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#21
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Richard Bell wrote: > In article > , > Bruce Chang > wrote: > > > >"sdlomi2" > wrote in message > ... > >> Help! Got a '55 Studebaker. Aftermkt. mp3/cd/amfm radio. If I set > >> stations, it remembers fine going from on to off to access to off. But! > >> When I start i, the starter seems to drop the "voltage threshold"(at least > >> that's what I visualize) such that all memory settings are lost. > >> Ideas, please? Thank you, s > >> > >> > >> > > > >Maybe a small 12V battery or large capacitor that's hooked up with a diode > >between it and the car battery so voltage to the stereo won't drop when the > >car battery voltage drops significantly. > > The large capacitor will cause a voltage > spike, just after the motor starts, so a choke coil will be needed to > protect the sound unit. Inductors generate spikes, not capacitors. The best arrangement has a choke in the supply line, with a capacitor between the hot side of the choke and ground. That'll swallow most interference. It would take a pretty big capacitor to keep a stereo happy. Spikes are generated by the starter solenoid and by the starter itself when the current is cut off. A capacitor cannot provide any higher voltage than it was provided with in the first place, but an inductor like a solenoid can make a 600-volt spike. I've measured it. It's the principle behind the ignition coil. > Here, in the Great White North (How's it goin', eh?), we would advise you that > your battery is seriously short of cold cranking amperage and that you > should buy the biggest battery that will fit in the tray. However, being > designed for old-tech batteries, you should accept anything that covers half > of your '55 studebaker's battery tray. While cold weather starting may be > the least of your worries, a larger battery is the simplest (if not the > cheapest) solution to your problem. A 35-watt electric battery blanket will make that battery perform. Some guys use them instead of a block heater. A battery loses something like 60% of its capacity at -25 degrees C. Dan |
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#22
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In article om>,
> wrote: > >Richard Bell wrote: >The large capacitor will cause a voltage >> spike, just after the motor starts, so a choke coil will be needed to >> protect the sound unit. > >Inductors generate spikes, not capacitors. The best >arrangement has a choke in the supply line, with a capacitor between >the hot side of the choke and ground. That'll swallow most >interference. It would take a pretty big capacitor to keep a stereo >happy. The car sits with the engine not running for a long time, so the voltage on the capacitor drops to the battery terminal voltage (hopefully 12.6v). During cranking, the battery voltage drops to below the keep-alive threshold (whatever that is), but the low load of the audio system keep the cap at about 12.6v. The engine catches, and the alternator now cranks out current at a voltage that will charge the battery (14+ volts). The cap draws a current appropriate to two or more volts divided by the wire's resistance in ohms, or a few tens of amps. As all wires in the real world, running close (or within) large steel objects have non-zero inductance, the circuit will ring. Without the capacitor, the switching transient is not as bad, as there is no apparent short circuit to a lower voltage, so there is no inrush overcurrent. Switching transients always happen. Capacitors make them worse. On some powerlines, switched capacitor banks are more serious than lightning strikes. |
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