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Re: [Shop-talk] led flashlight troubleshooting

To: "John Niolon" <jniolon@att.net>, "shop-talk" <shop-talk@autox.team.net>
Subject: Re: [Shop-talk] led flashlight troubleshooting
From: "Arvid Jedlicka" <arvidj@visi.com>
Date: Wed, 25 Dec 2013 08:40:09 -0600
Delivered-to: mharc@autox.team.net
Delivered-to: shop-talk@autox.team.net
References: <03BDFD91AA5A48CDB2F2AB299B932B3D@john5043a2d406>
I do not have that specific flashlight but have experienced intermittent and 
DOA with other small imported flashlights.

In all of my cases it was related to the way the PCB that the LED mounts to 
returns current to the battery. The PCB is a circular affair which has a pad 
in the center of it that presses against the center battery\battery holder 
post. This provides one side of the circuit. The outer diameter of the 
circular PCB also has pads on one side of it. Think single sided PCB, not 
two sided or with plating on the edges of the PCB, but only on one normal 
flat side of the PCB. These pads on the flat part are suppose to contact a 
"ledge" in the the metal body of the flashlight. This ledge provides the 
return circuit and also prevents the PCB from falling into the body of the 
flashlight.

The current flow is "battery -> pad in the center of the PCB -> LED and 
related current regulating circuits -> pads on the flat outer diameter of 
the PCB -> ledge on flashlight body -> flashlight body -> switch in the cap 
on the end of the flashlight body -> spring in the switch\cap -> other end 
of the battery.

In all cases my failures were related to the PCB being a very poor press fit 
into the flashlight body and the outer diameter pads not contacting the 
ledge. If I would press on the lens to push the PCB into the body they would 
light up. Note that "press ..." did not actually cause the lens to move any 
discernable amount. All is seemed to do was press the outer diameter 
contacts down hard enough to actually press against the ledge and work.

The pressing incantation seemed to permanently fix some of them but for 
others it was 'only worked while pressed'. Needless to say those that would 
only work if I had my finger on the top of the lens were classified as "did 
not seem to fit the spirit of useful flashlight design" and were trashed.

Yours may have much better construction techniques.

Arvid
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