> 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?

First, I would run the relay off the unregulated supply, although that won't
help with the overall voltage droop problem.  However, if you look closely
at the specs for your relay you will probably find a closing current and
holding current spec.  The closing current is the short term current
required to flip the relay from the off to the on state.  The holding
current is the current required to keep it in the on state once it is there.
The holding current can be several times lower.  To make use of this, tie
the unregulated supply to a resistor, then to the coil, then to the
switching transistor, then to ground.  Adjust the resistor to get the
holding current.  The closing current comes from a cap from the junction of
the resistor and coil, to the ground.  This will supply a higher current for
a short time when the relay is first switched on.  The exact value depends
on the coil resistance, the unregulated supply voltage with the relay off,
and the length of time the closing current is needed.

Oh, and don't forget the flyback diode!

********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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