LED torch (flashlight) conversion – design
I have this brinkmann 3xLED flashlight. I bought it in a walmart in the USA a few years ago for next to nothing. It isn’t very bright but the body is completely aluminium and the overall build quality is pretty good. I’ve been wanting to supercharge it for a while now so i bought some 3watt star LEDs from bestshop2008hk on ebay. My idea is to use one of these in the flashlight. I’ve put together a simple design to help accomplish this, including a way of heat sinking the LED to the torch case. Read more to see the design.
I’ve drawn a diagram of how the torch currently fits together:
The negative of the battery is connected to the case and picked up by the contact spring, which is pressing on the circuit board. The circuit board holds the +ve battery connector, a resistor and three LEDs
The power LEDs i have get quite hot so i want to heatsink them to the case of the torch. My design uses two aluminium discs cut from 3mm aluminium plate. I’ll cut these out using aviation snips and finish them using a flat file. I’ll also use the existing circuit board with LEDs and resistor removed for the power pick-up then simply run wires to the new LED and resistor.
To calculate the resistor value i had to look at two possibilities. The possibility of the torch using 3 standard AAA batteries and of it using 3 rechargeable AAA batteries. Standard batteries are 1.5v each (3 x 1.5v = 4.5v) but have a significant internal resistance to consider, whilst rechargeables are only 1.2v each (3 x 1.2v = 3.6v). The voltage drop across the LED is 3.2v and the current is 700mA. I did consider using a current regulator but decided there wasn’t room inside the torch.
Since the batteries have such different characteristics i decided to design with only rechargeable in mind. This meant a supply voltage of 3.6v. A quick trip to ledcalc.com shows that the nominal resistance would be 0.57 ohms. The nearest higher resistance is 1 ohm so i stuck with that. Another quick look at energizers data sheet archive shows that my rechargeable AAA batteries should last a little over an hour (possibly even 1.5 hours) at this discharge rate so I’m happy with that.
The design is quite simple, I’ll use arctic silver thermal compound between the LED base, heatsink plates (aluminium discs) and torch case and hold them together with hot glue. The thermal compound locations are market in red and the hot glue locations are marked in blue. I’ve removed the contact spring to save space. This has been replaced with solder blobs on the circuit board as this seems to provide a good enough electrical contact.
After the electrical modification I’ll also add a new lens to focus the LED.