PC power supply efficiency & running a car stereo.

I have a car radio I'd like to set up in the garage, powering 4 speakers from it. To power it I thought about a PC PSU as they have 12 volt output and can supply 10 amps.
1. I understand that they need some altering to 'wake up' the main power supply, so does this mean that the PSU would be unable to supply a low-current 12 volt supply to the accessory supply to maintain radio tuner memory, etc?
2. How much power do these PSUs draw when they are supplying nothing to whatever they are connected? Being a switched mode unit, I understand that they have a constant draw of power.
My understandings, of course, may be wrong.
Last idea is to run the car stereo from a small lead acid battery (which I already have, so no cost involved) and to connect a small trickle charger to the battery to keep it topped up. Any comments on this idea?
Thanks very much for any helpful info you can give me.
David Paste.
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On 13/01/16 16:34, David Paste wrote:

IIRC you need a load on the 5V
A lot of peole did this to create a 12v source for field battery chargers for models, but in the end I actually bought an ex CB radio mains power supply off Ebay.
It was cheaper and neater than spending hours on a lashup
http://www.ebay.co.uk/sch/Power-Supplies-/48708/i.html?rt=nc&LH_Auction=1&_trksid=p2045573.m1684
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On 13/01/2016 16:34, David Paste wrote:

I have a car stereo in the kitchen, it fits nicely under the cupboards with a slot cut out of the trim. I use a 12V 2A plug-in supply, which is perfect, but if you want loud you'll need more. Ebay has plenty of 12V 5A units for less than a tenner, and I'd go for this rather than a PC supply.
Being a car stereo, it's supply needs are fairly robust, so a switcher is fine. Mine loses presets if there's a power cut - I suspect that's usual as it saves a penny or two on non-volatile memory.
Cheers
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On Wed, 13 Jan 2016 08:34:51 -0800, David Paste wrote:

Do you need anything _close_ to 10A?
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Yes.

No. You can supply what the radio thinks is the battery and that will keep the radio tuner memory etc fine.

You have that backwards, it's the alternative linear supplies that do that.

It is.

The simplest trickle chargers aren't very good for the battery and so they don't last all that long.

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On 13/01/16 17:43, jack wrote:

Even they dont do that although quiescent draw may be higher

however the battery is really just a sort of big regulator and capacitor. This actually works. Even with a 90% destroyed battery.

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On 13/01/2016 16:34, David Paste wrote:

That's the one I'd go for - but I'd have the charger on a timer rather than on trickle all the time, or I'd just switch it on when the battery started to get flat.
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Depending on the charger design, you may well get mains hum from it on the radio.
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For a start, most of them never produce nothing, as the system powers usb sockets assoon as the unit is plugged in. It may well be that the on off switch wakes up the 12 volt supplies though but you can see this in manyweb sites which use such supplies for other uses.
Brian

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That's the quite separate +5V standby power line from the power supply. Usually done with a simple linear power supply because the current it can supply is bugger all.

There is no on off switch in a modern ATX supply, the motherboard tells it to start up.
And that is the whole supply apart from the +5V standby supply, not just the 12V rails. You don't get any of the other rails coming up either until it gets the startup signal from wherever, from the motherboard when its used to power a desktop PC.

Yeah, they are very decent high current supplies if you want 12V or 5V or 3.3V.
They aren't all independently regulated tho so that can complicate things if you want to use more than one rail.

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On Wednesday, 13 January 2016 16:34:57 UTC, David Paste wrote:

Thanks all. I think I'll go with the car battery option. Lidl are selling little smart chargers for car batteries, which will be useful to have.
Thanks again.
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