VOISS - Power

Written by Rich Morin.

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Precis:  an overview of VOISS (etc) USB power handling

Note: As of early 2020, the VOISS project and F123Light were discontinued. For details, see VOISS - Status.

The VOISS Classic systems include two components for USB power handling: a “power bank” and a “power source”. These devices work together to provide clean, reliable power for the computer and its USB peripherals. Let’s examine how these devices differ and how they relate to each other.

Power Bank

The typical USB power bank contains one or more Lithium-ion batteries and some electronics for monitoring and control. So, it’s basically a rechargeable battery that plays nicely with USB. Being able to use the VOISS anywhere you like (e.g., sitting on a park bench) is pretty awesome, but that’s just part of the story.

Even if you only expect to use the VOISS in a location with generally reliable and easily available power, I’d recommend running your power feed through a power bank. Because this acts as a dedicated uninterruptable power supply (UPS), your computer can keep working during transient power outages. (Like when someone (oops!) unplugs your power supply.) The power bank also provides quite a bit of protection against power line transients that could damage your system.

Description

A USB “power bank” is a sealed unit which contains some batteries and electronics. The bank can accept power (5 VDC) from a USB source, store it, and dispense it to other USB devices. The power bank, like the RasPi processor board, receives power via a micro-B USB socket. The power exits the bank via a pair of standard USB (Type A) sockets.

Power bank capacities are listed in mAh (milliampere-hours); a typical unit might claim to store 10,000 mAh. Removing three zeroes from the number lets us get rid of the “milli” prefix, resulting in a claim of 10 Ampere-hours (10 Ah). So, it should be able to supply 2.5 Amperes for about four hours.

A set of tiny LEDs (or even a tiny LCD panel) typically indicates the charging and operational status. This isn’t going to be particularly useful to most blind users, but it shouldn’t be all that big a problem in practice. If you charge up your power bank on a regular basis, you won’t have to worry about the details.

That said, if you find any power banks with an audible indicator or Bluetooth capability, be sure to let me know! So far, I know about the following products:

Requirements

When selecting a power bank, get one that can supply close to 2.5 Amperes. As the introductory blog post for the 3B+ (Raspberry Pi 3 Model B+ on sale now at $35) says:

Note that Raspberry Pi 3B+ does consume substantially more power than its predecessor. We strongly encourage you to use a high-quality 2.5A power supply, such as the official Raspberry Pi Universal Power Supply.

By way of clarification, the 3B+ only needs about 1.5A, even when all of its four procesing cores are working hard. However, any USB devices the computer powers (e.g., headphones, keyboard) must be added to the total current requirement. (Devices with their own power sources do not, in general, draw any power from the computer.)

Selection

Amazon lists power banks with various capacities, features, prices, and sizes. I haven’t found any that provide a full 2.5 A, but several claim to provide 2.4 A, which is probably good enough. For example, here are a 5,200 mAh unit for $10 and a 25,000 mAh unit for $35. As noted above, accessibility adds quite a bit to the cost.

Having a power bank fail isn’t likely to be a catastrophe, but I’d still pay attention to the reviews before making a purchase. Bear in mind is that you can carry around more than one power bank. That way, if one bank runs out of power, you can plug in another.

Power HATs

A variety of Hardware Attached on Top (HAT) boards can be used with the 3B+. One of the most useful types, which I’m calling a Power HAT, contains a Lithium-ion battery and some associated circuitry. This allows it to act, like a Power Bank, as a UPS for the computer.

However, because it is connected to the 3B+ using the General purpose input-output (GPIO) connector, it should be able to report its status to the operating system. If so, this information could be used to support power management functions (e.g., alerting the user, shutting down the computer). For details, see VOISS - Power HATs.

Power Source

The USB power source (aka “wall wart”) is a small, sealed power supply that plugs into an AC power outlet. It typically several feet of cord attached, ending in a micro-B plug.

Although the built-in plug will be designed to work with the sockets in a particular country, the power source itself doesn’t care much about the local voltage or frequency. A simple set of adapter plugs will let the device work almost anywhere.

Details? Because the device uses a switched-mode power supply, it is taking very short and rapid sips from the available power. So, as long as it gets what it needs, it’s happy. Thus, 110/220 VAC or 50/60 Hz are equally good in its estimation.

That said, be sure to get an appropriate “power source” for the system. As noted above, it needs to supply 5 VDC at 2.5A, via a USB micro-B connector. This one, for example, is designed to work with the 3B+.

Cables

You’ll need a few USB cables to hook things together. One will go from the power bank to the processor board. Another may go from a USB outlet to the power bank. Fortunately, USB cables are really cheap; so, get a variety and find out which one(s) work best for what purposes.