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Led help.


Jimrubbers

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Jimrubbers

I'm trying to convert a maglite torch to led and recgargable. 

I have a 1w led 350ma

I have a 5v battery.

Led voltage drop is 3.4v

I worked it out that it would need a 4.7ohm resistor.

I connected everything up to the battery and the resistor got that hot that it melted the insulation. 

 

Is there any reason this should be happening?

 

Any help greatly appreciated.

 

Heres the link to the led

https://www.ebay.co.uk/itm/141679345010

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Bruspark

So assuming the LED is as described and you are going to use the maximum current (this risks problems with heat sinking and thermal runaway) the first question is did the LED light?  if so then the wiring seems to be correct

At 5v  4.7R and  3.3V the resistor would need to dissipate 0.56 Watts as heat.   It will also need to be in pretty cool air to do that without overheating itself - not stuck in sealed box with a Hot LED chip for company. 
 In those circumstances a resistor rating or 1.5w or more could be needed to avoid resistor problems but the heat is still in there even with a bigger resistor size/rating.  Likely this solution won't fit

Also you are wasting 1/3 of your battery power heating the torch case in addition to the unavoidable heat out put of the LED itself.

Generally (cheap) LED Torch makers have only used resistors up to around the 0.5W to LED level (you can still use the same LED module but the whole thing is horribly power inefficient and 40% is dissipated in heating a 12R resistor with 0.3W

The resistor design is also very poor at maintaining light output as the battery voltage falls (or protecting the circuit if the battery is initially say 6V)

Most decent LED torches therefore use a digital power controller that uses Pulse Width Modulation to vary the power to the LED while dissipating little itself (it's mostly either on or off)  and including thermal protection for the LED and a better regulation curve as input voltage falls.

( dedicated torch driver controllers are recommended - Just going for 'constant current' may plunge you into darkness quite quickly at the end of the battery curve while many dedicated torch drivers will drop power steps or flash at battery end to ensure  a safe conclusion) 

Edited by Bruspark
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Bruspark

Also before going too far make sure the LED module you have will work in the existing reflector -  you'll need the LED chip in the position where the Bulb's filament was (which may be tricky) or you will have a very uneven /doughnut beam

I converted a couple of old-favourite maglight mini's years ago using digital modules and LED's with special replacement reflectors that were available back then.   

(These days I get my torches from this outfit and they do make some really superb ones https://www.fenixlight.com/ )

 

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Jimrubbers

I modified the original bulb and mounted the led in place. It works I have tested with the battery just not for long without heating up the resistor. So should I go for a torch driver then? Is there anything else I can do?

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Bruspark
On 21/11/2021 at 12:41, Jimrubbers said:

I modified the original bulb and mounted the led in place. It works I have tested with the battery just not for long without heating up the resistor. So should I go for a torch driver then? Is there anything else I can do?

If you reduce the design current to 100mA you can make it work with a resistor.    
 

if your battery was 3.6 V some cheap Chinese torches will just stick the led across it with a smaller value resistor as ‘sort of protection’ until the voltage drops 

(expect to ruin a few LED and batteries optimising  that though)


 

Above about 4v of supply you can’t make a ‘good’ stable, consistent light output design with resistors. They waste too much energy, overheat etc.  

 

Controller /driver boards aren’t all that expensive but I can’t guess how many you need to order to find one that fits, works as you require and has the appropriate input voltage range and output currents
 

 

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