Dropout controllable regulator

[matusel]

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?

 

[candhg124]

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

 

[matusel]

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

 

[erikl]

dropout_voltage = input_voltage - output_voltage

Perhaps you should reconstruct your question?

 

[matusel]

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 .

Regards

 

[D.A.(Tony)Stewart]

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

 

[matusel]

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 ?

 

[D.A.(Tony)Stewart]

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.

 

[crutschow]

................. 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?