[Diy_efi] Re: Looking for circuit to turn transfer pump off at

[Toyota Supra]

hey brian,

thanks for explaining hysteresis to me in normal terms. i had an 
understanding, but it has been furthered by your explanation! it'll probably 
take me a while to build a circuit (since i am a lowly novice) but i'll try 
and post something once it is done for critiquing.

-j






From: Brian Dessent <brian at dessent.net>
>Reply-To: diy_efi at diy-efi.org
>To: diy_efi at diy-efi.org
>Subject: Re: [Diy_efi] Re: Looking for circuit to turn transfer pump off at 
>  certainvoltages
>Date: Tue, 12 Nov 2002 22:34:06 -0800







>What you want is called a window detector.  It's basically two
>comparators, and two reference voltages.  Check out the datasheet for a
>basic comparator, such as:
>
>http://www.national.com/ds/LM/LM119.pdf
>
>Usually the two threshold voltages are generated from voltage dividers
>(i.e. resistors) -- you can use potentiometers if you want to be able to
>adjust the thresholds.
>
>The tricky part is what happens right along the thresholds?  If you
>don't use any sort of hysteresis, the output signal could oscillate or
>"dither" on and off quickly as the input signal always has some amount
>of noise to it.  Hysteresis solves this by making the circuit nonlinear,
>for example: say you want a 2V threshold, so that something turns on at
>2V.  With 100mV of hysteresis, the input would have to rise past 2.050V
>to turn ON and would have to drop past 1.950V to turn off.  There's some
>discussion of this in this application note:
>

oops, hit send accidently)

to turn ON and would have to drop past 1.950V to turn off.  There's some
discussion of this in this application note:

http://www.national.com/an/AN/AN-74.pdf

You will usually see hysteresis explained using a graph of output versus
input, with lines with arrowheads indicating the direction of
transition.  It makes a lot more sense then trying to explain it in
words.

Brian





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