Tank circuit design + help needed.

**Arathranar** · 11-06-2004, 08:44 PM

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?

**choyak** · 11-06-2004, 09:31 PM

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!!!

**lummox** · 11-06-2004, 11:03 PM

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.

**Curiosity** · 11-07-2004, 09:19 AM

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.

**Arathranar** · 11-07-2004, 02:37 PM

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?

**Curiosity** · 11-07-2004, 03:46 PM

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?

**ColdPhreze** · 11-07-2004, 05:36 PM

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

**Curiosity** · 11-07-2004, 05:53 PM

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.

**Ricky327** · 11-07-2004, 10:33 PM

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

**Ricky327** · 11-07-2004, 10:40 PM

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