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TANK CIRCUIT
Rectifiers (which are diodes rated for greater than 3-amps) hate transient voltage spikes with a purple passion. So do voltage regulator components. And every other solid-state component in a circuit. Including engine ECU, radios, you name it.
While chopping off a major load leaving a minor load (the chassis battery) isn't considered to be a true out-and-out LOAD DUMP it's ENOUGH of a load dump to cause problems, especially if the chassis battery is fully charged.
Here is a cut and paste excerpt of a description of LOAD DUMP
A load dump occurs when the load to which a generator is delivering current is abruptly disconnected. In automotive electronics, this applies to disconnecting a battery while it is being charged by the alternator. It is apparently well-described in this $65 SAE document; Wikipedia claims it can be "as high as 120 V and may take up to 400 ms to decay". This document claims a 12V system dump can be as high as 87V and 400ms long:
12V system 24V system
Us 65V to 87V 123V to 174V // maximum voltage
Ri 0.5? to 4? 1? to 8? // source resistance
td 40ms to 400ms 100ms to 350ms // pulse length
tr 10ms?? 5ms?? // rise time
end...
Deterioration (destruction) of solid-state components can happen instantaneously when subjected to severe voltage transients, and WILL BE CUMULATIVE when subjected to less severe voltage transients.
Here's an example...
145 amp alternator charging house bank at 90 amps
Charging chassis battery at 3 amps
The house bank is disconnected.
All of the load dump will be directed at the chassis. But the chassis battery is "full" presenting a relatively high resistance. The excess energy MUST go somewhere.
A ringing TANK CIRCUIT not only presents POSITIVE SPIKES. It also presents NEGATIVE ORIENTED SPIKES.
This issue causes no end to system failures. General Motors and many Japanese manufacturers deemed partial load dump to be severe enough to warrant a major design change. The inclusion of AVALANCHE RECTIFIERS in alternators. The value of the devices is 28-32 VDC. Excessive voltage dumps back into the stator because it is an AC source it can absorb this damaging voltage. Hundreds of millions of dollars are spent avoiding PARTIAL load dump. The same type of load dump that the opening of a switch to hard charging house batteries would cause.
Keep in mind. The word CUMULATIVE in this explanation means something. $$$$$$$$$
My load dump protection for fire engines and ambulances included a 20,000 micro-farad 100 volt capacitor, and three 50-amp rated avalanche rectifiers strategically located throughout the electrical system.
Does it work?
The city of Los Angeles wrote a letter to me in 1991 stating the suppression package reduces ancillary failures of electronic components by 80% over a six-year period. Because of KKK-1822-AA regulations, ambulances with the Sign of Life qualification must use Schottky isolators for the cargo bay. The failure rate for the finned isolator went from 10+ in 1986 to 0 in the following FIVE YEARS. This stuff isn't hocus-pocus. It really does work.
BTW The Kelly II has eight capacitors 220K uf caps, and a dozen avalanche rectifiers.
Excuse me while I go powder my nose...
