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voltage/current jump

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bowser22

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hey guys i have a household amp here that im going to run in my car, im goin to create all the different voltages in dc required to power the device so i can remove the ac power supply because i want to save money and save power so by not purchasing an inverter.just wondering, is the voltage and current output on the battery the same as what comes out of the cig lighter?when i start the car will the voltage or current jump, possibly causing damage to components in my homemade dc-dc psu?
thanks
 
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dknguyen

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Do your devices actually run on DC? They have a AC-DC power supply between them and the wall?

But yeah, battery and cigarette lighter voltage would be the same...that's one of the problems with automotive electronics- poor quality power. You have to design your PSU to handle that.
 
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tcmtech

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Your typical DC voltage on a "12 volt" automotive system can actually vary from as low as 10.8 volts (starting) to has High as 15.8 volts(winter charging conditions). The typical standard you should consider is a 12 to 14.6 volt variation between not running and running.

So what power level and voltage outputs do you need?
Do you have a particular circuit design in mind?

If its just to theoretically save money you will get a load of grief from many here but if its part of a hands on learning and self education experiment you will get a fair amount of help!
 

unclejed613

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car amps do this all the time. a switching converter drives a transformer at 50khz, and the secondary goes through high speed rectifiers to get +/- 65V or so.
 

smanches

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A cars electrical system is incredibly noisy and full of transients from 3v to as high as 90v. Just think of the inductive kick your starter lets off when it stops.

It's probably the largest difference between automotive and consumer electronics.
 
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bowser22

Member
hey guys thanks for all the info.what im trying to do is completely remove the ac psu then use the batterys 12v and step it down or boost it to the appropriate voltages and then id have to take care of the amperage.i havent taken apart the amp yet so i dont know if i have to boost voltages.i was planning on using a few voltage regulators and other small circuits to do the job.when i dissasemble the amp i will tell you the voltages required so you guys can make some suggestions.its a sony str k502p
 

unclejed613

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when transistors were young, car radios had a device made with two metal plates and separated by a mica washer. no, it wasn't a capacitor, it was called a spark plate. it was to keep the voltage spikes from causing damage inside the radio. it was usually soldered to the frame of the radio and the incoming +12V wire was soldered to the other side. now they use zeners. the voltage spikes tended to be higher because car voltage regulators were made using relays, not transistors. relay coils in a car voltage regulator could punch out inductive spikes of 100V or more. the alternator or generator field coil could produce some high voltage spikes when the relay contacts opened as well. these days it's done with transistors, and the field coil is linearly controlled.
 
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audioguru

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Sony has its owner's manual in Javascript but my pc dosn't do Javascript since I got new anti-virus software a few days ago.
Somebody is selling one and claims its total output is 500W. You need a pretty sophistocated and powerful voltage stepup circuit to do it, not little voltage regulators. It might not be able to drive 4 ohm car speakers.
 

Diver300

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Your typical DC voltage on a "12 volt" automotive system can actually vary from as low as 10.8 volts (starting) to has High as 15.8 volts(winter charging conditions).
I have seen voltages as low as 5 V when the starter motor is running. Admittedly, the engine management computer stopped working at that point.

10.8 V is a lot higher than I would expect when starting a car.

Load dump can be as high as 80 V.

Buy a amplifier designed for a car and let someone else do all the difficult stuff.
 

tcmtech

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I went by actual numbers I have seen on my own vehicles over the years.
They are what I have considered to be the realistic lows and highs you would see with a normally functioning charging system.;)

Below 10.8 volts many vehicles wont even start. And that is usually considered the near dead voltage of a LA battery.

All though there are definitely much wider voltages possible. -12 volts if you jump start your car wrong! Thats worse than the zero volts you get when your battery is disconnected! :p:D
And I have seen a diesel truck with a fried voltage regulator in a 120 amp alternator top out at 22 volts on a 12 volt system! :eek:
I generally dont count the spikes and pulses though. They are easily trimmed down with any standard snubbing capacitor or filtering circuit.
 
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