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  • Tank circuit design + help needed.

    I'm working on a tank circuit with very basic voltage regulation.

    The circuit uses a 7.2Ah sealed lead acid battery to fill in during low voltage windows from the main car supply. I'm intending to take over the cigarette lighter fused circuit and add an additional in-line fuse before it goes into the PC. The cigarette lighter output is off during crank and off when the car is switched off (actually it's grounded in both these cases).

    The diodes I will be using are 20A 0.25 average forward voltage drop schottky's.

    I'll be using 8 of these to bring the 13.4->14.6 voltage swing I've been seeing whilst driving down to 11.4->12.6 which is (just) within ATX specs. Then I'll send that into a PW-200-M.

    The problem is that I need a cheap and simple way to disconnect the tank battery from the PC when it's being supplied with a voltage greater than it's capable of supplying. I.e. when the car is > ~13V I want to automatically disconnect the tank battery from the PC and rely on the car voltage. But below that I want to be using the tank battery.

    Another question is - how big a capacitor is good to smooth voltage during the transition?
    Attached Files
    Progress: 80% - Permanent install left.
    Motion LS800 Tablet PC and dock.
    Vista, Bu-535 GPS, RoadRunner, MPT2006.

  • #2
    I would probably go a more complicated route, using 2 MOSFETS and switching on the gates whenever necessary (when alternator is on, switch off the tank MOSFET), this could also be implemented with a PWM controller to charge the tank battery when the alternator is on. Just verify the MOSFETs rds(on) is VERY TINY!!

    this one is good http://www.fairchildsemi.com/pf/FD/F...A0.html <br /> and PRICEY!!!

    you probably only need about 10 amps IRFZ44 is CHEAP!!!
    Carputer currently 'ghettoed' into car!!!

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    • #3
      I'm planning using the DIY relay controller being designed on the 'General Hardware' board, it supposedly will be able to control 8 relays (up to 0.5A switching current) and will have 5 digital inputs and 3 analogues. I'm gonna program it so that it will connect the tank to the alt when the car battery is above a certain min (~13.5V) and when the tank is below 12V, and disconnect when the tank is at 14V and thus fully charged. I might even rig it so that the tank can never be connected to the alt when the car is in neutral or park (its an auto) as this is when the output voltage from the alt is lowest.

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      • #4
        I wouldn't use the computer to switch the tank because it needs to be very fast, and needs to work when the PC is booting/shutting down for instance. I was trying to think of a simple automatic circuit to do this. Like use a comparator (LM339), or a 555 that's triggered instantly with a small delay but don't know if that's needed. Anyway, something with a 339 and a few resistors and zeners could hold a relay/MOSFET at a set voltage, plus the coil of the relay could be connected to a COM port and set up like a UPS, or connected to the DIY I/O board and you could monitor volts and know if the tank is in use at the same time. That would be perfect.

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        • #5
          That's exactly what I'm thinking of but I've hit the limits of my remembered electronics knowledge. I don't suppose you can point me at example circuits?
          Progress: 80% - Permanent install left.
          Motion LS800 Tablet PC and dock.
          Vista, Bu-535 GPS, RoadRunner, MPT2006.

          Comment


          • #6
            I'm way past my limits on most EE things too. That's what makes it fun. I just buy parts in bulk so I can replace them when they go POW.
            Anyway, something like this pic. The left diode is 5.1V (reference) and the right one is 12V (power). The arrow is the neg side of the output and goes to a transistor to control the relay. So as long as the other side of the LM339 input is higher than X (adjustable) the relay remains closed, and if it goes lower it opens or just kills the whole thing. Does that look right?
            Attached Files

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            • #7
              Curiosity, that should work.

              The only thing I would change is I would use an FET such as the IRF1404 at the output of the LM339. It is plenty big enough, has a low RDSon (.004ohm = lower heat losses), extremly high current rating, and doesn't cost too much (~$1.68 each at 10 qty Digikey.com). I would not use the IRFZ44, it has a high RDSon.

              Wire the arrow to the gate of the fet. Wire the + side of the tank battery to the FET's drain pin, and the FET's source pin to what you labeded PC+ in the schematic. You may also want to use an optoisolator to buffer the FET's gate pin, but it shouldn't be necessary.

              As for the filter capacitors on output, I would recommend 2 or 3 470uF caps, and maybe even a .1uf cap. The reason: small capacitors will filter the high frequencies, where the large capacitors will filter low frequencies and provide a power stiffening effect. In reality, the more the better. If I built this board, I would use a .01uF, 1uF, 100uF and two 470uF caps. Overkilling it is always better than the alternative, and caps are cheap.

              ColdPhreze
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              200wRMS inverter (for the screen), CarPC Pro v2.4 Power Controller,
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              • #8
                Thanks very much ColdPhreze. I always have trouble figuring out all the caps. Right now I have a 0 voltage drop relay (large diodes on the relay contacts) to switch between main and tank battery, so I was just going to control it with this. Then that goes on to the ghetto SDC. Almost done! I just hope that switching to the tank doesn't raise the main battery voltage back up. That might suck.

                Comment


                • #9
                  Originally posted by Curiosity
                  I wouldn't use the computer to switch the tank because it needs to be very fast, and needs to work when the PC is booting/shutting down for instance.
                  Thats right, the project on that thread is controlled by the PC, what is really needed is a dedicated controller to switch over the 2 batteries.

                  Originally posted by Curiosity
                  Right now I have a 0 voltage drop relay (large diodes on the relay contacts) to switch between main and tank battery, so I was just going to control it with this.
                  Just out of interest which circuit did you follow for the zero voltage drop? I have never built the one I posted here ages ago. The diode you used? tank battery? any limiting resistor? circuit diagram?

                  I think the idea of auto switching the zero voltage drop setup with the circuit above is great.


                  Let us know the results

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                  • #10
                    But just to add things...

                    If theres enough demand to have an auto switching between the battery and the tank using the serial card...the firmware can be modified such that one of the analog input is used to monitor the battery level of the main battery and then use one of the relays to switch over to the tank.

                    Of course one ADC and one relay is going to be tied up for this application...unless of course the firmware is reflashed again

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                    • #11
                      Originally posted by lummox
                      I'm planning using the DIY relay controller being designed on the 'General Hardware' board, it supposedly will be able to control 8 relays (up to 0.5A switching current) and will have 5 digital inputs and 3 analogues. I'm gonna program it so that it will connect the tank to the alt when the car battery is above a certain min (~13.5V) and when the tank is below 12V, and disconnect when the tank is at 14V and thus fully charged. I might even rig it so that the tank can never be connected to the alt when the car is in neutral or park (its an auto) as this is when the output voltage from the alt is lowest.
                      Its a great I dea, but from what curiosity said the PC need to be fast and switched on before anything can be controlled.

                      A much more realistic approach is to perhaps have a "tank/inverter manager" based on a small PIC with ADC and relay to control whats needed. As been said earlier, the PC can then read off the voltage/status of the batteries.

                      New project comming along... PIC based tank/inverter manager

                      Sound like a cool stuff, Ill look into it, when I have the time

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                      • #12
                        Ricky327, I used your zero voltage drop relay of course! Large 6A diodes. I figured it will just get current from the higher one if it doesn't switch properly. The battery is a 7A SLA and I have a 10 ohm 10 watt resistor between the two batteries. I guess it really needs a charging circuit/manager thing like you said to actually make it all work properly.

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                        • #13
                          Originally posted by Curiosity
                          I figured it will just get current from the higher one if it doesn't switch properly. The battery is a 7A SLA and I have a 10 ohm 10 watt resistor between the two batteries. I guess it really needs a charging circuit/manager thing like you said to actually make it all work properly.
                          OK assuming the switching between the 2 batteries happens correctly does it work?...if they were switched manually.

                          If they do work then I guess you can build that circuit to do it automatically.


                          Maybe you wont need the circuit you propose if :

                          Lets say the main battery is at 12.5V (not cranking) this voltage is used to power to relay coil that switch between the 2 battery.

                          So the main battery is being used to power the PC since the relay switch is connected to it.

                          As soon as the main battery drop below 10V (cranking) the relay coil can no longer hold and so would normally switch over to switch resting position...therefore connecting the tank battery to the PC.

                          I dont know how reliable this would be, but it may be worth a try.

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                          • #14
                            Yes, if switched manually it works fine. Well, I've only done it when cranking so it's always been switched to the higher voltage supply as far as I know.

                            Right. The relay would be powered for main battery. That way, even if it dropped to 2V or something the relay would be off, and it would be on tank supply.

                            No harm in trying.

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                            • #15
                              Cool...

                              You just need to adjust the relay such that it cannot hold the switch when the main battery voltage goes below 10V lets say...via a series resistor perhaps?

                              Ideally the circuit should switch over to the tank just before the car is to be cranked and this is what you are doing manually? or?

                              What happen if you switch over to the tank half a second later on starting the crank? The diodes are supposed to do the switching electronically and powering the PC, with the diode voltage drop of course. As soon as you switch the relay manually then the PC should receive the zero voltage drop.

                              Just wanted to know if the circuit you intend to add will cause any problems if it were to switch half a second later when the main battery had already droped to a low level.

                              If it work with the switching happening abit late then circuit should work fine.

                              Let us know what happen and post a new thread for all who may wanna try...something like "Curiosity's auto switching zero voltage drop tank circuit"

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