7812 and 7805 for powering a stamp and 2 IC''s
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Posts: 46,084
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=W6cOiZWypfuKdIWfOnttJbtmtqNWOuJKIirpyT7TpqZ5VgBBZqjqbb60xKDo305gfzLLImNoXJulxEQK_GBT]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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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=W6cOiZWypfuKdIWfOnttJbtmtqNWOuJKIirpyT7TpqZ5VgBBZqjqbb60xKDo305gfzLLImNoXJulxEQK_GBT]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/