Can somebody help a noob?

[Figmo]

GOAL: To continue to power my 12v DC automotive stereo for "x" seconds after power is turned off. Where X = 10, 15 or even 30 seconds - length is not vital at this point.

So in my noob-ness, I thought I could just get a big ass capacitor to do this job. Since I have no clue what amperage or wattage the device actually draws during normal use, my logic was just solve it by trial and error. Try one, if the delay was too short, try a bigger one. Too long? Try a smaller one.

My first attempt used this 12f SuperCapacitor device I found on eBay. Looks like below. The PCB has a "+" and a "-" so I hooked 12v source power to +, and stereo power wire to - Stereo did indeed power up when turning on the ignition - but instantly powered off when I switched the vehicle ignition off. No residual power effect after turning vehicle off (or so quick it was imperceptible).

I really expected this first try would be in the "too big" category. I kind of expected to have 12v power supplied to the stereo for TOO long after turning the vehicle off. Because this rack of caps just looks pretty big to me.

So now I'm thinking wiring it must not be as straight forward as putting it "inline" with the 12v supply power wire.

Did I wire it wrong? Or is 12f still not big enough to do the job? Or is my entire premise flawed?

Any help would be appreciated.

 

[betwixt]

Your thinking is 'almost' right but you missed one important point: the capacitors will also discharge into the rest of the vehicle electrics when the source (battery) is disconnected by the ignition switch. What you have done is essentially slightly boosted the battery capacity but still used it to power the whole vehicle.

The best solution is to wire a delay timer in series with the power line to the stereo and wire the feed side of it directly to the battery, avoiding the ignition switch altogether -or- if it must turn off eventually after the ignition, do the same but use the original power from the ignition switch to keep the timer in the 'on' state.

Brian.

 

[Easy peasy]

Even if the caps are directly connected to the amplifier and head unit ONLY and the key disconnects these from the rest of the car and everything - the cap voltage runs down with current drawn

So say we go from 13.5 to 10V ( where the car stereo cuts off ) and we draw 5 amps ( ~ 60 watts )

i/ C = dv/dt, so 5/12F = 3.5 / dt. => dt = 8 secs

less for more than 60W draw and considerably longer for power << 60W

However if you are supplying the rest of the car electronics - AND/OR - the key stops the head unit instantly

then = 0 secs.


Some re-wiring required to get the delay ...

 

[Figmo]

Would a diode keep the current from back-flowing into the rest of the car circuit? Thus ensuring the caps can only discharge into the head unit?

 

[betwixt]

A diode might help with that problem but it could also introduce others. A diode drops a voltage when current flows through it so the stereo would see a lower supply voltage. It may still be sufficient though, depending on the design of the unit.

A more general problem is what Easy Peasy pointed out, the difference between a battery and a capacitor is one releases power from a chemical reaction and the other only stores what is already there. The battery voltage will stay up as the chemical reaction takes place, maybe for hours but the capacitor voltage will start to drop from the instant you start discharging it. How long the stereo will function depends on how well, if at all, it can function on reduced voltage. It will start dropping as soon as the ignition is turned off and the rate of drop will depend on how much current the stereo draws from it. I'm afraid there is no reliable calculation to work out the time as it depends on things like the volume level and threshold at which the stereo simply stops functioning.

Brian.

 

[cupoftea]

You coudl try a PFET in there....but again, if the stereo is set up to stop working when its vin <11.5v say, then its not so goo deven then.......the obvious, but involved way, is to add a rechargeable battery pack and battery management system.....which switches the stereo to the recharge battery for x seconds, whatever you want, whenever the car voltage drops.

 

[cupoftea]

Sorry a PFET is bidirectional i forgot.....anyway, what about a schottky, and then the cap bank, and then a swamped boost converter....which boosts up to 12V.....so is in no load when car batt is supplying then, when it drops, the booster kicks in, and supplies 12v from your cap bank.

 

[Easy peasy]

With a diode there is also the issue of re-charge current to the cap when the key is turned on - the current peak will be very high - so an 100A 35V schottky is a minimum here - on a heatsink - also repeated high peak charge currents may well end up killing part of the ignition switch.

A relay timer seems the best way to go - as already suggested above by a poster.