Saturday, March 14, 2009

Running a PC off batteries

This (well, actually next) month's QST has an article on DC-to-AC inverters targeted toward the specific purpose of running a shack computer off the shack battery backup supply. While DC-to-AC inverters can be useful, there's a better solution to this problem and the QST article completely ignored it: replacing the PC's supply with a direct DC-to-DC converter.

Powerstream (a company with which I have no experience or connection) offers several DC-to-DC power supplies in the ATX form factor for very reasonable prices. Laptops and other devices with "brick" or "wall-wart" supplies typically have low-voltage DC inputs that can be accommodated either by adapting a car adapter (recall that 13.6 VDC "shack" power is essentially the same as automotive accessory power) or by homebrewing a DC-to-DC converter (buck/boost converters are one option for this). About the only devices that cannot be easily run by a DC-to-DC converter are CRT monitors and laser printers. CRT monitors should be replaced by LCDs anyway as the latter have far lower power consumption, and laser printers have such high startup currents that they probably can't be run off a DC-to-AC inverter either unless the inverter has a very high surge capacity. I've known more than one IT operation that had UPS problems caused by plugging a laser printer into the UPS.

Of course, most DC-to-DC converters use pulse-width modulation to generate the desired voltage(s), and hams will have to be careful to ensure that any RFI generated by such approaches is managed with care, and it might be difficult to do this. My main beef with the QST article is that it presented DC-to-AC inverters as the solution to the "how to power a computer off batteries" problem without even mentioning in passing that there are other solutions that at least deserve some consideration.