Simple & Tested Tank Circuit

**Jamez_Warner** · 11-27-2003, 02:05 PM

OK, seen a lot of dodgy tank circuits on the forum, especially the capacitor based ones - that will rarely work ( only if you have a v good battery and a 1farad cap will it make a difference )

This solution uses a Yuasa NP 1.2Ah 12V battery as the "backup" supply, charged via a 10ohm resistor, the manual for battery says you do not need this resistor as the battery internally limits charge current - but I want to be on the safe side.... ( the battery will charge untill it 'floats' at the same voltage as the main battery....

Always uses lots of fuses

Try to use 0.4V drop diodes, and a 10W+ resistor

This battery will provide enough power to keep an EPIA + TFT alive for a good 10minutes if it takes that long to crank your car though i'd get a new one !

This circuit works and has been tested - the battery could be charged better by using an L200 regulator - but for the £4 i paid for battery I don't mind a shorter life

IMPORTANT NOTE: while this circuit works fine for me - use your common sense whilst building such circuits - use this as a guide only. Incorrect circuits can cause overcharging and may lead to a fire - ( which is bad in a car

) Please take care and test under supervision on the bench....

**Ricky327** · 11-27-2003, 03:09 PM

Good post jamez...hope the capacitor tank discussion will disappear

Yup yuasa datasheet says you dont really need a limiting resistor, I didnt like that idea either just incase the battery have internal short and drains lots of current.

Does your battery get charged fully? since you have 0.4V drop before the voltage get fed to the backup.

**Jamez_Warner** · 11-27-2003, 03:14 PM

yes, resistor is a safety measure ( allthough swapping this for a 4A fuse would do same job ) recommended float charge is 13.5V, car gives out 14.5 when running - ( my diodes are 0.8V drop atm) which gives a 13.7V float ( near enough )

just installed in car - and has stopped pc rebooting - works perfectly...

**Ricky327** · 11-27-2003, 03:22 PM

Good to hear that...I only expect about 13.8V from a car running. What PSU you feeding that into? Im using a DC-DC converter and I cant afford to loose any voltage at all...it dies when the supply goes below about 11.5V

With the capacitor :

1 farad capacitor? I dont even think thats big enough...less than half a second you get on 1F at 5A drain. No one seems to have proved they managed to succede with it.

**Jamez_Warner** · 11-27-2003, 03:27 PM

yeah - the 1 farad only works if you battery has a low voltage drop while cranking anyway ( the bulk of current of a crank is drawn in first few milliseconds - cap compensates for this and battery is enough to keep pc alive through rest of crank) but car batteries are rarely this good and i haven't heard of anyone being succesful with a cap

im using a morex 55w DC-DC psu, it too trips out at about 11.5V ( but at below 12V the HDD usually spins down causing a crash )

this also means you can change you main car battery with your PC still on if you are particulary addicted ! rofl

**SuperSonic** · 11-27-2003, 04:18 PM

Correct me is I'm wrong, but this circuit will provide backup battery power AND main battery power right? What I am suggesting is that running the PC with the engine off is no problem (it's not just limited to the capacity of the backup battery alone in this setup)

So you could easily run the PC for say 2 hours with the engine off?
Won't the downside of this be that you will probably drain your backup battery in the process, thus killing the PC when cranking after that?

Nice design!

**Jamez_Warner** · 11-27-2003, 04:46 PM

the backup battery runs about 2v lower than the main battery - running the PC with the engine off will drain main battery for 90% of the time, but will switch to backup power when main battery is partially discharged ( then both will go down together )
I don't think this would ever be a problem though - and your backup battery can never be lower than your main whatever happpens

**OdysseyPC** · 11-27-2003, 04:58 PM

It is true that you would be able to run the PC while the engine is off and provided you have a big enough battery (12Ah+) it will last about 2 hours. I currently use a very similar circuit to this which also powers a Morex 55W DC-DC unit.

I have one extra modification in that i include an automatic shutdown that monitors the battery level if it drops below 11.9V (easily variable) it shuts down the PC to ensure i have enough juice for the next crank.

Because it is often on display i use an 18.2Ah which keeps it chugging for just over 3 hours.

I'm currently in the progress of designing a modified sproggy that has the tank circuit and automatic shutdown all on one board, it's taking a while because i like small things (i.e small boards) and Protel DXP's autorouter likes big things so it is taking some time. Hope to post it soon.

Just to summarise:
* Yes you could run the PC for long times with the engine off
* There is a solution to preventing complete battery drain

**Jamez_Warner** · 11-27-2003, 05:03 PM

odysesy - this circuit is just for powering the PC while you are cranking engine - for about 2 secs - need a tiny capacity battery - you are thinking of a full size dual battery system - if my key isn't in ignition my PC gets shutdown and relay turned off so no current draw....

**Ricky327** · 11-27-2003, 05:11 PM

Protel DXP's autorouter likes big things so it is taking some time.

Protel is cool, I been using it since 1994. But its still cleaner to manually reroute certain parts you are not too happy about...or route everything manually if the project is not too big. Im sure your circuit is not too complex.