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Voltage regulator with a buffer + voltage divider

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rprodrigues

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Hi guys,


I do need your assistance again.

This time I plan to get 5v from 7v, and to do it I will use a simple voltage divider to get 5V from 7V and pass it trough a buffer (opamp), so I get it stable at the output side of the regulator (opamp).

Is this schema a power safe one or it would wast a lot of power?
I plan to use the 5v to supply a lot of ICs in my circuit.

How could I know that ?

2ugi2cj.png




Thank you
 

FvM

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It's not a good method to design a voltage regulator. If you intend a linear regulator (you have another thread on VR efficiency), a industry standard 3-pin regulator is most simple, e.g. 7805 respectively 78L05 for lower currents. An OP limits the output current to about 20 or 25 mA, and it won't like bypass capacitors. An OP buffer can be a solution for a reference voltage supply.

But a 12 to 5V linear regulator wastes 140 mW for each 100 mW sourced at the output.
 

rprodrigues

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FvM said:
It's not a good method to design a voltage regulator. If you intend a linear regulator (you have another thread on VR efficiency), a industry standard 3-pin regulator is most simple, e.g. 7805 respectively 78L05 for lower currents. An OP limits the output current to about 20 or 25 mA, and it won't like bypass capacitors. An OP buffer can be a solution for a reference voltage supply.

But a 12 to 5V linear regulator wastes 140 mW for each 100 mW sourced at the output.


You are right.

That circuit is not a good one for the purpose I said you.

What I need is to get 5v from 7v but without using a switching regulator and achieving the better efficiency possible. A simple way to do this would be using two 1N4004 diodes in series so I can get about 5V when under 1A.

Any suggestion?


Thank you.
 

FvM

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A 7805 is able to create stable 5V with 7V input, a low-dropout regulator can work down to a few 100 mV voltage difference.
As said before, selecting a suitable linear voltage regulator hasn't to do with efficiency. It's very simple, the power
dissipation is basically (Vin-Vout)*Iload. So it's of course more efficient, to use 7V Vin instead of 12V Vin.
 

rprodrigues

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FvM said:
A 7805 is able to create stable 5V with 7V input, a low-dropout regulator can work down to a few 100 mV voltage difference.
As said before, selecting a suitable linear voltage regulator hasn't to do with efficiency. It's very simple, the power
dissipation is basically (Vin-Vout)*Iload. So it's of course more efficient, to use 7V Vin instead of 12V Vin.


FvM,

The problem is that I plan to achieve the better power efficiency I can since my circuit is connected to a battery.

And If I use a LDO and before running the power lead to it I passed it through a series diode to decrease the power voltage about 1V. Could it be a more power efficient solution ?

Any suggestion about a LDO ?
 

Mr.Cool

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original schematic is NOT a voltage regulator at all.. it is a voltage divider.

i agree with FvM.

another option is to use a 5.1V zener diode as your regulator but i don't know if that's any more efficient than the linear regulator.

Mr.Cool
 

rprodrigues

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Guys,


I decided to go with an LM2675 from National!!!!

It is a switching regulator with a wonderful efficiency (about 86% ~ 90%).


Thank you very much!

:D
 

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