·

Large capacitor
charge "controllers" (Car Audio)

(c) klausmobile 2000

One-farad capacitors require some care. It's a common knowledge
that they must be pre-charged with a current-limiting resistor before plugging
in. It is less common knowledge that those fifty-dollar 'smart cap controllers'
are in fact very simple devices, two dollars for parts and labor. Here's a
basic cap switch that will connect capacitor to ground only when it's charged
to within 0.4-0.7V from battery voltage (otherwise, it's charging through a 22
Ohm resistor, which can be a low-power 12V lightbulb,
too).

Note that when the circuit is open, and
a high current draw starts dicharging the capacitor,
T1 (N-MOS with low channel resistance - IRF1010, IRFP054, IRF048, P60N06 etc)
is actually conducting in reverse direction (as in syncronous
rectifiers: S->D not D->S). This is OK for a fully open MOS FET. The
circuit could be built on P-channek devices too but
they are inherently worse conductors than N-channels.

Such a cheap, simple circuit needs no heatsink. Just parallel as
many FETs as necessary (with a common gate control
network). Given amplifier (AB class) useful output P /Watts/ and each FET's Rds /Ohms/, and 1 Watt
maximum heat dissipation for each, number of transistors N>=1/6 * P * sqrt (Rds). Practically, it is
safe to cut N 2-3 times, so minimum N>=1/15 * P * sqrt(Rds).
That is, one IRF1010 with maximum (hot) Rds of 0.020 Ohms, can safely serve a 120W output amplifier. Commercial
controllers rely on many (some as many as 18 devices) small dirt-cheap
transistors in D2 or MiniDIP cases, each dissipating
up to 0.3-0.5W. Some of them have Rds actually better
than the best devices in 'large' TO220 and TO247 cases.

A step further is to disable capacitor discharge when REMOTE
control signal is low. The simplest solution is to power all gate control
circuitry from the REMOTE wire, or - better - with a follower. LEDs D3-D4 indicate charge, D5-D6 - switch is open. 0.2F capacitor
value shown is for example only - that's enough assuming a single transistor
serves a 120W amp.

Power loss in the switch depends on the FET's
Rds, and even more - on wiring. Thin, inefficient
power wiring can leave the whole device useless. DO NOT rely on PCB copper -
use thick, at least 2 sq.mm. copper
litz wire. For a DIY install, forget fancy terminals
and hard-solder short AWG8 wires that will connect to amp and distributor box.

Source:http://www.klausmobile.narod.ru