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Dropout controllable regulator

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matusel

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Hi all I need to design a cheap circuit (with bjt linear regulator) capable of regulating the voltage dropout on a power supply line independently from the supply current but dependently from the supply voltage.
The circuit should be able to generate a (for example) 5V dropout on the line if the input voltage is grater than 40V and should have a low dropout (ideally zero) when the input is smaller than 40V.
Could someone help me in making it in the cheapest way?
 

If you could design the circuit yourself, then it will save you a big amount of money.
 

If you could design the circuit yourself, then it will save you a big amount of money.

thanks for the response has been very helpful ..............improved my day
 
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dropout_voltage = input_voltage - output_voltage

Perhaps you should reconstruct your question?
 

Dear erikl I needed a circuit like one in the image attached (somethings that cuts voltage over 40V); my question wasn't clear, my bad. I wasn't able to make it work until now (i was simulating with a wrong LTspice library), so now the problem is solved thanx for trying to help 01circuit.jpg02circuit.jpg.

Regards
 

any solution is irrelevant without a current spec. otherwise you just need an Over voltage Protection circuit (OVP set at 40V.)

200mA are specification for current, what do you meand for OVP protection ?
 

OVP is to limit the output voltage.

Use a P MOSFET switch to pass the input to output with no voltage drop. Then use a PNP transistor to turn off the switch. It can be regulated using a comparator with a 2.5V reference and divider on the output. the open collector driver can now drive the PNP base for a precision 40V series pass switch with a regulated 40V maximum with 2% tolerance except for R tolerances. A small Cap may be useful to reduce noise near clipping.

Less precise designs can be made simpler. Such as a Zener controlled shunt to bring down voltage but NPN must handle 4W. (40V @ 100mA.
 

................. I needed a circuit like one in the image attached (somethings that cuts voltage over 40V); my question wasn't clear, my bad. I wasn't able to make it work until now (i was simulating with a wrong LTspice library), so now the problem is solved ......................... View attachment 105271View attachment 105272.
That circuit has a minimum forward drop of about 0.7V at voltages below 40V. Is that okay for your application?
 

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