Need help for 7.5v/800mA Regulated Power Supply

Hi everybody,

Jut want to seek for everybodys help. Can anybody give me a design of a 7.5V / 800mA regulated power supply using transistor and zener diode(pls include the exact part # and wattage and also the computation). My input voltage is 12V. I am going to use this to supply a network hub that i am going to attached to our machine here in my work.
your help is highly appreciated. :lol: :wink:

7.5V,800mA power supply

With the use of LM317T, this can be achieved with 2 resistors and a cap.
Visit www.national.com and find out.

Gajanan

Dear Mr. AMalonzo,
please refer to the attachement for the 7.5V regulatot circuit that u r looking for.
This is a simple, stable and reliable reg cct design.

please refer to the attachement for the 7.5V regulatot circuit that u r looking for.
This is a simple, stable and reliable reg cct design.[/quote]
Remember to attached a heat sink to the power transistor 2SD2396(K).
Bcos the power dissipated on it is:
(12-7.5)V* 0.8A= 3.6W
Just a small size heat sink will do (ofcos bigger is better).

gary_choo said:

Dear Mr. AMalonzo,
please refer to the attachement for the 7.5V regulatot circuit that u r looking for.
This is a simple, stable and reliable reg cct design.

Click to expand...

LM317 is a better solution, as gmphadte says, but if this zener/transistor regulator is used, the zener needs to be an 8.2v unit, to compensate for the Vbe drop of the transistor.

Ideally, we need to consider the Vbe drop across the Power Transistor, but in practically, Vbe can be ignore because it won't draw more than 0.3V.
By using the Transistor/ZenerDiode cct that i suggested, if u found that the o/p voltage is too low, u can change the Zener Diode (D1) to higher rank-->
from HZS7L(C2) to:
HZS7L(C1)--> the output voltage will be increase by about 0.2V
from the previous (HZS7L(C2) )
or
HZS7L(B3)--> the output voltage will be increase by about 0.4V
from the previous (HZS7L(C2) )
I just checked the part specification about LM317T. it is a power transistor or a Regulator IC. The output voltage will be determined by the input voltage ref. In other words. u can't get a stable output voltage at 7.5V if ur input voltage is not stabled on 12V.
The advantages for using Power Transistor plus Zener Diode for a voltage reg circuit here are:
1. the cost will be much more lower than the IC type Reg IC.
2. the output voltage will always stabled at ur desired voltage (7.5V) as long as the input voltage not lower than about 8V.

gary_choo said:

Ideally, we need to consider the Vbe drop across the Power Transistor, but in practically, Vbe can be ignore because it won't draw more than 0.3V.

Click to expand...

Vbe will be about 0.7V, not 0.3V. See fig.4 in the datasheet.

By using the Transistor/ZenerDiode cct that i suggested, if u found that the o/p voltage is too low, u can change the Zener Diode (D1) to higher rank-->
from HZS7L(C2) to:
HZS7L(C1)--> the output voltage will be increase by about 0.2V
from the previous (HZS7L(C2) )
or
HZS7L(B3)--> the output voltage will be increase by about 0.4V
from the previous (HZS7L(C2) )
I just checked the part specification about LM317T. it is a power transistor or a Regulator IC. The output voltage will be determined by the input voltage ref. In other words. u can't get a stable output voltage at 7.5V if ur input voltage is not stabled on 12V.

Click to expand...

This is patently untrue. The LM317 will have vastly superior input and load regulation when compared to a zener/emitter follower. Plus, it is short-circuit and over-temperature protected, which is not true for the zener/emitter follower. If you have to check to see what an LM317 is, and then misinterpret the specs, you probably shouldn't be giving advice on voltage regulators.

The advantages for using Power Transistor plus Zener Diode for a voltage reg circuit here are:
1. the cost will be much more lower than the IC type Reg IC.

Click to expand...

Wrong. Did you check prices before coming to this conclusion?

2. the output voltage will always stabled at ur desired voltage (7.5V) as long as the input voltage not lower than about 8V.

Click to expand...

It is true that the LM317 requires about 3 volts of headroom. However, regulation vs input voltage variation is many, many times better. Not only that, the circuit you posted won't deliver 800ma with an 8v supply. The current through R1 will only be 0.5ma, and this has to supply zener current and base current. With a minimum current gain of 400, you will need 2ma of base current.
I must say, though, that this is a nice transistor.