# Dual AC feed to power a DC unit (Raspberry Pi) with Diode and Polyfuse for protection

I wish to place some Raspberry Pi's in remote locations (data centres) for telemetry and other services like serial OOB access.

Typically dual AC feeds are presented to customers in a data centre which come from completely separate feeds from the local power grid. Raspberry Pi's take a single DC 5v 1mA input. In the event that one of the power feeds in the data centre goes down I want the Pi to stay up (and in the event they both go, it will have a battery pack).

This isn't a natural feature of the Rasp Pi (two power supplies) so I need to design a safe and fault tolerant system that will do this. My knowledge of electronics is limited and I will be consulting others and testing in a lab style environment; As a basic premise though, does the following sound sane or a have I grossly misunderstood electronics and risk exploding all the kit in my rack and potentially others;

• Dual 16A or 32A feeds per rack (A and B feed)
• Attach a regular 5v DC Rasp Pi transformer to each feed to present two 5v DC feeds
• Attach a diode to each feed to stop any current passing back towards the source A or B feed
• Attach a polyfuse in case of a surge coming in from the source A or B feed
• Attach A feed to Rasp Pi battery pack, then to Pi 5v inpit
• Attach B feed as the power supply to a powered USB hub, which is then connected to Rasp Pi USB port (A Rasp Pi can be powered by either it's main USB mini power input, or voltage coming from a powered USB hub, both together as far as I have read, won't harm it)

  A Feed                    B Feed
|                         |
DC                        DC
Transformer               Transformer
|                         |
Diode                     Diode
|                         |
Polyfuse                  Polyfuse
|                         |
5V DC 1mA                  USB Hub
Battery Pack               Power Input
|                         |
Rasp Pi                   Rasp Pi
5V Input_________________USB Socket


I understand this question is somewhat Rasp Pi specific, but in terms of general electronics, would this work? What should I be looking out for on the Rasp Pi that could cause this plan to fail?

My main concern is that I will connect the A and B feeds directly together and blow everything attached to both feeds. Also, what if the A and B feeds are out of phase as they are coming from two different sources?; I assume this doesn't matter as they are both being converted to DC?

Many thanks and sorry if this seems silly, my knowledge of electronics is very limited.

Given the relatively low cost of a Raspberry Pi, this could be one approach:

• Power the Pi with an off-the-shelf online charger -->> battery pack -->> UBEC such as used in remote control model aircraft. This provides a regulated voltage from the UBEC output, takes care of charging and safeguards for the battery, and also, with some chargers, will provide a visual alert when the battery has aged beyond a specified extent.
• Power the charger through any failover mechanism of choice from both raw power lines.
• Set up two such independent Pi + power units, in separate racks, to duplicate each other's function - since the RPi is not an industrial reliability product, failure during continuous operation is not unlikely.
• Incorporate remote heartbeat code on each RPi, polled from your central Network Operations Center or equivalent, so the failure of any RPi generates an alert on the monitoring panel.

I personally would power the raspberry from a battery which normally would be charged from A and by using a relay I would continue to charge the battery from B in case of a blackout.

That way you do not have to worry about blackouts and you will not have any power from B to A or B to A.

I run a hosting company and we have to think about dual power redundancy a lot. If I were to build a solution to your setup I would simply use an ATS (Automatic Transfer Switch). These are AC devices that have two power inputs and one output. Connect one input to circuit A and the other input to circuit B. The output would then power your 5V rectifier. If circuit A dies, the ATS seamlessly switches loads to use circuit B, without any interruption of the loads.

APC makes a whole line of ATS devices. You can find several rackmount versions here:

http://www.apc.com/products/family/index.cfm?id=14

• I am aware of ATS' but they cost far too much though to be practice here, the cheapest I have seen is around 10 times the price of a Rasp Pi, but thanks though :) – jwbensley Dec 3 '12 at 15:30

Examining all common sources of failure to find solutions; I see the following opportunities;

• power line transients from lightning on both feeds. Power meters protect only >6kV with built-in arc gap, transformers are usually rated at 1.5 to 3kV. Line filter reduces that peak voltage substantially for both common mode and differential mode.
• Some use MOV for extra surge suppression (not polyfuse) as long as impedance protected.
• People who don't design their computer systems to 6kV are at risk in exposed power line areas. That was our Corporate Std for all equipment at Unisys.
• Choice of power supply must have demonstrated reliability (not any old Sparkfun transformer)
• Battery pack must have capacity over worst case power outage duration and demonstrated reliability over environmental range, otherwise take your chances.
• Polyfuse is for thermal protection from permanent overvoltage or short-circuit failure.
• Connectors are a common point of failure and may need additional strain relief from inadvertent damage or pulling on cables.
• Consider HALT-HASS testing with vibration and low voltage to ensure stress operation of units after high temperature high humidity cycles while operating.
• if there is a fault do you want to detect it or just have redundant power? Raspberry
• Pi's may not have demonstrated reliability yet so Halt/HASS helps immprove confidence level. - EMI is a common point of functional failure
• Consider Ingress from sensors, ESD, industrial noise crosstalk on sensors and communication. Typical tests are 15kV ESD, 1kV radiated pulse dump from sheet metal, pulsed radar, PLT test, HIPOT etc and...
• use CM chokes on all sensor cable pairs. Ethernet already has that.
• consider any other single point of failures in your system and risks.