flashlight barrel so it does not contact the center pole, fill with hot glue, hold (or brace) until glue is cool
-wrap up everything with electrical tape

Now, I will need to add the part about adding a resistor... LOL
 

·
Registered
Joined
·
1,329 Posts
Fearington... Yes, every LED needs a resistor somewhere in there. Some, like those flickering tea lights, have the circuitry built into them, so you can't actually see the individual components, but they are in there. :)
 
1 - 17 of 17 Posts
Top