[sdiy] CV source - just checking its as easy as it seems...

[harrybissell]

Ok I'm on the case

Your idea is essentially correct, but has some big practical problems.

The 1M pot is going to be a loser... most CV inputs are 100K ohms. The
1M pot will be loaded VERY heavily.

I'd go this route.

Make the pot 1/10 of the load value, so that would be 10K.

The top of the pot should be 5V.  So thats 5V / 10,000 ohms
(ohms law...  E/R = I) = .0005A or .5mA

Now you have a 12V supply... so 12V - 5V = 7V  That's the voltage
across an unknown series resistor between the top of the pot and the 12V

7V / .0005A = 14,000 ohms  (14K)   (ohms law... E / I = R)

Put this resistor in series with the pot.  Bingo, all done.  Now 14K is a weird
value so you can use some standard value that is close...  13K will put you
a little OVER 5V... 15K will put you a little under.  Choose your poison.

Another possible solution is to use a 5V regulator from the 12V supply and drive
all the pots in your circuit.  Or you could use a 5.1V zener diode added to the
series circuit I described above.  In this case, you would want the voltage at the
top of the pot, without the zener added... to be above 5V.   A 10K resistor and a 10K
pot would give 6V... the zener will eat the extra 1V to leave you with a regulated
5V at the top of the pot.

In all these cases, you will reach 0V. All of them will be limited in current so they are
short circuit proof.

Your series solution is not real good. It is very dependant on the load. I don't recommend it.  (but it would indeed do sort of what you
want...)

The math of ohms law is really not that tough... get a calculator and it will work fine.
Almost all the problems you will encounter will fall before the mighty LAW !!!

H^) harry


Julian wrote:

> My modualar power rail is +12v.
>
> If i connect straight from this, to a resistor (to cut the voltage to 5v), and then through a, say 1M pot, to a jack socket, is this
> going to be fine?
>
> I know that the voltage will never reach zero on the output here.  Thats ok with me, but do i have the general idea ok?
>
> I know this is real basic, but, even though im carefull when i build other peoples boards, i really dont get much of the math....
>
> Ohms law, right?  But all this talk of current...
>
> Just trying to build a manual CV source for use with other modules.
>
> Cheers, Julian