Common sense dictates that your car’s electrical system never rises above 13.8 volts DC, but is it equipped with an alternator load dump filter nonetheless?
By: Ringo Bones
It seems like 1992 was only yesterday when “mainstream” electrical and electronic engineers where dumbfounded on why their car audio / car high fidelity system gave up the ghost after only eight months or so. Well, these tenured “mainstream” engineers seems to have failed to research one of the nasties that’s been lurking inside every automotive electrical system called alternator load dump and how it could shorten the lives of solid-state integrated circuit based electronics of a typical car stereo / car hi-fi system.
An alternator load dump occurs when the load to which a generator is delivering current to is abruptly disconnected, causing a brief but relatively high spike in output voltage from the generator. In a typical car’s alternator-car battery system, this applies to disconnecting the car battery while it is being charged by the alternator. Ever since automotive technicians equipped themselves with electrical measuring equipment, it has been found out that during alternator load dump situations, voltage measurements can be as high as 87 to 120 volts and may take up to 400 milliseconds to go back to 13.8 volts.
Given that spec sheets of automotive solid-state integrated circuit based power amplifiers - like the AN214 - that operate in the 12-volt DC environment of the car can only handle a power supply voltage spike of only 48 volts for up to 500 milliseconds or so, even a “brief” alternator load dump that more often than not exceeds this voltage is enough to shorten the lives of solid-state integrated circuit based car hi-fi. Is there a solution to protect one’s car hi-fi from the inherently hostile automotive electrical system?
There are commercially made / ready made alternator load dump filter boxes available out there and due to the prevailing ignorance of a majority of do-it-yourself electronic enthusiasts of what constitutes such filters, they are priced as if they are made of gold by unscrupulous manufacturers. But a reliable alternator load dump filter can easily be made by using unused parts lying around in a typical DIY electronic enthusiast’s electronics workbench.
A typical alternator load dump filter consists of an inductor-capacitor or L-C filter. Usually a 100 to 300 milliHenry inductor and a 1,000 microfarad 16 volt electrolytic capacitor whose negative terminal is connected to the car’s negative ground. To further save costs, one can even wind his or her own 300 milliHenry inductor using a 30-foot piece of number 20 AWG magnet wire into a ½ - inch plastic bobbin with a ½ - inch E-I silicon steel core to increase its inductance. One can even over-engineer the alternator load dump filter by using a 10,000 microfarad 35 volt capacitor for the L-C filter. This type of filter can also has the benefit of further minimize the hum to inaudibility of an AN214 IC based amplifier connected to a 12-volt full wave center tap power supply plugged into a typical home’s 220-volt 60-Hz ac current.