7812 and 7805 for powering a stamp and 2 IC''s
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Posts: 46,084
Kevin,
Yes, I need 12v for a relay. I should have used a 5v relay then I'd
be all set to use just 5v. But droping 20v down to 5v seems, like a
large drop. But since the current is so low, I could problally get
away with it. I may replay the 12v relay with a 5v and get rid of the
7812 all together.
Tim
--- In basicstamps@y..., Kevin Olalde <kolalde@h...> wrote:
> Can the 7812 be skipped? I'm not saying that's what the original
problem is,
> just wondering why the whole curcuit can't just be run from the
same 7805 (or is
> there a specific need for 12V)?
>
> Kevin
>
> "James R. Parish" wrote:
> >
> > Put a 100uf 16v electrolytic on the input and output of the
7805. Also put
> > a .1uf on the output to ground. On the 7812 do the same thing
but use 35vdc
> > caps. IF the 20VDC supply has good filter caps, say 10,000uf at
50VDC then
> > skip the cap on the input of the 7812. In fact, you can use all
35VDC caps
> > if you have them. That should fix your problem.
> >
> > James R. Parish
> > J-Tech Internet Services Division
> > J-Com Enterprises, Inc.
> >
> >
Original Message
> > From: timevex@o... [noparse][[/noparse]mailto:timevex@o...]
> > Sent: Thursday, September 27, 2001 1:27 PM
> > To: basicstamps@y...
> > Subject: [noparse][[/noparse]basicstamps] 7812 and 7805 for powering a stamp
and 2 IC's
> >
> > Hi,
> >
> > I'm using a 7812 regulator to power a stamp from a 20v source.
I'm also
> > using a 7805 to power to IC's from the 12V from the 7812.
> > My problem is that my 5v from the 7805 keeps drifting low, it
starts at
> > around 4.5 and will drift slowly down to 3.0v . I can't figure
out why this
> > is happening, I've tried other 7805's and I get the same result.
The circuit
> > works fine off my bench 5v supply. I'm thinking that this could
be happening
> > because I'm not drawing enough current that it can't keep a
stable voltage
> > with a real low power draw.
> > The only thing the 7805 is powering is 2 IC's (7 seg driver,
RS232 level
> > converter) 2 digit 7 seg display and one bi-color LED. I'm
guessing my total
> > current load is less than 100ma perhaps more like 50ma.
> > Is their a minimum current load for the 7805 or 7812 regulators?
> > Should I just hang a shunt resistor on the 5v to load it down?
> > Thanks for you help
> > Tim
> >
> > To UNSUBSCRIBE, just send mail to:
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Yes, I need 12v for a relay. I should have used a 5v relay then I'd
be all set to use just 5v. But droping 20v down to 5v seems, like a
large drop. But since the current is so low, I could problally get
away with it. I may replay the 12v relay with a 5v and get rid of the
7812 all together.
Tim
--- In basicstamps@y..., Kevin Olalde <kolalde@h...> wrote:
> Can the 7812 be skipped? I'm not saying that's what the original
problem is,
> just wondering why the whole curcuit can't just be run from the
same 7805 (or is
> there a specific need for 12V)?
>
> Kevin
>
> "James R. Parish" wrote:
> >
> > Put a 100uf 16v electrolytic on the input and output of the
7805. Also put
> > a .1uf on the output to ground. On the 7812 do the same thing
but use 35vdc
> > caps. IF the 20VDC supply has good filter caps, say 10,000uf at
50VDC then
> > skip the cap on the input of the 7812. In fact, you can use all
35VDC caps
> > if you have them. That should fix your problem.
> >
> > James R. Parish
> > J-Tech Internet Services Division
> > J-Com Enterprises, Inc.
> >
> >
Original Message
> > From: timevex@o... [noparse][[/noparse]mailto:timevex@o...]
> > Sent: Thursday, September 27, 2001 1:27 PM
> > To: basicstamps@y...
> > Subject: [noparse][[/noparse]basicstamps] 7812 and 7805 for powering a stamp
and 2 IC's
> >
> > Hi,
> >
> > I'm using a 7812 regulator to power a stamp from a 20v source.
I'm also
> > using a 7805 to power to IC's from the 12V from the 7812.
> > My problem is that my 5v from the 7805 keeps drifting low, it
starts at
> > around 4.5 and will drift slowly down to 3.0v . I can't figure
out why this
> > is happening, I've tried other 7805's and I get the same result.
The circuit
> > works fine off my bench 5v supply. I'm thinking that this could
be happening
> > because I'm not drawing enough current that it can't keep a
stable voltage
> > with a real low power draw.
> > The only thing the 7805 is powering is 2 IC's (7 seg driver,
RS232 level
> > converter) 2 digit 7 seg display and one bi-color LED. I'm
guessing my total
> > current load is less than 100ma perhaps more like 50ma.
> > Is their a minimum current load for the 7805 or 7812 regulators?
> > Should I just hang a shunt resistor on the 5v to load it down?
> > Thanks for you help
> > Tim
> >
> > 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/
> >
> > 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/