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Why does my LED flash light has a transistor?

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Kibelon:
I took apart this flashlight because it was very dim despite having a good battery. And I believe I've found the reason. The connection between the aluminum chassis and the circuit was terribly designed.
But apart from that... I'm completely surprised by the circuit I found. It has a transistor and I can't figure out what it is it doing there! It is a simple, dumb flashlight. I was expecting just a resistor.
The circuit seems to be this. The transistor is just there, parallel to the LED.

I'm no electronics engineer, and to me, it looks like is just wasting power. What is it doing? Is there any valid reason to have it there?



Thanks in advance.

golden_labels:
Are you sure this is the entire circuit?

In that configuration it doesn’t even matter how the transistor is placed relative to the LED. As soon as the circuit is powered, there would be a short through transistor’s base, leading to its destruction.

Also, please post images as attachments. The “Attach” section in the bottom of the posting window. This way posts’ integrity is preserved and you do not leak information to 3rd parties.

Gyro:
If your battery is just a single cell then it won't have enough voltage to power a white LED. The "transistor" might be a boost converter IC and the "resistor", an inductor.

golden_labels:
I was thinking about that not being a transistor at all too. Things like AMC7135 come in the same package. But it wouldn’t be connected in that manner, as it must be in series with the LED. If the “resistor” is not actually a resistor, Gyro may be right. How did you determine the nature of each component?

MrAl:

--- Quote from: Kibelon on September 28, 2022, 07:56:02 pm ---I took apart this flashlight because it was very dim despite having a good battery. And I believe I've found the reason. The connection between the aluminum chassis and the circuit was terribly designed.
But apart from that... I'm completely surprised by the circuit I found. It has a transistor and I can't figure out what it is it doing there! It is a simple, dumb flashlight. I was expecting just a resistor.
The circuit seems to be this. The transistor is just there, parallel to the LED.

I'm no electronics engineer, and to me, it looks like is just wasting power. What is it doing? Is there any valid reason to have it there?



Thanks in advance.

--- End quote ---

Hi,

You should really take a GOOD picture of the circuit board and post it here so we can take a look and then tell you much better what is going on inside the flashlight.

If the light takes only one AA for example then there MUST be a boost circuit of some kind in there, either transistor or IC chip.  The cheapie transistor type boost circuit usually has two transistors though, but i believe if there is a small transformer in there they may get by with just one transistor and other small components.
IC chips can look like transistors so it could be an IC chip that does the boost function.  Because an AA battery has a nominal voltage of just 1.5v and the white (presumably white) LED takes 3v or higher, the voltage has to be stepped up to at least two times the battery voltage, but as it runs down the boost will be even greater until the battery runs really really down.  At that point when you turn it off it may not turn back on right away, you may have to wait a little while for the battery to 'recover' from the low voltage.  The voltage will climb back up a little and it's usually enough to get the light to turn back on and stay on for a while, but when you turn it off again you have to wait to turn it back on.  That's a typical boost circuit for ya because it takes a little more voltage to start the circuit than to keep it running.

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