Good elecs have discharge tau's in days.
So turn the pot.
The op "knows some basics" and wants a chart recorder to sweep. So you're suggesting he learn to program a PIC and lay out a board with DACs and stuff?
The RC thing is something anybody can make in a few minutes from a few RatShack parts, and once it's running it will be easy and obvious how to tweak it.
John
The linearity of a charging cap depends on how close to the battery voltage you allow it to go; the higher the cap voltage goes, the lower the slope and the worse the curvature. If you use a 9-volt battery to charge a cap through a resistor, and only use the first volt of the resulting voltage ramp (namely set your chart recorder to sweep over the 0 to 1 volt range), the ramp error will be roughly 5%; to be exact, the slope at the and of the 1-volt ramp is 11% down from the slope at the start.
If you use 0.1 volts of a potentially 9 volt ramp, the linearity error is below 1%.
Try it!
John
Thanks , I looked for one like that, and it was on sale for $53,000. I will fool with the RC and see what happens.
pot----------*--------*---------to recorder+
*-----------------------------------------*--------*--------- recorder -Yeah, good idea, he did say he had MV input ranges, good reduction!
Rocky
Okay, here's a sketch, but I do NOT have time to support it, sorry. Ignore any shwarma or mango stains.
Best regards, Spehro Pefhany
Hello,
Yes. For example, the Wavetek function generator in our lab is digitally controlled and operates from 10mHz (milli-Hertz) to 12MHz. So this one can do a 100-second ramp. Not quite long enough for your application but I am sure you could find one that goes below 1mHz. It can do ramp-up, ramp-down, triangle, sine and all kind of other tricks. Great linearity. Everything is programmable, including output amplitude and offset which is importannt with x-y recorders. Most of them also display and store the programmed values so it'll be the same when you turn it on the next morning.
But it must be a digitally controlled version, not some cheap RC solution. So look around for one with a digital readout (LED or LCD).
Regards, Joerg
I read in sci.electronics.design that " snipped-for-privacy@sover.net" wrote (in ) about 'ramp time to voltage', on Fri, 29 Apr 2005:
No, both charge and discharge are exponential.
It's rather simple to charge (or discharge) your cap though a constant-current source, which gives a linear ramp. You can even buy one, looking like a transistor. But you can also make one with two transistors and two diodes or an LED. I don't have time to draw you a diagram at present, sorry.
Hello,
No idea where that price came from. Refurbs are often around $1000, on Ebay they can go for a few hundred, with some luck under $100. Here is one that supposedly goes down to 100 Mikrohertz, but you'd have to check that and whether it does what you want:
These are often found cheaply on Ebay.
Regards, Joerg
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