Help with small home project

[ljubo700]

Hello,

I'm succeded with connecting a computer PSU to a car stereo and all works fine. I'm now interested how to wire/connect a 12V battery so if I don't want to power it from AC current through PSU. And only power it from 12V DC battery so it's "portable". I was thinking of using swithces or diodes, but I'm tottaly new to electrical stuff.

Thanks in advance

 

[Gryd3]

What size/kind of battery do you want to use, and how long do you want to be 'mobile' with your portable automotive stereo?

Diodes will allow you to prevent current flowing backwards through the battery or PSU depending on their charge.
A switch will allow you to manually select either power source. The choice is yours

What was your question exactly?

 

[ljubo700]

I was thinking about using a car battery as the DC power, so 12V 40-70Ah. I was asking how to i.e. wire a switch (and what kind of switch) to change from AC (PSU) power to DC (battery) safely. Autonomy as much as I know depends on the size of the battery (Ah).
Current setup is: PSU Yellow(12V)-Car stereo Red,Yellow; PSU Black,Green-Car stereo Black.

 

[Gryd3]

I am familiar with how you wired your PSU into your car stereo
I have done so myself when I was putting together a Car PC.

It's almost too bad the PSU didn't have a slightly higher voltage, you could have used it to charge the car battery.
I'm willing to bet that a diode solution is out of the question. Car batteries are usually a little over 12V, so it would continue to drain to a certain extent even when the PSU has power.
While in use, will the car battery ever be connected to any other equipment? ie.. battery charger/tender?
If not, you could leave the neg from the battery and ground from the PSU connected together to the Car stereo and use a switch to select your 12V source.
Very simple idea really. You could take a look at some marine or trailer dealers as many of them have a switch that will allow you to select which battery to use. The only difference is that your swapping out the battery for a PSU.

Regardless of the source, you need a single pole, double through switch to make it easy
http://en.wikipedia.org/wiki/Single_pole,_double_throw#Contact_terminology

 

[ljubo700]

The battery would be used just to power the stereo. So negative PSU and battery together, and positive PSU and battery through switch ? Switch must be over 220V and 10A ?

Thank you

 

[Gryd3]

Switch will be used to select +12-14V from the battery, or roughly +12V from the PSU.
It does not need to be rated for mains voltage, but should be rated for more current.
Is there an amperage rating for the stereo you are planning to use? What can we expect to draw?

 

[ljubo700]

Well it has a 10 or 15A fuse don't know from top off my head, is that enough for the switch ? Switch must be single pole on/on I assume ?

 

[Gryd3]

You can find a switch that is rated for 10A. Try to make sure it's higher than the fuse rating, and it would be a good practice to not have the stereo pumpin when you switch over.
The switch would be an on-on switch yes. Should have 3 legs, one is common, which gets connected to one of the other two depending on the switch position. You may get an on-off-on switch as well if you want to put the switch in the middle position to cut off power altogether.

http://www.radioshack.com/product/index.jsp?productId=24431266
If you cant fine a SPDT switch, you can use a DPDT switch instead. (But half of the legs will be unused... a DPDT is essentially two switches in one package)

 

[ljubo700]

Thank you, got it now how and what to do. I'll just use On/On and switch it to PSU power, and then switch off the PSU from its switch.

 

[Arouse1973]

What size/kind of battery do you want to use, and how long do you want to be 'mobile' with your portable automotive stereo?

Diodes will allow you to prevent current flowing backwards through the battery or PSU depending on their charge.
A switch will allow you to manually select either power source. The choice is yours

What was your question exactly?

Hi Gryd3
Sorry if I seem a bit confused I may have not understood you statement , but how is the diodes blocking capability dependant on the charge of the battery? Doesn't a diode block the majority of current (excluding drift current) when reversed biased? Obviously whilst under the breakdown voltage.
Cheers
Adam

 

[Gryd3]

Hi Gryd3
Sorry if I seem a bit confused I may have not understood you statement , but how is the diodes blocking capability dependant on the charge of the battery? Doesn't a diode block the majority of current (excluding drift current) when reversed biased? Obviously whilst under the breakdown voltage.
Cheers
Adam

If the battery voltage is higher than the +12VDC from the PSU, then current flow will originate from the battery and not the PSU. The diode would work fine though if the battery was less than the +12VDC from the PSU because then the diode would be reverse biased.
Was I wrong to assume a voltage range anywhere from 11.5 - 13V on a car battery?

 

[Arouse1973]

I thought he was on about diode or-ing the battery and the PSU. So either could supply current, maybe I got that wrong. But thats how I would do it. May draw a circuit so it clear to all what you/he means.
Cheers
Adam

 

[Gryd3]

I thought he was on about diode or-ing the battery and the PSU. So either could supply current, maybe I got that wrong. But thats how I would do it. May draw a circuit so it clear to all what you/he means.
Cheers
Adam

That is what he wants, but if the battery and PSU were diode or'd together, would there need to be a switch to cut-off battery power? This would not be an issue if the PSU supplies a slightly higher voltage than the battery, but if the battery were of a higher voltage than the PSU, the circuit would not draw from the PSU until the battery voltage drops.
I'll draw one up real quick.

 

[Gryd3]

This is what I had in mind.
Left section will allow power source to be selected.
Right section would automatically use the higher voltage source, would it not?

 

[Arouse1973]

It will draw from which ever is higher voltage first but who cares. Does it matter?

 

[Gryd3]

It will draw from which ever is higher voltage first but who cares. Does it matter?

In the grand scheme of things, probably not. The device will work either way.
The device is going to be 'portable', so I assumed the op would not want the battery discharging while he was using the PSU.
If that was the case, the only other solution I can think of to auto-select would be a relay.

 

[Arouse1973]

Yes correct, in this case the battery is most likely to have a higher voltage than the PSU. So a switch of some kind will be needed.
Adam