low voltage cutoff

I am working on a project where I need a variable voltage from 4-12V. to make I can regulate input voltage to keep the voltage below 12V, and I am thinking of using a zener diode combined with a transistor to block current below 4V. Will this work?

note: adjusting the voltage works, I only need help with the cutoff.

schematic:**broken link removed**

Just add a resistor between ground and the lower end of the pot1/wiper junction. Select a value that will limit the output of U1 to 4V when the pot1's wiper is at the top of pot1.

Ken

KMoffett said:

Just add a resistor between ground and the lower end of the pot1/wiper junction. Select a value that will limit the output of U1 to 4V when the pot1's wiper is at the top of pot1.

Ken

Click to expand...

Yes.
All the stuff downstream of U1 kind of defeats the purpose of using an IC regulator.

KMoffett: like this?

**broken link removed**


Willbe: what do you mean? from what I understand, as long as there is a high enough voltage across the zener to keep the transistor open, the current passes freely from Vout, through the transistor and into the load. R2 is only there to keep the zener from shorting the circuit.

No, like this. You don't need anything beyond the LM317, 2 resistors and a pot. Actually you need small capacitors on the input and output of the 317.

A quick calculation: top resistor=156Ω, the pot=1KΩ, and the bottom resistor=343Ω. Depending on how accurate you need to be, the top resistor could be a 200Ω trim pot, and the bottom resistor could be a 500Ω trim pot. Or you could use the parallel resistance calculators on various sites to parallel standard resistors for these two odd values.

Ken

thanks. about accuracy, I don't need very much, I am going to use it to manually regulate the speed of a dc fan. I considered using microcontroller, but as I do not know any c, (beside a few of the most basic terms, and those are more memorized than understood) it would be too complicated to write the program.

Then use 150Ω for the top resistor, 1KΩ for the pot, and 330Ω for the bottom resistor.

Ken