I do know the question in the title is really stupid, but it is the best phrasing I could come up with. Let's say I am building a circuit and the schematic says to use a BD139 transistor. Would there be any issue in using any other NPN transistor? What would I do if I could not find this transistor? I can't buy any parts online, and I do not have access to a store where they would sell this stuff, so I am limited to what I can salvage from old broken electronics, most of which have older, outdated parts that have limited information online.
Electronic – Are transistors interchangeable
transistors
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Your basic problem is PNP1/NPN2. With the bases tied together, you are getting maximum current flow through the two emitter-base junctions from +12 to ground. Any real transistors in this configuration will die instantly.
If you must use BJTs and want to keep your circuit ideas as much as possible, replicate NPN1/PNP2, and use a fifth transistor to invert the base drive signal between the right and left sides of the bridge. Also, you'll need to either use a 12-volt coil voltage or provide a 6th NPN as a level shifter. But be aware that this will not work well, since the NPNs will not get enough base voltage to turn on hard, and you won't get very good performance.
A better idea is something like
simulate this circuit – Schematic created using CircuitLab
You will have noticed, with some frustration, that none of the resistors have values. There's a very good reason for that: you haven't specified your load current requirements. At any rate, Q1 - Q4 are power transistors, able to handle the load current, while Q5 - Q9 are smaller signal transistors able to handle about 1/10 the load current.
To get the I/V-Curve of the solar panel (and therefore the Maximum Power Point), I would suggest you use a circuit with feedback to change the current sourced from the solar panel. This is possible with a variable current source like this:
simulate this circuit – Schematic created using CircuitLab
Vref determines the current sinked from the solar panel. The current is calculated as follows:
\$I=\frac{V_{ref}}{R_2}\$
Therefore, from the knowledge of the Vref and the measurement of the solar panel voltage, you can easily determine the maximum power of your panels
For reference: The circuit is from "A collection of Amp Application by James Wong (Analog Devices): http://www.analog.com/media/en/technical-documentation/application-notes/28080533AN106.pdf
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Best Answer
If you are missng a screw, can you add any screw ?
Would it depend on how long is the screw ? What is its thread is? How large is the diameter ? What is the material ?
Similarily, all electrical components have electrical characteristics and parameters. Different components can tolerate more voltages and currents than others. Others are set up for a particular application even though they are all part of the same family of components (capacitors, resistors, inductors, transistors, diodes).
So yes transistors are interchangeable, if the type (npn/pnp) and required specs match.
You compare the parameters for the transistor you can have access too, and compare it to the one in the you want to have. You keep searching until you can find one that can handle it. Now, someone might have used an a transistor that was overkill for the project, and so knowing a bit about the circuit would help. If the current through a transistor was only 10mA and they have a part that can tolerate 1A, well, that's a bit much, and you can find a part that is more suited to what the circuit it. But if you do not have knowledge with how to analyse circuits, then you should probably match the component (to be on the safe side).