Introduction: Converting a PAPR Respirator to USB Power
There are plenty activities where one should wear a respirator but one doesn't because they are uncomfortable, hot, and make your safety glasses fog up.
A Powered Air-Purifying Respirator (PAPR) combines sealed face protection with a battery-powered fan that drives air through filter cartridges. They were developed for use in mining, asbestos removal, disaster response, etc., so they are not cheap, but they work very well.
I've had a 3M Breathe Easy unit for a number of years and the NiCd battery is worn out from use. This Instructable shows how to convert a PAPR (or any 5V device, really) to what has become the new universal rechargeable power source: the "USB battery pack".
Step 1: Identifying Voltage Requirements
This is the old battery, a 3M 520-01-15. It is a made of four NiCd cells in series yielding 4.8V nominal output and is rated at 5.7 amp-hours capacity. The battery is very ruggedly constructed, important because it hangs off the belt of the PAPR and tends to take a beating in the course of normal operation.
Step 2: Replace Stock Connector With USB a Connector
I am not exactly sure what the original connector is on this device, but it is probably an XLR connector. I cut off the existing connector and replaced it with a ruggedized USB connector, the DCC-USBAB-180 from Conxall/Switchcraft. This connector is designed to make a watertight connection with a matched USB receptacle, but I only picked it because it seemed rugged. And black.
Step 3: Solder New Connector
The conversion is straightforward; we want to make the respirator into a USB "cable", with power on pin 1 and ground on pin 4. Shown also is a typical USB device to help identify the pins correctly. (In this case red is power and white ground.)
The soldering is pretty easy (once I borrowed some reading glasses, sigh) but the DCC-USBAB-180 comes without any directions. I wish I could provide better instruction beyond "put the connector housing on the cable before soldering and have faith that it will all fit together tightly when assembled in the right order", but that's all I can tell you.
I would like to make a pitch to use lead-free solder in projects. With the right flux, it is not much harder to work with than regular lead-tin solder, and it keeps lead out of your household . Shown is 96% Tin/ 4% Silver solder which I use with Superior No.30 Supersafe Liquid Solder Flux.
Step 4: The USB Battery Pack
Originally I planned to make my own battery pack from individual LiFePO4 cells. I'd also need to construct a charging circuit, low-voltage cutoff, and some kind of case. But it is really not worth doing this: the widespread use of USB-powered devices like the iPhone has created a whole industry of "USB battery packs" that provide 5V output on a USB connector. Amazon has hundreds of these for sale; I chose the Anker Astro E4, mostly because a lot of people seemed to like it and it is a great deal at ~$30. (Spec-wise, it wipes the floor with the old battery, which weighed 28 oz/ 800 g and was rated at 5.7 amp-hrs, while the replacement is only 10 oz / 290 g and rated for 13 amp-hrs!)
The Anker comes with a little mesh bag. I don't know why anyone would use this in normal life, but for me it solved the problem of how to secure the battery: I just sewed the bag directly onto the belt of the PAPR. The bag provides the battery will a modicum of protection and it has a little drawstring that will keep the battery from falling out when I am working at some weird angle.
I have to go off on a tangent to marvel at these batteries. Normally if you have some battery, it outputs a voltage associated with whatever chemical reaction is at work, and the voltage slowly drops with use. To charge the battery, you have to apply some overvoltage to overcome the kinetic barriers associated with the charging reaction. But with these batteries they both charge at 5 V and discharge at 5 V. It's like magic!
Internally these batteries have a DC-DC converter that boosts the output voltage from the 3.6-3.7 V characteristic of Li-Ion batteries to the USB standard of 5 V. As Horowitz and Hill say, "voltage isn't everything": the boost circuit draws more current from the battery than it outputs at the 5 V terminal. The output voltage will hold steady, drawing increasing amounts of current, until the discharge circuit notices the battery has hit its low voltage limit and shuts down. While there is some power lost in the DC-DC conversion (the efficiency is around 85%) it is a worthwhile tradeoff. Having a steady voltage means the airflow to the PAPR will be held at the right level, critical to its proper functioning.
Step 5: Ready for Action!
I let this run for about 4 hours yesterday and the battery pack still claimed "75-100% full". This suggests I'll be able to get in a full day of scraping paint or killing zombies with my Pulaski. I don't know about you, but I find hand tools are really the most effective way to dispatch living dead: the downside is that always-annoying problem of "splashback". Wearing a PAPR really cuts down on the "yuck factor".
Since the battery has two USB ports, one could even recharge an iPhone while killing zombies. Let's go!