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From: "Bill Sloman"
Subject: Re: Linear voltage change over a 30 minute period?
Date: Fri, 13 Dec 2002 23:56:22 +0100
Organization: Planet Internet
NNTP-Posting-Date: 13 Dec 2002 22:56:20 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Spehro Pefhany" wrote in message
> On 13 Dec 2002 11:26:33 -0800, the renowned firstname.lastname@example.org
> (Justin) wrote:
> >I have a basic electronics question (or I would think it's basic). I
> >want to build a simple DC circuit that will supply a linearly
> >increasing voltage from V=0 at t=0 to V=10 volts at t=30 minutes. The
> >circuit would be powered by either a battery or an AC/DC adapter.
> >If linear is too complex I would be okay with just getting the voltage
> >to ramp up over a time period of about 30 minutes. Can anybody
> >enlighten me?
> 30 minutes is a fairly long time for an analog circuit. You'd
> either be dealing with very low currents and a film capacitor or
> more reasonable currents and a potentially leaky and otherwise
> troublesome special electrolytic cap. Resetting it accurately
> could also be a problem (I've used reed relays in some cases).
> I suggest you consider using an oscillator (a 555 will do), a
> CD4040B 12 bit counter and a R-2R ladder to generate it digitally.
> If you use up one output to halt the 555 at the end of the ramp,
> you'll have 0.05% resolution (steps every 0.88 second), which
> leads to quite reasonable timing components for the 555.
I looked at this in some detail for a magetic dilution refrigeration scheme
a few years ago, and Burr-Brown had a 20-bit sigma-delta DAC with a half-way
decent set of DC specifications - most sigma-delta DAC are exclusively aimed
at audio and their DC performance is terrible.
You could do a very nice ramp with their part. IIRR it was the DAC1220.
Bill Sloman, Nijmegen
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