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PSU Needed

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 28 October, 2007 » PSU Needed « Previous Next »

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terry
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Username: terry

Post Number: 430
Registered: 05-2005

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Posted on Thursday, 18 October, 2007 - 09:59 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I need to supply a regulated 5v at 4A max and 12v at 2A max from a car battery (not connected to the car). Size is the most important thing as it needs to be compact but effecient too.

Thanks, Terry
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grab
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Username: grab

Post Number: 580
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Posted on Thursday, 18 October, 2007 - 01:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

(Advance warning: I'm going to round to convenient numbers where possible, so don't expect numbers to N decimal places...)

At 5V, a linear regulator has 28W heat to dissipate, so you'll want a 2 degC/Watt heatsink just for that. That's rather energy-inefficient though, so you might be better with a switch-mode if you can find a suitable one at that current rating. If you want a smoother supply than a switch-mode gives you, use the switch-mode to get maybe 7V and then use a linear regulator for the rest.

The regulated 12V is going to be a problem though, because you don't know whether that's above or below the battery voltage. The best plan is probably to drop it down (to somewhere below the lowest battery voltage) and then step it back up, because you can then combine this with the step-down for the 5V. Alternatively you could step it up to somewhere higher than the highest battery voltage, say 15-16V, and drop it down from there - this isn't any better efficiency-wise, but the higher voltage means you'll be dealing with lower currents which might make components cheaper. A further alternative of course is to run your higher-voltage stuff off 9V instead.

Suppose you go for the easiest solution of stepping everything down to 5V, and then stepping that back up to 12V for your second supply. Let's say the 12V step-up is 70% efficient, then you need to give it 35W at full power. And let's say the 5V step-down is also 70% efficient. Then the 5V supply needs to give out 11A total, and needs to be rated at 80W. You've also got a total of 35W of heat to get rid of.

Graham.
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terry
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Username: terry

Post Number: 431
Registered: 05-2005

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Posted on Thursday, 18 October, 2007 - 04:45 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks Graham, the numbers I quoted are from the mains PSU. It seems that the real numbers are a lot less. I connected the mains PSU to an inverter powered from a car battery and the current is 2.2A from the car battery. I dont know how much is the 12v and how much is the 5v but it all looks a lot easier.

Terry
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terry
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Username: terry

Post Number: 434
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Posted on Saturday, 20 October, 2007 - 03:25 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Has anyone got a favorite step down 5v switcher that can handle 2A ?

Terry
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gordon
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Username: gordon

Post Number: 329
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Posted on Saturday, 20 October, 2007 - 04:45 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The MIC4576BT step down converter can handle 3A, and needs few external components. Rapid currently stock it. I think National also do a range of simple switchers that might also be suitable.
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terry
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Username: terry

Post Number: 435
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Posted on Sunday, 21 October, 2007 - 09:58 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks gordon I will look it up.

Terry

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