As the title explains it, I have
Kn2222A x2 transistors connected as a Darlington pair connected at the ground side of a [JQC-3F (T73) 5v] relay
The relay is connected to a 5v power supply.
the problem is, when I feed the base of the Darlington pair with low current, it won't switch the relay, it seems that the minimum current needed for the base is 0.3mA, less than that won't do a thing, looking at the data-sheet I should get at least 100*100 gain, 10000 * 0.3mA = 3A I am sure that's more than enough for the relay, unless I am missing something
I'm having hard time getting Hfe from the datasheet, and knowing the amp needed by the relay.
Transistor datasheet
another question is, can the relay function at higher than 5volts? (safely)
I'm only experiencing with electronics so any more info will be appreciated
Best Answer
I'm assuming your circuit looks like
simulate this circuit – Schematic created using CircuitLab
Assuming this is true, I think you're running up against a characteristic of the Darlington - high saturation voltage. In this configuration, the high gain does not carry over to the situation where you want to turn the transistor hard on. Instead of a transistor voltage of ~0.2 volts, something like 0.8 is more likely. This means that the relay is only getting about 4.2 volts, and this is close to the operating limit. As a result, you have to drive the Darlington harder than you think you need to.
Instead, try this:
simulate this circuit
This will allow more voltage across the relay.
In any event, compound transistor connections aren't magic. The problem is that at the very low current levels you expect with the first stage, current gain goes way down, so the overall gain is less than you expect from reading the data sheet.
Also note the catch diode. Always use one of these. If you don't, you'll get a turn-off voltage spike which may well kill your transistor.