Application of 7805

[lramirev]

The emitter-to-ground resistor in CDRIVE's diagram is actually labelled "Load", not RL. It is intended to represent the resistance of the device or circuit that is being powered from the regulator.

But! You should use a resistor from the transistor's emitter to ground, unless the circuit you're supplying will always draw a significant amount of current (e.g. at least 10 mA).

Without any current being drawn from the transistor's emitter, the output voltage will rise due to leakage current in the transistor. Also the transistor's base-emitter forward voltage varies somewhat with the collector/emitter current, so ensuring a minimum load current will improve regulation, which is not very good with this design.

If your load doesn't draw significant current at all times, I suggest using a resistor from the emitter to ground. A 470 ohm resistor will draw about 10 mA and dissipate about 50 mW.

P.S. Have you changed your username from rai.abhishek to lramirev or are you a new user joining this thread?

Kris:
Thank you for the adivise. I'll add a resistor to the design as suggested. So a 1/4 W, 470 ohm resistor must be enough then. Again thank you and CDDRIVE for the help.

PD. I'm a new guy to the discussion. Re starting my electronics after 20 year of not working on this. Long time away of this kind of things will make you forget some stuff.

 

[KrisBlueNZ]

Yes, a 470 ohm resistor should be enough load to prevent the output voltage from rising greatly when no load current is being drawn.

You can check the output regulation by measuring the output voltage with no load (apart from the 470R resistor that's in the power supply itself), and measuring the output voltage with a heavy load. For example, a 4R7 (4.7 ohm) resistor connected across the output will draw about an amp (Ohm's Law: I = V / R). Calculate the difference between the two measured voltages as a percentage of the nominal output voltage. If the regulation isn't good enough, you can use a different regulation arrangement using an op-amp. Let us know if you need to go that way.