Add-on for WM-1

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From: RICHARD HIEBER (SZ0026@daphne.rrze.uni-erlangen.de)
Date: Tue Oct 04 1994 - 09:52:32 EDT


Hi gang,

I had a short exchange with somebody from the QRP mailing list some
weeks ago. I told him about a little circuitry that I added to my
beloved WM-1 wattmeter from Oak Hills Research. The problem was, I
forgot to switch it off and drained the battery. If not, I was afraid
that I might have forgotten to switch it off and was much too occupied
to check the position of the rotary switch.

One day then I decided to install the following circuitry:

 +9V (Battery)
 O------------------------------------------------------+
        | | |
        | | |
        | | X
        | | X R=5M
        | | Zener diode X
        | | E | 6.2 Volts X
        | C | +-->| B K+- A R=5M |
        +---| B |--------| xX-------xxxxxx------|
            |-----------| | |
        +<--| C | +--------+
        | E | | |
        | | |
        | C --- \ push button
  +-----------+ e.g. --- |
  | OHR | 10myF | |
  | WM-1 | | |
  +-----------+ +--------+
        | |
        | |
        | |
        +-----------------------------------------------+
                          |
                        ----- Ground

My apologies for the very crude design of the ascii drawing. The
transistors are bipolar ones, not FETs. I found it incredibly hard to
do this ... must have a closer look at one of the better examples of
this art :-)

The essential element in the circuit is the push button which empties
the condenser. As long as the condenser gets loaded, the two
transistors are giving way to feed the battery voltage to the
wattmeter. I am getting a switch-off time of about 4 minutes with a C
of 22 or 47 micro Farads (can't remember the right value). In the last
minute or so the wattmeter readings are getting inaccurate since the
voltage gets cut off gradually, not like with a switch. I've thought
about better designs, e.g. with a NE555 (uses too much power standby)
or with a low power thyristor (current consumption of the wattmeter is
too low), but wasn't successful. Standby consumption is NOT zero: My
digital ammeter shows something around 1 micro amp. I think it's
tolerable. Time will tell :-)

If anybody else has a better solution or ideas to improve on this one,
I'd like to hear about it.

72 & 73 de Richard, DL8MFQ @ DB0SIF.DEU.EU (alias AA8CP)


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