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.

 

[rjenkinsgb]

It must be connected "across" the power supply, not in series with it.

Also, the power source should be switched and only feed the load (the stereo), so it's not trying to power everything else that is switched by the ignition etc.

Also note that the charging current may be extremely high when power is connected, enough to blow fuses or melt cables; even possibly set them on fire.

The formula for capacitor value vs voltage drop is: One Farad drops one volt per second, per amp.

So 12F would drop 1/12 volt per second at 1A or 2V per second at 24A etc.

Also, is the does the stereo have both a main power feed and an ignition (on/off) control input? If it does, then it's the ignition switched wire that needs the power delay & that would better be done with a timer relay than with a capacitor.

 

[ronsimpson]

My old radio gets its power from the battery through the key switch.
The new radio gets it power directly from the battery. It uses the old power to tell the radio to turn on/off but does not take power from that wire. When the radio sees the key switch power is gone it keeps the radio on for 5 minutes.

With this idea in mind, I think the radio's power must come from the battery not the key-switch and we need to add a 30 second timer relay.
Just my first though on it.

 

[Diver300]

Many car stereos built in the last 20 years have a permanent power wire and an ignition wire, and they take most of their current from the permanent feed. If yours is one of those, you would only need a moderate size of capacitor to keep the ignition feed supplied for 15 seconds or so.

If you have to supply the full current to the stereo, you would be far better to use a timer relay. It's not practical to supply a stereo for 15 seconds from capacitors.

 

[DrDoggy2]

Car stereos get power even when the ignition is off due to the clock. (mentioned above)
Use your capacitor on the "ignition wire" that triggers the power. (mentioned above)
A resistor on both sides of the capacitor would be a good idea too, on the ignition side to protect from over draw on the ignition circuit when charging, and on the stereo side to slow down the discharge rate. An amps reading on the trigger circuit would help to establish the values required.
(to get that reading cut the ignition wire and put your amp meter on both ends of the cut wire)

 

[Diver300]

A resistor on both sides of the capacitor would be a good idea too, on the ignition side to protect from over draw on the ignition circuit when charging, and on the stereo side to slow down the discharge rate.

A diode in series with resistor between the ignition side and the capacitor could help. It would stop the capacitor discharging back to the ignition side. If there are other loads on the car fed by the ignition supply, it won't be possible to keep all of those going for a few seconds without a hugh capacitor, so a diode would be needed.

 

[DrDoggy2]

I suspect the ignition trigger will be low power compared to the actual V+ that is used to pump the base. I never measured the amps on the trigger but a guess would be in the mA range.

 

[Diver300]

I suspect the ignition trigger will be low power compared to the actual V+ that is used to pump the base. I never measured the amps on the trigger but a guess would be in the mA range.

The problem is if something in the car forces the ignition signal to zero volts when the ignition is turned off. If that is the case, a diode would allow the capacitor to keep its charge for a few seconds.

 

[shokjok]

Many car stereos built in the last 20 years have a permanent power wire and an ignition wire, and they take most of their current from the permanent feed. If yours is one of those, you would only need a moderate size of capacitor to keep the ignition feed supplied for 15 seconds or so.

If you have to supply the full current to the stereo, you would be far better to use a timer relay. It's not practical to supply a stereo for 15 seconds from capacitors.

The majority of car stereos installed over the last thirty years have three power wires: a hot feed for clock/ station memory, a switched feed for tuner/ player/ amplifier power, and a backlight wire from the light switch dimmer feed.
Figmo can connect to the hot wire for the radio source power/ lighter socket, headlight switch, etc., after Googling "delay timer circuits" for ideas.