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'[EE]: power supply rises too slow'
2003\03\09@131910 by ?c2FtbyBiZW5lZGnoaeg=?=

Hi!

I have very simple power supply, consisting from transformer 220/12V,
bridge, 2200micro cap, 7812 regulator, another 2200micro cap to get 12V.
From 12V I have 7805 regulator and yet another 2200 micro cap. The problem
is, that 5V rises too slow to reliably start my 16f877. Is there any simple
way ( except smaller caps) to make 5V rise faster?

Regards, Samo

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2003\03\09@132735 by Wouter van Ooijen

face picon face
Maybe could you use a separate bridge to feed the 7805, maybe using a
smaller cap, and a smaller cap after the 7805 (why 2m2 there?).

Did you enable the brownout protection in the F877?

The real solution is to use an external reset chip....

Wouter van Ooijen

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> {Original Message removed}

2003\03\09@133741 by erholm (QAC)

flavicon
face
First, connect both the 7812 and the 7805 directly to the diod bridge
(and the primary 2200uF cap, of course).

(In you curcuit, the 7812 have to deliver the full current of *both*
the 12V *and* the 5V rail.)

Second, I don't think you need that large caps *after* the 78xx's.
Try with, let's say 10-100uF (and a 100nF cap to stabilize things).

Note that the 78xx have builtin current limiting, and this will create
a voltage "slope" over the cap on the output rail, even if the *input*
to the 78xx has a short rise time.

I don't thing there is any way to get a faster rising 5V rail
with the current caps on the output rails.

Jan-Erik Soderholm

samo benedicic wrote:

>Hi!
>
>I have very simple power supply, consisting from transformer 220/12V,
>bridge, 2200micro cap, 7812 regulator, another 2200micro cap to get 12V.
>From 12V I have 7805 regulator and yet another 2200 micro cap. The problem
>is, that 5V rises too slow to reliably start my 16f877. Is there any simple
>way ( except smaller caps) to make 5V rise faster?

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2003\03\09@152433 by Jinx

face picon face
Do you need those big caps for something special in the
circuit ? It sounds like the sort of arrangement you'd try
because the power supply is collapsing at some point. I
have an application for driving 4 ohm solenoids. The regs
have the normal low value caps and the solenoids have
their own high value cap supply current-limited through
an LM317. The big caps store enough charge to pulse the
solenoids without affecting the PIC's supply. Would you
be able to add a separate 78L05 just for the PIC ? It could
have a D-C filter on its input to isolate it from the rest of
the PSU

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2003\03\09@163016 by David Harris

picon face
Hi-
How about a pass transistor configured to turn on when the supply gets
up to voltage?
David


samo benedihih wrote:

{Quote hidden}

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2003\03\10@072036 by Rick C.

flavicon
face
Shouldn't happen. An often overlooked design error is to put large
capacitors on the output of a 78xx regulator. Should the input drop
below the output (large filter on output, filter and heavy load on input
senario), the regulator will blow. A circuit as you described with a
chain of two or more regulators, with each supplying a load, will
develop this problem. Some datasheets suggest a 1N4001 across each
reguloator in/out with the cathode to the input. This insures the input
will never drop more than .7 volts below the output as the circuit
discharges.
Rick

Hi!

I have very simple power supply, consisting from transformer 220/12V,
bridge, 2200micro cap, 7812 regulator, another 2200micro cap to get 12V.

From 12V I have 7805 regulator and yet another 2200 micro cap. The
problem
is, that 5V rises too slow to reliably start my 16f877. Is there any
simple
way ( except smaller caps) to make 5V rise faster?

Regards, Samo

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