starfox

Thanks to Ricky327 and none for their posts on this topic.

New User's guide to tank circuits

What is a tank circuit?
-A tank circuit is a circuit which attempts to stop your computer from rebooting when you start your car.

Background
When you start / crank your car, the starter motor in your car spins to initiate combusion in your engine. The starter motor is a DC motor which needs to spin fast, and in turn drains a lot of power from the battery. The problem with this is that the battery's voltage drops and becomes unstable (the current drain is large enough that the internal resistance of the battery starts to matter). If the voltage drops too much or is not stable enough, many power supplies and inverters will shut down momentairily when you start your car.

Many amplifiers and audio head units will already do this. You may have noticed that when you start your car, these switch off momentairily until the voltage is stable again.

This is bad for a computer, as power loss can lead to data corruption on your hard drive. It's also very annoying as your PC has to reboot from scratch again, and that means no music for maybe 30 seconds!

Which tank circuits have been shown to work?
So far, the only tank circuits that have been shown to work are sealed lead acid batteries. These can provide enough current to keep your car computer power supply happy while the altenator starts.

See this thread for more information about a battery based tank circuit:
http://www.mp3car.com/vbulletin/showthread.php?t=20359

What about using capacitors?
In a tank circuit, the role of capacitors should be to smooth out ripple and noise in the car's electrical system. They cannot by themselves be used to keep the voltage to the PSU high enough to prevent the computer's PSU from shutting down. A car computer PSU drains more current than a capacitor can supply.

But people use large capacitors in car audio systems for their amps!
These capacitors (typically 1F or larger) are used to buffer power to amplifiers for very short periods of time. Massive capacitors are usually used with subwoofer amplifiers to prevent them going quiet for a few seconds after heavy bass hits. These bass hits individually last for a much shorter time than it takes to start your car!

In short: Capacitors are used to buffer power for short periods of time. Starting your car is not considered a short period of time. Capacitors are not an appropriate device to use.

More information about capacitors:
Not everyone should be using massive capacitors in their cars. Typically these are only used in SPL competition audio, and should only be installed by people who really know what they are doing (if you don't know what ESR is, know how much charge 1F is, or know how to charge a 1F capacitor then don't install one).

Massive capacitors are very dangerous and discharging one of these quicky by accident may cause the capacitor to heat up and explode.

More threads with information:
http://www.mp3car.com/vbulletin/showthread.php?t=20279
-Where to use capacitors in a tank circuit

http://www.mp3car.com/vbulletin/showthread.php?t=20359
-Battery based tank circuit

(If there are any corrections to be made, or comments please PM me so i can edit this post)