I'm using this darlington transistor as a switch. When I supply 5 V to the base, while the collector is connected to 12 V, the voltmeter reads ?5 V on the emitter in respect to the same ground. Why? I think maybe the problem is that I should connect the load to Vcc and the collector, as shown in here and the emitter to ground. Is it?
Yes ... Ground the Emitter and connect load between the Collector and Vcc. Also, a Current Limiting resistor in the Base circuit ?
Your second link to PNG does not work for me.
Thank you! There is a current limiting resistor in the circuit. The PNG just showed what you suggested. Why do I have to connect the load that way? Is it only related to the behaviour of NPN transistor?
You collect the load that way because NPN circuits like to sink current from a load, and PNP circuits like to source current into a load. What is the part number you are using? The first link is giving me trouble.
It's D2493. Could you please tell me how to find more detailed information about this behaviour? Thank you.
What do you have, some old obsolete parts?
See the sections “Active-mode NPN transistors in circuits”
Basically saying that the base voltage must be higher than the emitter voltage for current to start flowing; when the emitter is grounded, the base only needs to be ~0.7V above ground to start turning the transistor on. When the emitter is sourcing current into a load, then the base voltage has to be 0.7V Plus the voltage across the load. If you want high voltage across the load, then the base voltage has to be higher. Much easier (efficient) to have a lower base voltage and sink current thru a load.
Similarly for PNP - much easier to pull the base lower than its collector to let current flow from collector to emitter into a load.
cancelliere: It's D2493. Could you please tell me how to find more detailed information about this behaviour? Thank you.
BJT Common Collector configuration: http://en.wikipedia.org/wiki/Common_collector
BJT Common Emitter configuration: http://en.wikipedia.org/wiki/Common_emitter
If it is really a darlington, you need a Vbe of about 1.4 V to turn it on since you have two Vbe junctions to feed.