What is your supply to the circuit gonna be? a wall-wart, batteries...... sounds like you need basically a power supply unit (psu)? what is low current, for me it'd be about
5ma, but for others it may be 50 amps? and what are you supplying by it?
It is a circuit I am building using transistors - 2 LM324's and one IC - AD633JN, so basically milliamps.
These are supplied by a linear voltage ramping circuit, made up of a 555 timer connected to a VDAC to provide 0-15V. I want to limit the output voltage of that to 12V.
I can send a picture of the circuit if it's easier.
Have you looked at lm317 voltage regulator or similar? They only go down to 1.25 volts or so with a 2volt or so overhead, some are lower dropout (overhead voltage). in the data sheets there are schematics that allow for regulation down to 0 volts. I'm not sure if it would work for you though, I've never used those multiplier chips
If you're thinking of putting the LM317 in series with the output, you'll find that it drops some voltage e.g. if you are outputting 4.4V, the output of the LM317 might be 3V. The voltage drop is not necessarily constant, nor does the LM317 have a complementary drive output, so basically it will bugger up your signal.
Thanks all. I have attached a copy of the circuit. It is a hypothetical circuit to practice circuit building, not to drive a particular load or anything.
The left hand side is the power supply. The bottom right is to create a variable voltage supply. The rest is to create the function I have written at the top. For Vout(max) to be 12V, I can either limit the output, or the Vin from the DAC to 7.9V, which is what I tried with the LM317.
Yeah, the LM317 isn't designed for such usage. I see you have a couple of spare opamps; have a look at the attached limiter circuit, which functions as an ideal zener, followed by a buffer. The 12V reference voltage can be obtained from the 15V rail using 2 resistors (& a cap for stabilisation).
I have read your reply a few times today. Would that circuit be suited to go on the output of the 12V? Is that 12V reference what makes it limit the overall voltage to 12V?
Thanks again for your reply
The point marked "12V" on the circuit is what I was calling the reference voltage, and yes, the voltage at that point sets the maximum output voltage.
I'm not sure what you mean by "output of the 12V". It is suitable to go in place of the LM317 currently shown on the schematic or at the output of the last opamp. Adjust the reference voltage accordingly.
Sorry, I meant the overall circuit Vout that I indicated on the far right of the diagram, where 12V can be the maximum. The output of the initial LM317 can be a maximum of 7.9V due to the equation stepping that up to 12V at the output.