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Wood Lamp / High Power UV LED Flashlight

 

Anatomy of a High Power LED flashlight

(would also be a fitting title here)

 I went with components I had because I made this in a haste. 

The DC to DC converter is massive Overkill but what you have is usually the cheapest and the yellow lantern body afforded the space easily.



Lithium Ion Batteries and Protection.

The photo bellow is a larger version of what's wrapped in painters tape above. It's an 18650 cell holder wired in series. The pic above is a 2 cell bellow is 4. The pic bellow is to show the primitive BMS I included. Essentially the same thing but a little smaller than the one bellow.
The purpose of a board seen above is to prevent over discharge of the cells. On the left side of the pcb the wires attach between the cells that comprise the battery pack. This allows the chips on the board to read the voltage of each cell as the battery is discharged. 

Why is this important? Buy x batteries made by same plant same day and they will all have slightly different capacities. Also lithium ion has this nasty tendency to misbehave(boom) if overly drained. The key is turning off the battery before any of the cells goes bellow 3.2v*

 Bellow that voltage it causes little stalactite like formations called dendrites inside of your battery cell. If those grow long enough they internally short positive to negative you have a battery fire. 

The board above shuts off the power when the first cell drops bellow 3.2v. the one in the flashlight does the same but with 2 cells instead of 4.

Voltage indicator / Battery Gauge

For further protection and at a glance battery level indication; I included a 3 wire voltage indicator/meter. 

(Nice and level lol)
2x series cells is:
 8.4v at full
7.4v nominal
6.4v dead/cut off
Aka in this photo I'm almost out of juice.

Amazon wants 10 usd for 5x. They can be had  cheaper on ebay if you are willing to wait. I used to have a bunch of these in stock 


This also functions as crude  battery gauge. The main reason it's there is the protection board costs about a buck fifty from china. I trust it accordingly. Or in other words this is a way to double check. But it also adds the benefit of being able to estimate how much longer the light will run for.
 

In general you assume one lithium ion cell(a single 18650) is 4.2v full, 3.2 empty.

 They are sometimes labeled 3.7 or 3.6 because that's the voltage they sit at the longest while in use. Another name for that is the nominal voltage 

In the light above I have 2x in series the positive side of one is connected to the negative of the other. In this config the voltage = #of cells * volts per cell.

Or in other words the pack is full at 8.4v spends most of it's time around 7.4v and gets cut off at 6.4v.

The 3 wire voltage indicator is an at a glance measure of battery remaining.

Here's the cells removed to recharge.

There's a lot I could have done to make this
particular construction a lot safer. That said it was thrown together from my UV lithography rig in a hurry. But that's a story for another day.


Driving a high power LED

(With out a current limiting resistor or rediculous ly priced flashlight driver board)

LZ1-00UV00



The flashlight community has a term driver board. 

 A more accurate term would be a DC to DC converter with constant current mode.


Which is exactly what I used and is pictured above.

To be fair a lot of the ones intended to run flashlights come with a little bit of logic/mcu(micro controller unit) to do cool things like read a button press to select a power level.

 Meh.

Why do LEDs Need a Driver?

 LEDs have this really nasty tendency. the nasty bit is the hotter they get the more current they draw (internal resistance drops).

 current is expressed  as e/r=i. Aka volts/ohms = amps/current




If E (volts) remains constant and R (ohms) approaches 0 (decreases)

what happens to I (amps)?

 I = Bigger number?
fried LED = yes.

CC mode allows you to preprogram the driver/psu board with a current limit and a max voltage. 

The logic incorporated into the dc-dc power supply will see the circuit resistance drop 

(as the LED heats up)
 and it responds by dropping the circuit voltage.

This keeps I stable at the programed max.

LEDs are current driven devices so you won't notice brightness change with voltage fluctuations from the controller. 

All of that was a high level overview not intended to be exact but rather..the gist of it.

What was the old way?

A current limiting resistor. 

Basically a guaranteed min ohm/r value infront of the LED.

Ie if you know cold led is going to be (numbers out my ass/for max illistration)
0.5 ohms and hot is going to be 0.1 ohms..
Stick 10.0ohms in front and you've made the variation somewhat meaningless.

You've also made a hot resistor at high power levels.

This works well for a few ma but when your led runs on batteries or draws amp(s) you get a lot of power wasted into a hot resistor. You also get very little in terms of battery life.

The DC to DC CC mode driver's are vastly more efficient

I've entered all of this on a smartphone otherwise I'd give more realistic numbers or do calculations. I'm literally sitting in what's essentially a mental ward haven't been stripped of most of my property and most of my rights.



This particular dc to DC board with cc
Was sourced from eBay.
 unlike many ebay parts the IC that powers that powers this one comes from a very reputable company.(also one that's very awesome to work with) The company I speak of is called LTC or linear technologies.

If I get back into digikey or eBay accounts I could look up the exact part number but I think it was like the 37xx or something along those lines. It's a nice little board for what it does. I was using the same chip in one of my commissioned pcb designs 






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