Some info regarding capacitors please

[trekster]

Hello friends,


Thinking I would like to stick a capacitor in line of my head unit so when I switch between acc and start the car it doesn't turn off everytime then turn on again, reason being it takes a good 10 seconds to come back on and if I need the reversing camera it won't come on straight away.

Not sure if its the power line or the trigger +12v line this would need to be put in, but would need enough juice to keep power there for around 5-10 seconds.


Any ideas/comments would be greatly appreciated.


Cheers,
Eric.

[mudnut]

A capacitor would need a resistor in series with it to slowly discharge, ergo it would need time to charge as well. It's called an RC timing circuit. Different values of resistance and capacitance give different discharge times. I don't know if there is a cheap, capacitor that can sustain a current large enough to run the head unit for that amount of time, but technology is changing all the time.

[Yendor]

I reckon you could achieve this with 2 diodes and some wire.

Connect a wire to the starter motor feed wire at the ignition switch and run it to your head unit. Fit the diode in this wire with the band of the diode towards the head unit and then connect to the acc feed at the head unit.

The second diode will go in the original acc feed wire to the head unit with the band of the diode towards the head unit. So it would be the head unit then the first diode spliced in and then the second diode inline in the original acc wire.

This will supply power when cranking to the head unit only.

This should stop the head unit from going through its reboot procedure.

[Alitis007]

I'm not sure if this will help but i know how capacitors work. The capacitor drains over a time constant by 50% every time constant for eg fully charged at 100% over 1 time constant it will drain down to 50% then after another time constant it will drain to 25% then 12.5% etc etc. at a guess the capacitor your looking for should have a time constant of 10 seconds. You would also need to tap into the Accessory wire not the 12v constant because thats the radio trigger wire, the only problem i see is when you turn the ignition off you will need to wait out the 10 odd time constants untill the radio turns off.

[mudnut]

Good idea Yendor. Would the circuit need a small capacitor to take away any spikes from the starter circuit?

[Yendor]

Good idea Yendor. Would the circuit need a small capacitor to take away any spikes from the starter circuit?

I don't think it would be necessary. The head unit should have adequate protection.

If you were really concerned you could fit another diode as close as possible to the starter solenoid. This would redirect any voltage spike that occurred when the solenoid field collapsed back through the solenoid.

[Yendor]

Good idea Yendor. Would the circuit need a small capacitor to take away any spikes from the starter circuit?


I don't think it would be necessary. The head unit should have adequate protection.

If you were really concerned you could fit another diode as close as possible to the starter solenoid. This would redirect any voltage spike that occurred when the solenoid field collapsed back through the solenoid.

Or you could replace the first diode (the one in the starter motor feed circuit) with a relay.

[GQ TANK]

You need to put a meter on the power to the radio and work out what is dropping.

I have seen diesel 4wd & trucks where the battery voltage will drop to around 9 volts on crank, resulting in the radio resetting.

If this is causing the problem with your radio, then a capacitor and inline resistor will work - but the resistor may get hot depending on the wattage the radio is pumping out. If you don't put a resistor in line - then the rest of the vehicle will be hanging off the cap. If you have a second isolated battery, consider connecting the radio to this unit, and use the ignition wire to control the unit.

If the problem is due to the ignition wire removing volts to the radio while you are cranking - you might need to look at where you are getting the IGN volts as you want a supply that will power the radio will you are cranking, acc and running- this is where the comments about using diodes come from.

[Garth]

Why not run a switch wire from your acc on the ignition to a relay, pull the power for the radio from batter to the 2nd battery, so wheyou start the car rather than using the same battery your car starts with, your radio is powered from the 2nd battery? That's how mine is done, and I don't have any re starting issue with radio anymore