I would consider driving the relay directly with the unregulated voltage.
This is easily done with an NPN transistor or logic FET and a resistor or
two.

This technique will not overload the 78L05, and the load on the wallwart
should be well under the 380ma spec leaving plenty of headroom for the
78L05.

What is the actual voltage coming out of the wallwart at low (or no)
current?

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: "John Waters" <john_fm_waters@HOTMAIL.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, July 09, 2001 1:49 PM
Subject: [EE]: Voltage regulation for circuits with relays, how?


> Hi All,
>
> I'm using a 78L05 to keep a stable 5V supply to a simple microcontroller
> circuit, the power comes from a standard home-use 9V, 380Ma power adapter,
> it works fine so far. However, I want to add a relay to my circuit, which
> will easily take up to 80Ma when being turned ON, the 78L05 is now bearly
> powerful enough to maintain a stable 5V. Other than that, when the relay
is
> drawing current, the power adapter output voltage will drop considerably,
> this sudden drop and rise in input voltage to the 78L05 will affect its
> regulated output as well. What is the usual way to handle this situation,
> where a low power consumption circuit is driving a relay, with all power
> being supplied by a domestic grade power adapter?
>
> Thanks in advance!
>
> John
>
>
>
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