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7805 regulator transistors equivalent

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neazoi

neazoi

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Hello,
I have a portable battery operated receiver circuit in which a 78L05 regulator is used.
The battery is 12V and until it gets discharged to 8V the regulator can provide a "stable" 5V supply regardless of the battery voltage drop.

What I need is to get a relatively stable 5V voltage out of a 5-12v, that is not affected by the battery voltage drop.

Is there any way I can implement this behavior using just a few transistors instead of the 78L05? Or just use a 5V zener diode will be ok?
 

A classical circuit is combining a zener diode and a transistor voltage follower. The basic advantage is in a much lower idle current respectively a larger input voltage range, depending on the dimensioning. The zener voltage has to be increased by Vbe of the transistor, e.g. 5.6V for 5V output.

But no simple voltage regulator circuit can't work down to 5V input voltage, also a 7805 doesn't. Only a LDO regulator can provide drop-out voltages down to a few 100 mV. A discrete LDO circuit would involve a larger part count. You can analyze the basic cicrcuit of a LDO IC regulator and try to find a low part count discrete equivalent.
 
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    neazoi

    neazoi

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The zener and transistor approach seems fine to me. I am making this one https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/Ten-Tec_1253_Shortwave_Receiver.gif
In this particular I do not need a following transistor because the MPS6514 will do the job. So If I connect a 5.6v zener from the collector of the MPS6514 to the ground will it be ok? or better something like this https://schematics.circuitdiagram.net/images/rgh1261962999i.jpg


Also can the MPS6514 be replaced by say a 2n2222? why he has used such a rare transistor there?
 
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So If I connect a 5.6v zener from the collector of the MPS6514 to the ground will it be ok?
Click to expand...
Basically yes. T5 acts as capacitance multiplier to achieve a better filtering for the input stage supply. If you replace IC2 by a zener/transistor regulator, it would be still usefull to keep the T5 filter stage. The 7805 circuit has an output voltage of 8 - 8.5 V in the present dimensioning, by the way. It's misleading to name the output node "+5V".

Details like this raise doubts, if the circuit has been working exactly as the schematic tells. You can't know.
 
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    neazoi

    neazoi

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It is best to use a real regulator chip. The LM2941S can work down to a 0.5 volt differential Vout-Vin and still regulate, and provide 1 A.

If you just want to hook up some parts lying around, a NPN transistor with the collector to battery V+, Base to battery V+ thru a say 1 K resistor, 4.3 V zener between ground and base. Hook up radio to emitter, and you will get roughly +5V out.
 
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    neazoi

    neazoi

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