🚀 go-pugleaf

I've been working on this circuit for a while now, and I thought I was almost
finished, but it turns out that what I thought would be the easiest part of
the design has  become one of the strangest problems I've had.  Here's what
it's about.

I have made an 8W audio amplifier for my presently-speakerless monitor, and
I've made a video switch that selects one of two video signals coming from
my computer and sends it to the monitor, depending on a TTL control signal that
I control from my computer.  Both of these circuits are going to be put
together in the same box.

I protected both sections with normally-open relays.  I'm doing that so that
when my audio and video signals are on, but the power supply for my circuit is
off, that the signals don't harm the CMOS IC's.  The relays close when the coil
voltage is 5V.  I measured a resistance of 50 ohms across the coil terminals,
and I measured 100mA flowing through the coil when it's closed (makes sense).

I bought one of those '9V AC adapters' (the kind that claim to replace 9V
batteries; it turns out that mine puts out 14V, but that's OK for my
application - and it's rated for 1 amp) with the intention of using a 7805
voltage regulator to feed 5V to the portions of the circuit that need it.  The
7805 is also rated for 1 amp.

Here's the funny part.  When I power the circuit with my bench power supply,
it's fine.  Then when I try the AC adapter instead, the output of the 7805
drops to only about 1V, instead of 5V!  Immediately, I thought that my AC
adapter was the culprit, but when I go to measure the input to the 7805, it's
a healthy 13V.  Nevertheless I tried another '9V' AC adapter that I happened
to have (it's only rated for 500mA, but it does output about 14V).  Exactly
the same thing happened (Vout dropped to 1V).

So then I thought it was a bad 7805.  Yet, when I took off the
AC adapter and replaced it with my bench supply, it worked fine again.  Ugh!

The next thing I did was to test the current capability of the AC adapter and
the 7805.  I took out the 7805 from the circuit and loaded it with some
resistors (see the upcoming diagram).  To my regret, it worked.
Here are some measurements:  For the 7805: Vin=13.04V; Vout=4.7V, with a load
of five 130 ohm resistors in parallel (equivalent to 26 ohms).

Now, when I replace the parallel resistors with the relay, everything goes
out the window.  I thought that if I can put 4.7V across 26 ohms, I should
be able to put at least 4.7V across 50 ohms (that's the relay's
coil resistance).  Much to my astonishment, it doesn't happen.  Vin stays at
about 13V, but Vout drops to a little over 1 volt!  How can this be?

To summarize this maddness, here's a diagram:

                                  ______
                                  |7805|
                       ~13Vdc     |    |   4.7Vdc
                         |        ------     |
                         v        | | |      v    _________
120V AC ---- AC adapter ----------| | |-----------|26 ohms|------
                                  V G V           ---------      |
                                  i n o                          |
                                  n d u                         Gnd
                                      t

Now if you replace the 26 ohms with the relay coil terminals, Vout drops
to about 1V.  What's wrong here?  Does it have anything to do with the 
inductive nature of the coil?  Is the coil resistance that I measure with my
multimeter a bogus value?
                                             Jean Goulet
                                             Electrical Engineering
                                             Class of '89
                                             Concordia University
                                             Montreal, Canada