six50joe
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Powering a PI from 12.5V in?

Tue Feb 13, 2018 5:08 pm

Planning my first hardware project with a Pi. The home we purchased a year ago has a security system with a Napco controller. I'd like to replace this replace this controller with a Pi that will monitor the dry contacts for the existing devices.

The current controller is in a closet with no outlet. It is powered by a line running down to an 12.5V AC adapter in the basement.
I only have light experience in powering electronics, but always willing to try more. Is there a recommended way to convert this line so that I can power my Pi with it?

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mahjongg
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Re: Powering a PI from 12.5V in?

Tue Feb 13, 2018 5:15 pm

Use a Diode-bridge for AC to DC capable of at least 50V and 1.5A, with 10.000uF 16V elco (or larger) smoothing cap to get (about) 14V DC with ripple , then a cigar plug adapter with an USB output ( a cheap 12V to 5V switching regulator), best use a 2A version.
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you can unscrew the cigar plug adapter, and inside you will find a red wire running to the TIP, and a black wire running to the side springs.
Connect the black wire to the negative of the elco (GND) and the red wire to the 14V (or so) DC.

six50joe
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Re: Powering a PI from 12.5V in?

Tue Feb 13, 2018 8:04 pm

Wow, thankyou mahjongg for providing a complete solution including a schematic so soon after my post. It's as if I was just given a kit with instructions and only need to assemble it :)

In my case, this will be a lot less trouble to do than running conduit up to the closet and installing a receptacle.

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mahjongg
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Re: Powering a PI from 12.5V in?

Tue Feb 13, 2018 8:20 pm

Yes, but mind a few potential pitfalls:
the 12.5V AV would normally after rectification result in square root of two times peak DC, so 1.41 times 12.5 = 17.6V, which is a bit much for a cigar plug adapter, but as the AC wiring probably has some resistance, the DC voltage will probably be (a lot) less, which is good in this case. but don't connect the adapter if the voltage across the elco is much more than say 15V (average), or whatever the adapter specifies as itś maximum input voltage.
Also, if the 12.5V source is weak, and cannot deliver say 0.8A or so, then it might not work.

This is really a very simple minimal solution.... so don't expect miracles.

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Lob0426
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Re: Powering a PI from 12.5V in?

Tue Feb 13, 2018 9:19 pm

A LM2596 board is a useful thing to have around. I have used them in several DC/DC projects. The newer digital version boards are nice as you do not need to dig up a voltmeter to set them.

But the cigarette lighter adapter guts is simple and I have used them before also. Best is they are ready available everywhere!
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Re: Powering a PI from 12.5V in?

Wed Feb 14, 2018 2:11 pm

+1 for using the LM2596 DC-DC converter they will work with a much wider input voltage range and still provide a stable 5v output, which may be relevant with you possible variation of input voltage.

The ones I have will work down to 7v and up to 30v input.
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