Miata Mailing List: April 1995, Message #151
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   From: dkkim@uiuc.edu (David K. Kim)
Subject: High/Low beam together
   Date: Tue, 4 Apr 1995 12:48:39 -0400

(Copied to the list for suggestions.  Jay and I have been having a
discussion about adding a diode to make the low beams come on when the high
beam lever is engaged...)

Jay, don't use the 12 volt diode!

>Hi.  I got the Diode from Radio Shack.  The largest one I found was 1Kvolts
>(1000V), 2.5A Silicon Diodes.
>  But since I found a 12volt,21mA with Maximum Power Dissipation of 1.0w, I
>got this one as well.  (Doesn't cars use 12 volts?)
>
> Do I use this 1000V, 2.5A Silicon Diodes?  or 12V, 21mA Zender Diodes?
>
        The voltage is a *breakdown* voltage (beyond which it will conduct
        in *both* directions).  You want a breakdown voltage
        higher than what you'll ever use (so it will act as a "one-way wire").
        You also want the largest current rating possible.

        So use the 1000 V, 2.5 Amp diode, definitely.

        A "zener diode" is designed to break down at a precise voltage for
        regulation purposes.  Since the car will run a few volts above 12,
        this would be the equivalent of a short circuit (until it burned out).

>  I think I know how to hook this up.  Just attach theAnode side to high beam
>(Hot wire) and Cathode side to low beam right?
>
>Any suggestions?
>
>Thanks!
>TAKAHARA

        I believe you are correct about the direction (I always get confused
        by "anode" and "cathode" and the little arrow symbol, not to mention
        "electron flow" vs. "current flow"...)

        In theory, yes, *but* you want to hook it up so that no more than 2.5
        Amps flows through it, which means you want to use the voltage from the
        high beam switch to turn on the RELAY for the low beams.  The low beams
        themselves would draw too much current but the relay will not.
        (My guess: 80 Watts/12 volts = ~6.6 amps for both low beams together)

        Unfortunately, after looking at the wiring diagrams it seems that
                -the high beams are turned on by applying +12 volts to them
                        (the other side is already grounded)
                -the low beams are turned on by applying 0 volts to the
                        relay which controls them ("grounding the relay")
                        (the other side of the relay is permanently
                        attached to +12 volts)

        So, it seems that what we need is to add a relay between ground
        and the high beam "hot" wire which then controls an extra connection
        between the low-beam relay and ground.

        I do not have a FAX machine handy but I will FAX you the relevant
        manual pages as soon as possible.

        Anyone else on the list have a more elegant solution or know if Jay's
        newer car (94, I think) differs significantly in the wiring setup
        (I.e. is my 91 manual a bad thing to go by?)

                                                -Dave Kim

    David K. Kim                                 dkkim@uiuc.edu
"Next to knowing when to sieze an opportunity, the most important thing
  in life is to know when to forego an advantage."  -Benjamin Disraeli