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  • Will this work?

    Does this circut look like it will work? (Mainly the 12V section)
    http://a_channell.tripod.com/powersupply.jpg

  • #2
    doesn't display.

    last i check tripod doesn't allow image hosting, so to get around that make a single page with the image and post a like to the page.
    [SIZE=1]'91 Nissan Stanza

    [XXXXXXXXXXXXXXXXXXXX] XX% Completed -
    Car Totaled and i bought a 20G mp3 player

    http://machs-fuel.tk
    a DIY video projector page

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    • #3
      Cutting and pasting the image url to a browser works to view the image.

      That circuit is crazy. I can see what it's trying to do, but it's not a good plan to use a 12v zener to switch a relay like that, the 12v zener gets you a 12v drop, and you are left with no voltage to switch the relay. It's not gonna work. Basically the 12v output is always gonna be a straight through battery voltage.

      It looks like it's trying to eliminate the voltage drop across the 7812 when the battery voltage is too low by bypassing it using a relay. A better way to do this would be to use a comparator (op-amp with no feedback) to switch the relay. Hook the + input up to the output of the regulator and the - input up to a 12v reference (below which you want to bypass the regulator).


      Rob
      Old Systems retired due to new car
      New system at design/prototype stage on BeagleBoard.

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      • #4
        I was under the impression that when the zener sees +12v or more, the breakdown voltage is reached and it lets current pass. If voltage falls below that it stops current flow again. Is this incorrect?

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        • #5
          No, that is correct. But it will get you effectively a 12v drop across the zener. So the relay will "see" the excess volatge over 12v. A relay needs some voltage to push the current through. So with a fraction of a volt across the relay (which is at best what you will get), my money is on the relay not actuating.


          Rob
          Old Systems retired due to new car
          New system at design/prototype stage on BeagleBoard.

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          • #6
            Hey, what did you use to draw this circuit? I need a good schematic editor. The 50,000 dollar ones we use at work won't satisfy my hobby needs.. :-)
            MPEGBOX - Plexiglass Computer
            www.mpegbox.com

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            • #7
              What are you trying to do with the relay anyway? Is he right? you want a fall back plan when the voltage drops? If that is the case, I would use a transistor to drive the relay. Then use an adjustable a resistor divider tied to the input to drive the base of the transistor. That way you could tune when the relay kicks in and out without using a comparator or any logic. You could also do it with a comparator, but a standard comparator might not be able to supply enough current to drive the relay, you'll need the transistor too.

              Also, your not putting any capacitance on the output of the 5 volt section, and the output cap is backwards on the -12 volt section. You probably also want some capacitanc on the input to the 765, but it will probably work without it. Just not at high current
              MPEGBOX - Plexiglass Computer
              www.mpegbox.com

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              • #8
                Ohh okay, my understanding was slightly incorrect, thanks for correcting me . I tried it that way because I have never used logic components before (except microcontrollers). I used Circuitmaker 2000, it is really easy compared to most I've used, and it will do simulations as well. I don't know how much it costs as I aquiered it from a "secret" source.
                Thanks again!

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                • #9
                  Originally posted by Jeff Mucha:
                  <STRONG>Hey, what did you use to draw this circuit? I need a good schematic editor. The 50,000 dollar ones we use at work won't satisfy my hobby needs.. :-)</STRONG>
                  www.protel.com
                  astalavista.box.sk

                  That's all I'm gonna tell ya.

                  Presslab

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                  • #10
                    Originally posted by Jeff Mucha:
                    [QBThen use an adjustable a resistor divider tied to the input to drive the base of the transistor. That way you could tune when the relay kicks in and out without using a comparator or any logic.[/QB]
                    Could you explain this a little better? I'm not sure I understand how this would work.

                    Thanks a lot,
                    Milesg

                    PS: Presslab, you are soooo bad. (not to say that I didn't do that same thing a long time ago )
                    Near Completion: Intel P166 MMX, 32MB ram, 13GB Hard Drive, Keypad, 4x40 LCD. Sproggy MK2.6 ATX PSU. Win98SE with Winamp and Mark Zehnder Plugin. (Web-site: very soon)

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                    • #11
                      Originally posted by milesg:
                      <STRONG>

                      Could you explain this a little better? I'm not sure I understand how this would work.

                      Thanks a lot,
                      Milesg

                      )</STRONG>

                      It would be a lot easier for me to draw it but I don't have a good editor, so I do everything on paper. You have a npn transistor that is connected to the relay. Emitter to ground, collector to coil, other side of coil to input. you have a resistor divider between the input and ground. you connect the middle of the resistor divider to the base of the transistor. If you play with the values of the resistors, you can change at what voltage the relay turns on and off. if the relay turns off at 12 volts, it will connect the input to the output and you won't have any drop. If the input goes above 12 volts, the relay turns on and the regulator is connected to the output.

                      you could also use a zener to set the value the relay turns on and off. Think of it like this, if the input to the base of the transistor is above .7 volts, the npn is a switch and is turned on, if it is below .7 volts the npn is off.


                      This is a messy way to do it, but you save a volt when the car is off, you will still have problems with restarting the car..

                      [ 12-18-2001: Message edited by: Jeff Mucha ]
                      MPEGBOX - Plexiglass Computer
                      www.mpegbox.com

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