transistors
I have a flashlight that malfuntions. I opened it up and traced the circuit, came up with this schematic
Power input 3V, LED bulb dia. 9mm
I checked the transistor with a multimeter. One side has reading, the other side none. So, I know there's something wrong with it. I want to replace it with a good one. I searched the web with the number on the housing which is 0118, and found nothing. Does anyone know what kind of transistor(part #) I can use to replace that defective one?
STOP
Your enthusiasm is commendable but you are trying to do something that is potentially lethal. Before you use 120 (or) 110 VAC you need to understand what you are doing.
The transistors need DC to operate. As Oli says, a TRIAC will work for AC and isolation is "a good idea" at least.
Some additional clarification is required:
If you are using 120 VAC with a 2N3055 as you now state, and no rectification (which you may be doing but have not mentioned) then your promises are worse than useless. 120 VAC ~= 160 V peak is far above a 2N3055's rated voltage.
Added:
Konsalik's solution is a good one. It is not the only way but it is a useful solution, the cost is relatively low and it provides proper isolation.
This diagram is copied directly from @Konsalik 's answer.
Please give his answer an upvote now :-).
It provides a good electronic solution with
While he shows it as switching only 12 VAC it is suitable with almost no changes for operation of 110 VAC or 230 VAC.
His suggested MOC3023M TRIAC driver is available from Digikey for under $1 in 1's.
This is a "random phase" TRIAC driver which means it will turn on the load as soon as it receives a turnon signal.
You can also get "zero crossing" drivers which turn on the load when the mains voltage is at the zero voltage point. This decreases electrical interfenece from switching the load BUT means you can only get integral mutiples of a half wavelength turn on period.
Which sort is best depends on your application.
In many cases zero crossing switching is OK
and is preferred if switching at zero crossing points is an acceptable limitation.
"Random phase" switching is useful for fastest possible turn on control.
The MOC3023 driver requires 5 mA drive current, the lowest in its "family" of members, making it a good choice for driving with most microcontrollers.
The MOC3023 driver has a 400 VAC output rating, making it suitable for both 100 VAC and 230 VAC operation.
Example only:
A potentially good TRIAC is the ST2050H TRIAC
costing under $1 at Digikey.
It is rated for 600 V peak operation, 20 A continuous operation.
It requires 50 mA gate drive which is 'a bit heavy' but accommodated OK by the driver.
There are less well rated TRIACS at a somewhat lower price that would 'do he job,' but this one appears to be more robust and capable than many at an OK price.
Bipolar junction transistors, due to their nature and operation, tend to have a relatively large forward voltage. Field-effect transistors use the depletion or doping zone to pinch the current pathway closed or lever it open and so have a much lower forward voltage. They are much more sensitive to voltage surges such as those caused by static electricity though, so it is advisable to always use static protection when handling them.
Best Answer
Found it. That is indeed NOT a transistor.
It is a CL0118 LED driver. It uses an inductor to boost the voltage of a single cell high enough to drive a white LED. It will operate from cells that are very close to empty - input range of 0.8 to 1.5V.
Link to the datasheet for the closely related CL0117
Here's the example circuit from the datasheet:
It looks almost exactly like yours. The part you thought was a resistor is actually an inductor. They look a lot alike, but @sstobbe is right that it is an inductor. Inductors in axial packages tend to be green, unlike beige or grey for resistors.
I've posted information and links for the CL0117 because I could find good links for it, but the CL0118 should match it pretty well. You'll want to buy a replacement from someplace that can give a correct datasheet, anyway.