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Need Minimum Drop out Voltage in this Circuit

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Suraj143

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If I use 7805 it needs minimum input 7V to operate correctly. There is a drop out about 2V.

If I make a regulator with discrete components like below also it has a voltage drop.

In this diagram if I give input 9V the output gives about 3V.not 5.1V why is this?
(I don’t have low drop out type regulators)

**broken link removed**

Thanks a lot
 
Last edited:
It should give about 4.4V out (0.7V Vbe drop across the transistor), how much current are you trying to draw off it?, and what is the voltage on the base?.
 
Hi Suraj,

You need to add the Vbe of the transistor to the zener voltage.
If you want 5 volts output the zener voltage should be
5 + 0,7 =5,7 volts. The closest practical value would be 5,6 volts.
Note that near saturation a transistor needs ( a lot ) more base
current.

on1aag.
 
Hi thanks for the input.

I don't need much current need about 50-100 mA.

If I calculate with the Vbe also there is a gap about 2V.

In my circuit it should come about 4.5V but its coming only 3V why is this?
 
Hi Suraj,

Probably a transistor with a very low HFE which draws too much
base current which makes the zener voltage collapse.
Check also the 9 volts if it's a battery.

on1aag.
 
Suraj143 said:
Hi thanks for the input.

I don't need much current need about 50-100 mA.

If I calculate with the Vbe also there is a gap about 2V.

In my circuit it should come about 4.5V but its coming only 3V why is this?

Like I said, measure the voltage on the base - if it's only 3.7V then you're probably drawing too much base current.
 
There are too many low-dropout regulators out there offered by Maxim and Linear Technology to justify screwing around with reinventing the wheel. In fact, the bulk of Linear Technology's advertising is geared toward battery applications where "efficiency" is the key word.

Dean
 
I have been using the LM2931Z-5.0 low dropout regulator for years. Its minimum input voltage is about 5.4V.

All semiconductor maufacturers make low dropout regulators.
 
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