Car Radio Power Switched & Unswitched

[lpii]

My car Radio/GPS sytem recieves its power from the fuse panel which is switched by the ignition key. I would like to have the ability to turn the radio on without using the ignition. I cannot wire the radio directly to an always on connection becuase it runs a fan on the unit when power is on the line regardless if the radio is playing and it would eventually drain the battery.

I would like to run an always on power supply (through a toggle switch) to the power lead of the radio, but at the same time allow the radio to be powered through the normal ignition switched power supply so if could be powered by the switch (when closed) or by the ignition being on. If I do it though the toggle switch alone, I am afraid my wife will forget to turn the toggle switch off and drain the battery.

My concern is if I run power from the toggle switch at the same time the ignition switch is on will I get a back flow of power to the fuse box, or too much power & amperage to the radio?

If I use a diode, do I need one on each line one to protect the ignition switch and one to protect the toggle switch?

If the always on is 20amps and the radio needs 10amps will the radio only use what it needs or will the 20amp circuit burn out the radio?

+12V 10A Ignition Switched o_________________o__________o Radio
|
+12V 20A Always on o________/ _______|


or do I need to add diodes (not sure if I have the diode in the right direction)

+12V 10A Ignition Switched o_____________|<__o__________o Radio
|
+12V 20A Always on o________/ ___|<__|

If the radio only needs 10amps and the supply gives 20amps do I need to add anything to the circut to reduce the amperage?

Thanks,

Les

 

[lpii]

sorry the diagram didn't line up on the post like it did on the entry screen.

+12V 10A o __________o_____o Radio
+12V 20A o___/ ______|

+12V 10A o _______|<__o_____o Radio
+12V 20A o___/ ___|<__|

 

[crutschow]

The current rating of a circuit or fuse is just the maximum the circuit can deliver. It's the voltage that typically determines the power a circuit will draw. The radio will take whatever amps it needs so the extra circuit won't make any difference.

If both +12V supplys are from the same source then you don't need any diodes. You're just connecting them in parallel which has no effect. But note that any other accessory that is on the radio circuit will also be powered by the toggle switch. This probably isn't a problem, just something to be aware of.

 

[lpii]

All the power comes from the car battery ultimately. I am assuming that the power from the battery is split to different connectors through the fuse panel so is it all from the same power, yes.

If the power from the battery is stepped down for different voltages and amperage would that constitute different sources?

The ignition switched fuse is only for the radio. The other line is the power for the drivers seat height adjustment (which is always live) so it would not turn anything else on that isn't already on. So as long as I'm not adjusting the seat up/down while the radio toggle switch is powered I shouldn't tax the system.

 

[lpii]

Also, to be completely safe, would you add the diodes or since it all comes from the car battery it is without a doubt not needed?

 

[crutschow]

If the power from the battery is stepped down for different voltages and amperage would that constitute different sources?

Normally the voltage is not changed unless you use an inverter to power something that runs on 110VAC such as a TV. That would be a different source.

As I noted, the amperage is just a maximum rating. The actual current is determined by whatever loads you have connected. A separate line doesn't consitute a separate source, just a different path (the battery is the source for both wires).

Since it's all coming from one source, the diodes are not needed.

Think of the source as a water reservoir. Adding a pipe doesn't add a source. And the amount of water flowing through a pipe is not determined by the size of the pipe (at least for large pipes), it's determined by how many faucets are on and how open the faucets are. And connecting two pipes together doesn't affect the water flow through the faucets (expect to increase the maximum allowed flow).

 

[lpii]

Thanks, that answers all my questions, I will bring a sponge with me when I connect the pipes LOL.

Thanks.

Les