Hello,

I am building an ammo can boombox, and I want to be able to run it off of main power (laptop plug) when plugged in, and battery power when not plugged in.
Below are the parts I am using:
Lepai LP-2020 Amp
Toggle Switch (panel mounted)
Battery
DC Power Jack

How do I wire up the switch to change from mains to battery? Also, if I used a bigger power supply (12v 5 amp) would it fry the amp? Here is the wiring diagram I came up with.

Last thing, since the ammo can is metal, will there be grounding issues?

 

I can't follow your wiring diagram sorry....
I would do this - see schematic.

The switch you've shown is single throw, you should be able to get a double throw version. I would put a master power switch in also, especially if I didn't have a centre-off SPDT switch as shown.

A bigger power supply won't hurt your amp as long as it is the correct voltage, don't worry about current - just get one that can comfortably deliver more amps than the amp uses (I would for double myself....).

Lee

 

If you were to put diodes in the path of each power supply, instead of the switch, you don't need the switch. I'm in a parking lot now, so can't draw a schematic. Will reply later if you're interested. By the way, I agree with samuel.whiskers in that a master power switch is required, especially in this case otherwise the battery will drain.

 

Thanks for the information. Here is a wiring diagram that I am using now. It should be a lot better to follow than the first one I did.

Would I be able to use the current switch I have as a master switch?

 

The wiring of your switch looks wrong. It looks like in one position, you'd have a direct short. It's a problem with pictorial diagrams. It does not show the internal switch action.

With three terminals, it is likely a SPDT switch. One terminal is the common point, that is switched to one or the other. Which terminal in your diagram is the common one?

 

The switch is actually a SPST Illuminated toggle switch (ie. led lights up when switch is flipped on). The switching of power sources happens at the DC power jack w/switch (or is supposed to).

 

With that piece of information, the schematic makes more sense. One more question. Are you sure of the polarity of the jack? That is, is the outer connection the positive supply? I've seen the outer connection being ground more frequently.

 

You are right, the outer sleeve of the jack is ground (sleeve). It is tip positive. I got the names incorrect.

I have had this wiring working, but I think the grounding might be wrong. The DC jack is panel mounted on the metal, and so is the panel mount 3.5mm headphone/Aux jack (plugs into to amp to play mp3). Can these be causing problems?

I think I am going to use the diagram from samuel.whiskers to update the ammo can boombox. This is my final wiring diagram that I am going to base it off of. Im adding a bluetooth module to connect wirelessely, and a usb charger for phones as well. These both have the 12v to 5v step down built in.

 

The switch you show to select between the power sources appears to be a SP3T switch and you intend one of the throws to be an off position. if this is the case, your master on/off switch is redundant.

Of more concern is that the diagram shows the switch miswired. It is different than the diagram supplied by samuel.whiskers. The common terminal should go to the master on/off switch or directly to the load.

You could still use the internal switch in a switched jack as you had proposed earlier or wire it with diodes, but if this is the way you understand the circuit - go for it!

You are right to be concerned about the grounding of the headphone jack to the power jack. Amplifier outputs often cannot have their negative terminal tied to ground without letting some magic smoke out. I cannot tell with your amp, but would recommend not doing this anyway. I'd mount the output jacks on a piece of plastic, and then mount that in a rectangular/square hole in the metal, insulating the jack completely.

 

Here's your earlier diagram modified to use a power jack with a switch...

 

Wow, I didnt even realize I had that put in backwards. This is the first time Ive tried to use somewhat proper schematic symbols. Thanks for all the help. Hopefully I can get this thing working without it messing up my amp again.

 

I'll often make a copy of my schematic and then with different colored pencils/markers trace the positive and negative paths. My load should always have a positive and a negative connection. At no time should positive and negative be on the same wire. Trace the paths for all positions of switches. Sometimes more than one schematic copy is useful. Complex circuits can be analyzed like this too, up to a point. In that case, you might want additional colors to represent signal paths. I've used this technique on simple digital circuits as well.

 

The only problem I see with the updated diagram you made is that the pin is going to the ground. I am using a tip positive/sleeve negative power supply (plugs into jack).

 

As drawn, the "pin" connection (your label) is connecting to the sleeve. Hence, it should go to ground. Like you say. The diagram showing the switch connects to the tip of the jack, and it should go to the positive connections.

I agree with your description and my diagram shows the same connections. Perhaps there is some confusion as to the switched jack schematic symbol?

 

Well, I wired it all up and got nothing.

 

Can you supply a photo and tell us which drawing you were using? I can't quite follow what has been done. Hopefully it's something simple. It's surprisingly easy to make a mistake.

 

does the amp work when just directly connected to the battery??

 

The amp works when plugged directly into the battery, and when plugged in via the dc jack. I just cant get it to work with the switch. I think my grounding is the issue. I will put it all together again, and take a picture.

 

Which switch? the illuminated one?

 

Nope, I am just using the SPDT center off switch. Here is a picture of the it wired together, but not working. The amp is on the left with the positive of amp going to the middle terminal of switch (top of pic). The silver thing is my dc jack with the positive going to position 1 of the switch. The battery hook up is at the bottom right with positive going to position 3 of switch. As you can see, all the negatives are together.