Electronics question for the experts

[Amonra]

I have a few 2AA flashlights containing 11 led's and some kind of boost electronics.
Suddenly the led's on some of them turned green. Upon inspection i found that an IC or some sort of chip in the electronic board is becoming very very hot and that heat is being transferred to the led's turning them green. I removed this IC in one of them and they went white again and back to normal but the led's are now dimmer since i guess the electronic is no longer boosting the voltage as required due to the missing IC.

So now i'm wondering what could have caused this IC to become so hot as i did nothing abnormal to these flashlights.

Here are a few pictures of the electronic ( sorry for the bad pictures but phone cameras are not so good ):

There are 2 IC's, 2 Resistors, 1 Capacitor? and 1 inductor.

The IC's : Q1 has D9D written on it. Q2 has 2A written on it

The Resistors : R1 has 102 written on it. R2 has 103 written on it

There is nothing written on the capacitor

There is nothing written on the inductor but it has a total resistance of 1.1 Ohms. The wound wire is 0.14mm thick ( including enamel ) and 62cm long and is wound on a 'bobbin' with 1.6mm diameter so i guess it's about 125 turns.

The led's are all wired in parallel and each have a resistor in series with 000 written on it.

I hope someone can understand how this electronic works and can shed some light as to what is going wrong with it as i would like to fix these flashlights since im very fond of them.

Thanks for your help.

 

[altis]

The names Q1 and Q2 suggest they are transistors and not ICs.

The D9D might be an HMBT8050 NPN transistor:
http://www.hsmc.com.tw/pdf/HMBT8050.pdf

and the 2A might be an HMBT3906 PNP transistor:
http://www.hsmc.com.tw/pdf/HMBT3906.pdf

I suggest you create your own circuit diagram by following the tracks.

 

[Amonra]

Thanks for the suggestion, ill post a circuit diagram tomorrow as i dont have it here atm. I dont know much about electronics you are right about them being transistors. Any idea why Q1 transistor is getting so hot ? It gets very hot instantly as soon as you turn on the flashlight.

 

[Amonra]

Schematic posted
Thanks

 

[Amonra]

Anyone know ?

 

[MikeLip]

What you seem to have is a simple series regulator circuit. The transistor getting hot is the one driving the LEDs. The other is simply driving the driver. If the transistor was not getting hot before and is now, there are a couple possibilities. One is that the transistor is shorted. You'd need a meter to check that, and you'd need to take it out of circuit. Also possible is that it is getting hot because the LED circuit is drawing too much current - there might be a short out there.

Pass transistors can run hot too, but my suspicion is that the transistor is bad.

 

[Amonra]

Thanks.
I took it out of the circuit and tested all the legs and it does not seem to be shorted.
The ones that are working properly are drawing about 80ma out of 2AA batteries.
Dunno

 

[MrAl]

Hello there,

What you have looks like a very common two transistor unregulated
driver circuit like this one:

http://hometown.aol.com/xaxo/index.html

(Scroll down for the circuit)

Notice that this circuit is driving only one LED. I am surprised to see
them driving 11 LEDs with your little circuit. I would not be surprised
at all if the main transistor got very hot.

What you could do is try mounting the LED farther from the transistor
if the heat is actually bothering the LEDs.
You could also try soldering (carefully) a small piece of copper foil to the
collector of the transistor that gets hot (Q1). The collector is the lead
connected to 'OUT'. You would need to angle the foil up a bit so that
it gets plenty of free air circulation.
Another idea is to replace Q1 with a TO92 package device, which would
handle the power a little better.
As i said, i am surprised to see them trying to drive 11 LEDs with only
that one small transistor.
Also, do you know what the value of the resistors are (measure them
perhaps)?

If you cant get anything else to work you can increase the value of R1
a bit, which will reduce the output but will help the transistor stay cooler.

If you want to try a higher power transistor for Q1 you could try that too,
but you most likely will have to change R1 too unless you get very lucky
and get a transistor with almost the same gain as the old one.

 

[MikeLip]

That was my thought as well, I was going on the assumption that the pass transistor was running cool before. Those "0" marked zero-ohm parts could also be bumped up in value to decrease current consumption at the expense of brightness.

The circuit does not look all that well designed for the job it has to do!

 

[Amonra]

It was working correctly before i.e. not heating up. i dont think there is too much current going through as only about 80mA is going out of the batts to supply all 11 led's so they are getting like 8mA each.

By removing the transistor i get dimmer light anyway and it's easier than replacing the resistors.

R1 is 1K ohm and R2 is 10K ohm.

I guess it boils down to faulty transistor or something else is messing it up.

Thanks again for your help

 

[MrAl]

Hi again,

Yes, if it was working before then something went wrong with one of
the parts. It might also be that one or more of the LEDs became
defective for some reason and is drawing more current than before.
You could try replacing some of the LEDs and see if that makes any
difference too, or just remove a few that look like they might have
changed in some way.