Why does my LED flash light has a transistor?

[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]

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.

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.

[james_s]

I bet the thing that looks like a resistor is actually an inductor, that bluish green color is common. The thing that looks like a transistor is almost certainly a simple boost converter IC, the wiring would make sense for that.

[langwadt]

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.

yeh, something like this: https://www.circuitsdiy.com/hh004f-led-driver-circuit/

[Kibelon]

Ok. Thanks to all that answered.

First of all... Yes, the LED is white. And it does in fact only use 1 AA.
I took a better look at that "transistor" and it has the code SC6202C. And I found some datasheet of a Chinese led driver meant for 3V as SC6202, So it is most likely a low voltage version of that. And it is paired to an inductor in the example circuit. So, most likely that is what that "resistor" is. I didn't know those existed. I'm gonna have to look at how those work. Does the controller do some switching, like a switching power supply?
And about the circuit... I would have to take it apart to show more. But you wouldn't see more than what I showed. It is just those 3 components.

I've also managed to fix it. It had a spring that was supposed to rest on top of the 3 solder bulges at the edge of the PCB. Clearly a terrible Idea. And I have soldered the spring directly. Now shines bright as it should.

By the way, I leave the datasheet I've found here. Don't know if the site is safe. Visit it at your own risk.
https://html.alldatasheet.com/html-pdf/1146369/FUMAN/SC6202B/174/1/SC6202B.html

[Gyro]

Yes, it is a switching power supply (a boost converter). It's a low current one so all circuit elements, except the inductor, can be contained withing the small single 3 pin package.

Well done on fixing it.

[james_s]

Axial inductors are very common for low power applications, they look a lot like resistors but usually they're a bit stubby. That same sort of IC is frequently used in those cheap solar powered LED garden lights.

[golden_labels]

So, most likely that is what that "resistor" is. I didn't know those existed.

For educational purposes see miniature inductor specimens on Wikimedia. Here they are beige, but much more often they have distinctive cyan or lime color. The color code works the same as with resistors, except that the base unit is usually microhenries instead of ohms. The values will often make no sense for a resistor in a given location, which is another hint it’s an inductor. And, finally, they have low resistance.

I'm gonna have to look at how those work. Does the controller do some switching, like a switching power supply?

In a similar fashion. First it shorts pins 1 and 3, charging the inductor. Then it cuts that path off, causing the inductor to dump energy into the LED. Since that’s only possible if the LED is conducting, high enough voltage is ensured.

[MrAl]

Ok. Thanks to all that answered.

First of all... Yes, the LED is white. And it does in fact only use 1 AA.
I took a better look at that "transistor" and it has the code SC6202C. And I found some datasheet of a Chinese led driver meant for 3V as SC6202, So it is most likely a low voltage version of that. And it is paired to an inductor in the example circuit. So, most likely that is what that "resistor" is. I didn't know those existed. I'm gonna have to look at how those work. Does the controller do some switching, like a switching power supply?
And about the circuit... I would have to take it apart to show more. But you wouldn't see more than what I showed. It is just those 3 components.

I've also managed to fix it. It had a spring that was supposed to rest on top of the 3 solder bulges at the edge of the PCB. Clearly a terrible Idea. And I have soldered the spring directly. Now shines bright as it should.

By the way, I leave the datasheet I've found here. Don't know if the site is safe. Visit it at your own risk.
https://html.alldatasheet.com/html-pdf/1146369/FUMAN/SC6202B/174/1/SC6202B.html

Hi,

The basic action of an inductor in a switching circuit is to act like a current source for a short time.  Because it looks like a current source, if there is very high resistance across it the voltage will shoot up very high because it tries to keep the current constant, and a current across an open circuit (LED not conducting yet) means the voltage shoots up high until the LED starts to conduct, then the LED clamps the voltage at the nominal LED voltage (3v or so for white LEDs).
All this happens after the inductor is 'charged' and that is done with the voltage of the battery.  The battery is connected across the inductor for a short time and during that time the inductor current increases.  When the battery is removed via the switch, the inductor current tries to maintain that original charging current and so it shoots up to the LED voltage and the LED turns on.
This action is similar to how a car keeps rolling even when you take your foot off the gas pedal.  It tries to keep rolling at the same speed as before you took your foot off the gas but eventually it does slow down due to friction.  The inductor tries to keep the current flowing but Ohm's Law tells us that a current thought a very large resistor (open circuit) will generate a very high voltage, enough to turn the LED on.
For example say we charge the inductor such that it then has 1 amp flowing through it.  Now we remove the charge source and connect a 100 Ohm resistor across the inductor (very very quickly).  The voltage jumps up to:
V=i*R=1*100=100 volts
and it stays like that for a short time, just enough for the LED to light up (although it gets clamped to 3v or so with an LED).
And this could all be because we charged the inductor up with as little as 1.5 volts to begin with.
Once the designed time period has passed, the inductor has to be charged again.  During that time the LED may be off, but even though it turns on and off you dont see the pulsing because it's too fast.  Other circuits use a rectifier and capacitor to hold some of that voltage so the LED stays on during that time too.  So those are two different kinds of circuits.  The minimum component type will not have a capacitor so the LED turns on and off very rapidly.  They usually overdrive the LED too so that during the 'on' time it is much brighter than usual, so during the 'off' time it averages to a lower level of light output.  Your eyes act as a light intensity integrator so it averages out the high and low light to an average value that is the intended brightness of the flashlight.  You may see it pulse though if you turn your head very fast or sweep the light across your field of vision because then the light output varies with time and distance and your eye may pick that up unless the frequency is very high then it will be hard to notice.

Just to be complete, inductors are also used in filter circuits and resonant circuits but that's not usually a principle that is used in switching circuits, at least not until of late where the idea of switching in step with the resonant frequency was found to waste less energy so the flashlight or other device would be more efficient.  That's a more modern design though not sure if it appears in any low power circuits like a flashlight circuit yet it's mainly more of a concern with high power converters.