Wednesday, December 26, 2012

Does Your Car’s AN214 Amplifier Have An Alternator Load Dump Filter?

Logic dictates that the car’s 12 volt DC system never rises above 13.8 volts DC, but does your car’s AN214 amplifier equipped with an alternator load dump filter, nonetheless?

 By: Vanessa Uy

Some “mainstream” electrical and electronics engineers may consider the practice of installing an over-engineered alternator load dump filter for your car’s audio system to be a “tweak-extravaganza”, but have you ever wondered what these “mainstream” electrical and electronic engineers complaining about back in the early 1990s when their do-it-yourself installed car audios went kaput in just eight months after installation?
Well, maybe they should have done their research about one of the “electrical nasties” that can be encountered in a typical automotive 12-volt DC electrical system called alternator load dump. Alternator load dump occurs when the load to which a generator is delivering current to is abruptly disconnected which causes a very brief – but sometimes very high – spike in voltage. In a typical automotive electrical system, this applies to disconnecting the car’s battery while it is being charged by the alternator.

Automotive technicians using electrical measurement equipment have since found out that the voltage generated by an alternator load dump can be as high as 87 to 120 volts and could take up to 400 milliseconds to decay or return back to 13.8 volts. This can be enough to burn out the solid-state electronics of car stereos and their associated power amplifiers. But is there a solution? A typical alternator load dump filter consists of an L-C or inductor-capacitor filter where the inductor is rated at around 100 to 300 milliHenries at 1 ampere while the capacitor is a 1,000 microfarad 16 to 25 volt electrolytic capacitor connected to the car’s negative ground.

An over-engineered alternator load dump filter for an AN214 based automotive audio amp can be advantageous because an over-engineered L-C filter can also filter the hum of atypical full-wave-bridge 12 Volt DC power supply tapped in your home’s 220-volt 60-Hz AC wall socket. A knowledgeable electronic enthusiast can even wind his or her own inductor for a fraction of a cost of a commercially made one. Just wind a 30-foot long piece of number 20 AWG magnet wire on a ½-inch plastic transformer bobbin then place the laminated E-I silicon-steel cores to increase its inductance into the 100 milliHenry range.

To make the capacitor part of the L-C filter alternator load dump filter last almost indefinitely, use an electrolytic capacitor in the 35-volt range. An alternator load dump filter consisting of a 300 milliHenry inductor and a 10,000 microfarad 35-volt capacitor can limit the voltage spikes to 18-volts during an alternator load dump – which is more than necessary to protect the AN214 IC and related electronics since according to their spec sheets, automotive audio IC amplifiers can supposedly withstand voltage spikes as high as 48 volts for 500 milliseconds.