Holding voltage for a long time

[kristjan]

Does anyone know a simple way to store voltage for a long time? Best would be forever:) I have been thinking how to do this and searching for something but haven't found anything. I want to be able to apply voltage to something and make it hold until voltage is applied again. Thank you.

[petemoore]

Does anyone know a simple way to store voltage for a long time? Best would be forever:)
  Battery gets as close as anything I know of.
  If you're using it as a reference voltage, and have it seeing only a really high impedance like a Mosfet gate, a cap can hold voltage for a while.
  I have been thinking how to do this and searching for something but haven't found anything.
  I want to be able to apply voltage to something and make it hold until voltage is applied again.
  How long for the hold ?
  What voltage do you want to be able to apply to something and how much current does something draw.
  What type of ramp do you want between the voltage you apply and the other voltage you apply ?
  Will the second application of voltage always be greater than the first ? 

[frequencycentral]

Sample and hold. Like in analogue synths. Though the cap will slowly drain.

[R.G.]

digitize and remember the number

Anything else is subject to analog leakage and degradation. Even polystyrene, teflon, and glass dielectrics in caps leak some. Even if they didn't the humidity in the air around the hold cap would leak it down eventually.

[kristjan]

Yes I have tried with the caps but most the voltage leaks out in less than a day. I think I have to go digital but I just wanted to see if it was possible with some simple solution:) I want to control a VCO with a sequencer that I set the voltage for each step with a single pot. Then use a analog switch to select the step.

[R.G.]

If you're trying to do more than a day hold time, you're way past what can be done with analog.

[JKowalski]

I've seen a number of circuits that sample and compensate for leakage, but for a day, I second going digital.

[MoltenVoltage]

http://en.wikipedia.org/wiki/Memresistor

[PRR]

"Forever" is a looooong time.....

"One Day" is rough for a cap. (Depending on the precision.)

If you are allergic to digital: motorized potentiometer. Put a fixed reference voltage across the pot. Compare wiper-voltage to input-voltage. Use that to turn the motor to zero-difference. Disconnect the motor. It will hold "forever"; or until you bump it, or your reference voltage drifts.

This would be silly, except that several 1980s hi-fi rigs used motorized volume pots, and the Panasonic replacement is listed at DigiKey. A few bucks for the pot, a small power op-amp, some power rails.

[kristjan]

Yes I just want the voltage to hold in the setting that I have set them for the sequencer as long as power is on. I think I will try to go digital. I haven't done much diy digital stuff but it is fun to try:)

[R.G.]

Note that a $2.00 PIC can hold those values in EEPROM even when the power's off, as well as reading them to a nominal precision of 0.1% with the built-in A-D.