7812 and 7805 for powering a stamp and 2 IC''s
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Posts: 46,084
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:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=uctWULnxGC8F-r5H9fdiHsrcH210Yx0KYAvz8WXFjRqR4UdgAU2aVXJqFwB_OecTKxVHKi0Byio0Sryx]timevex@o...[/url
> Sent: Thursday, September 27, 2001 1:27 PM
> To: basicstamps@yahoogroups.com
> 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
>
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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:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=uctWULnxGC8F-r5H9fdiHsrcH210Yx0KYAvz8WXFjRqR4UdgAU2aVXJqFwB_OecTKxVHKi0Byio0Sryx]timevex@o...[/url
> Sent: Thursday, September 27, 2001 1:27 PM
> To: basicstamps@yahoogroups.com
> 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@yahoogroups.com 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@yahoogroups.com
> 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/