transistor voltage regulator

[walid]

please I need information about how to build a transistor voltage regulator without zener diode i use a small sig PNP transistor like 2N5401 and want to regulate 10 votls DC to 7.5 VDC. please tell me how can i clculate resistor svalues.
thanks

[audioguru]

Hi Walid,
You can't make a voltage regulator without a voltage reference like a zener diode or a voltage divided from a stable reference voltage. Since you are using a PNP transistor, input and output voltages in my descriptions are negative:
1) If the 10V is stable, then the 2N5401 can be used as an emitter-follower voltage reducer with the 10V divided down to about 8.2V with a couple of resistors. Since you need 1.8V across one resistor and 8.2V across the other, their ratio must be 1.8:8.2. Use 1.8k and 8.2k. With a lw load current, the emitter's voltage will be about 7.5V.
2) You can use the reverse-biased base-emitter junction of a silicon transistor as a 6V to 7V zener diode when it has avalanche breakdown. If the voltage is too low, the transistor's forward-biased base-collector diode can be added in series with it for about 0.7V more. A forward-biased silicon diode can also be added in series for about 0.7V more. Like a zener diode, the current must be limited with a resistor in series. ;D

[Kevin Weddle]

Something worth noting about regulators. I have this idea that major power supply manufacturers subscribe to this idea. If for every current you can maintain the same voltage, you are a winner. I have often wondered how you can purchase a supply and be guaranteed of the voltage at virtually any current. This is definitely an engineering marvel. Virtually no switches or ranges that are found on multimeters.

[audioguru]

Hi Kevin,
A linear voltage regulator has a reference voltage generated by a temperature-stable zener diode or band-gap device that is amplified and buffered from its load by a high input resistance opamp. The opamp drives a series pass power transistor that is in series with the load and operates like a variable resistance. The opamp uses its extremely high gain to compare the output voltage with its reference voltage and adjusts the output voltage to be the same. ;D