transistors
I wish to use a transistor for switching.
The datasheet says it allows a maximum collector current (continuous) of 200mA.
If my application just requires pulses of current every 1 second, would the transistor be able to pass through an Amp?
Is there a way, I can calculate or figure out the maximum collector current, when its not continuously flowing through it?
BC337 is a silicon transistor. Silicon PN junction starts conducting between 0.6V and 0.7 volts applied to it with P terminal being more positive than N. In your circuit, when current in 2.7ohm resistor reaches 250ma, it causes 0.675 volts across it turning on BE junction of transistor T2.
With the switches closed you would be trying to turn the relay on. As your circuit stands it doesn't work because the emitter voltage could never rise enough to power the relay. Even if it could it means you would be powering the relay most of the time - a waste of energy. Also, if the door/window was opened momentarily as soon as it was closed the alarm would stop.
By adding a second transistor Q2 (PNP) and a push to break switch you can improve your circuit (and get it to work).
The closed switches turn Q1 OFF (base connected to ground). As there is no current flowing through the 1K resistor Q2 is also OFF. No current can flow through the relay. The only current to flow is through R1 so the circuit will only draw 1.2mA.
If any switch is opened Q1 is turned ON and draws a current through R2 and R3. The voltage across R2 will be clamped by the emitter-base of Q2 to about 0.6V and will turn Q2 ON. With Q2 turned ON its collector current will turn the relay ON.
The diode (D1) is connected across the coil of the relay to prevent damaging Q2 from the back emf when the relay is turned OFF.
The relay switch over and closes the circuit between its common and normally open contacts.
The reset switch (push to OPEN) keeps the relay turned ON even if the door/window switches are closed after opening. It LATCHES the relay. Just omit the reset switch if you don't want this feature.
Best Answer
How to find the pulse current rating? LOOK IN THE DATASHEET, of course.
I don't understand what is so ambiguous about "maximum". If the datasheet says the maximum current is 200 mA, then that is all you can put thru the device and expect it to adhere to the other specs. Some datasheets may provide a additional maximum pulse current spec. If so then there is your answer. If not, the "maximum" current spec is still your answer. There is nothing ambiguous here.
If you are asking what you can get away with despite what the datasheet says, then you've come to the wrong place. We do engineering here, which wishful thinking and religious beliefs have nothing to do with.