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I have an electronics question. I am no electrician and am trying to build an alarm pi Mark2. Currently I have a 12v power supply running through a relay which the pi turns on and off to trigger my 12v RGB led strip. Along side this I have a standard micro usb powering the raspberry pi. What I would like to know is A) would it be possible to leech power from the 12v power supply using a t-joint solder into the power cable, and a 12v-5v buck converter, and still run both systems? Further to this, in the next generation of my project I would like to include a wireless charging dock and a set of USBs so that the Alarmpi could become an all in one bedside alarm and charge station. So B) how do I figure out the required power supply for 4 charging USBs, 1 5v qi mod, 1 5v pi cable and the 12v rgb leds? Please as for clarification if needed I know this is a little garbled but any advice would greatly help! Just to note the pi in turn will be running a 20x4 rgb LCD screen, 2 small adafruit speakers and an led clock screen as well as a luminosity sensor and a small amplifier

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A) would it be possible to leech power from the 12v power supply using a t-joint solder into the power cable, and a 12v-5v buck converter,

Yes, absolutely possible. However, it must be a switching mode DC-DC converter, linear regulators like the hearty 7805 have too much loss for this application

B) how do I figure out the required power supply for 4 charging USBs, 1 5v qi mod, 1 5v pi cable and the 12v rgb leds?

Add up their power by multiplying current draw x operating voltage

Simple Estimate, we will build the margin into the initial estimates

  • Raspberry Pi with Hat and peripherals 2A @ 5V = 10W
  • USB Charging 2A @ 5V = 10W x 4 = 40W

The unknown is the LED current, which you have not provided. However our simple estimate says we need about an extra 50W above the power consumption of the LED strip. 50W @ 12V ~ 4.25A , that is the extra current overhead you need from the 12V supply.

I would estimate that a reasonable LED strip is itself another 2A, so using typical grades I would pick a 10A @ 12V supply for your test. One such turnkey option for reasonable scratch is in the Mean-well series of power supplies (Vendor Link )

If you relax the charging requirements to 1A or fewer devices, or for your initial test, a 5A supply would probably work for most situations. But it will likely be starved if the lights are on and all four charging ports are bing used

Other Options

The problem with this setup is that you have quite a high current draw at your system supply voltage. This is probably reasonably OK for most users, but there are things to keep in mind, like the gauge of the wire used for the main supply hookup (at full current draw).

There are a few things to consider

  • Use a 24V voltage at a more reasonable 3-4A with two 12V LED strips in series with one another.
  • Use independent 12V and 5V supplies from the same mains plugs, turnkey switching supplies are cheap , reliable, and easy to use, this would guarantee your power needs.
  • Thank you for this it is fantastic, looking over this, I'm wondering, if I removed the USBs and, certainly for now, kept those separate, that would reduce the draw; USB Charging 2A @ 5V = 10W x 4 = 40W therefore I should be able to reduce the power supply right? The thing i'm aiming for is something with a fairly low power consumption which peaks only when in full use (the alarm going off, with the light on, whilst charging a phone and playing music! This is going to sound strange, but if you were to do this would you re-jig it and do it a different way to be more efficient? Thanks!! – Will Marks Sep 18 '17 at 16:52

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