voltage follower mystery
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I am trying to make a very simple voltage follower that generates an
output voltage equal to its input voltage (over a 0 to 5 vdc range).
I tried using an LM358 (from the analog/digital Basic Stamp kit) and
wired it up as a voltage follower, as described in the Analog and
Digital manual (page 13. i.e., 5vdc power to pin 8, ground to pin 4,
connected pin 1 and 2, put input voltage to pin 3 and read output at
pin 1). THe circuit works fine up to 3.75 volts, but the output will
go no higher then 3.75, even when the input is a full 5 volts. What
am I doing wrong, and/or can I use this op amp to follow the voltage
across the whole 0 to 5vdc range?
output voltage equal to its input voltage (over a 0 to 5 vdc range).
I tried using an LM358 (from the analog/digital Basic Stamp kit) and
wired it up as a voltage follower, as described in the Analog and
Digital manual (page 13. i.e., 5vdc power to pin 8, ground to pin 4,
connected pin 1 and 2, put input voltage to pin 3 and read output at
pin 1). THe circuit works fine up to 3.75 volts, but the output will
go no higher then 3.75, even when the input is a full 5 volts. What
am I doing wrong, and/or can I use this op amp to follow the voltage
across the whole 0 to 5vdc range?
Comments
voltage (usually ground), but the amount of current you draw will push
the upper limit away from the supply voltage. If you want the voltage to
vary between 0 and 5, you need (for a normal op amp) a larger supply
voltage like -5 to 10V or something. However, as I said, many op amps
can go to ground so a 0-9 or 10 or 12V supply would be OK for those.
The LM358 is a "single supply" op amp. If you get the data sheet look at
http://www.national.com/ds/LM/LM158.pdf and read the chart labeled
Output Characteristic Current Sourcing.
Hope that helps.
Al Williams
AWC
* Floating point A/D
http://www.al-williams.com/awce/pak9.htm
>
Original Message
> From: tomatlarge [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=B5tuY0EvMDm20CZgJSdi6018EVaB_XjpIxaTmR_yL_s_nSB7SdfuEis-ubAkHoG94iqkXQJMpXOJ0XtFGQQ]tomatlarge@y...[/url
> Sent: Sunday, December 16, 2001 12:48 PM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]basicstamps] voltage follower mystery
>
>
> I am trying to make a very simple voltage follower that generates an
> output voltage equal to its input voltage (over a 0 to 5 vdc range).
> I tried using an LM358 (from the analog/digital Basic Stamp kit) and
> wired it up as a voltage follower, as described in the Analog and
> Digital manual (page 13. i.e., 5vdc power to pin 8, ground to pin 4,
> connected pin 1 and 2, put input voltage to pin 3 and read output at
> pin 1). THe circuit works fine up to 3.75 volts, but the output will
> go no higher then 3.75, even when the input is a full 5 volts. What
> am I doing wrong, and/or can I use this op amp to follow the voltage
> across the whole 0 to 5vdc range?
>
>
> To UNSUBSCRIBE, just send mail to:
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> Subject and Body of the message will be ignored.
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>
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> http://docs.yahoo.com/info/terms/
>
typing and indeed a 9 vdc power supply fixes the problem--as verified
on the breadboard. Thanks!
--- In basicstamps@y..., "Al Williams" <alw@a...> wrote:
> Most op amps will not go "to the rail". Some can go to one supply
> voltage (usually ground), but the amount of current you draw will
push
> the upper limit away from the supply voltage. If you want the
voltage to
> vary between 0 and 5, you need (for a normal op amp) a larger supply
> voltage like -5 to 10V or something. However, as I said, many op
amps
> can go to ground so a 0-9 or 10 or 12V supply would be OK for
those.
>
> The LM358 is a "single supply" op amp. If you get the data sheet
look at
> http://www.national.com/ds/LM/LM158.pdf and read the chart labeled
> Output Characteristic Current Sourcing.
>
> Hope that helps.
>
> Al Williams
> AWC
> * Floating point A/D
> http://www.al-williams.com/awce/pak9.htm
>
> >
Original Message
> > From: tomatlarge [noparse][[/noparse]mailto:tomatlarge@y...]
> > Sent: Sunday, December 16, 2001 12:48 PM
> > To: basicstamps@y...
> > Subject: [noparse][[/noparse]basicstamps] voltage follower mystery
> >
> >
> > I am trying to make a very simple voltage follower that generates
an
> > output voltage equal to its input voltage (over a 0 to 5 vdc
range).
> > I tried using an LM358 (from the analog/digital Basic Stamp kit)
and
> > wired it up as a voltage follower, as described in the Analog and
> > Digital manual (page 13. i.e., 5vdc power to pin 8, ground to pin
4,
> > connected pin 1 and 2, put input voltage to pin 3 and read output
at
> > pin 1). THe circuit works fine up to 3.75 volts, but the output
will
> > go no higher then 3.75, even when the input is a full 5 volts.
What
> > am I doing wrong, and/or can I use this op amp to follow the
voltage
> > across the whole 0 to 5vdc range?
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@y...
> > from the same email address that you subscribed. Text in the
> > Subject and Body of the message will be ignored.
> >
> >
> > Your use of Yahoo! Groups is subject to
> > http://docs.yahoo.com/info/terms/
> >