Path: utzoo!telecom-request Date: Tue, 4 Jun 91 10:42:44 PDT From: Dave Platt Newsgroups: comp.dcom.telecom Subject: Re: Cellular Modems Message-ID: Organization: New Technologies Group, Inc. Palo Alto CA Sender: Telecom@eecs.nwu.edu Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 11, Issue 424, Message 5 of 8 Lines: 69 In article is written: > On the subject of cellular modems and laptops, I've run into something > that others might have encountered a fix for. It seems the > high-powered laptops such as the Toshiba 5200 don't have any provision > for running off of a 12-V DC input. I can imagine that your typical > low-power 12-V input might be insufficient to run something like this, > with a hard drive, 16 megs of memory, 25 MHz 386, VGA screen, etc. > However, we need something that powerful (and we need the slots). > Seeing as how a car battery can provide a substantial amount of power, > I was wondering if anyone knew of any product that would allow us to > run something like this off of the car's lighter jack, or even to hook > directly to the battery if necessary. Or of any portable with that > much power and a slot that you can take along with your cellular > phone. There may be a very real, defensive reason (or two) for the lack of a +12 VDC power input. [1] Most of the "guts" of a laptop or cellular modem probably run on +5 volts. To be able to accept +12, one would need either a linear voltage regulator (which would dissipate, as heat, more than half of the power fed into it), or a switching regulator (which would add size, weight, and cost). [2] 12-volt power drawn from a car's electrical system is hazardous. If the engine is running, the accessory voltage is likely to be quite "dirty", with sags and spikes and high-frequency noise from the ignition, other loads cutting in and out, etc. The auto environment is considered to be "electrically hostile"... audio amplifiers, CD players, etc. which are designed to operate in this environment must include a good deal of overvoltage/spike suppression, etc. The CMOS components in a laptop are quite vulnerable to spikes and surges. If you want to run a laptop, cellular, etc. from an auto's electrical system, you'll probably have to buy an external high-amperage 12-volt- to-5-volt stepdown regulator (probably a switcher, with extensive surge protection and some big filter capacitors), and a "cheater" which will let you plug the regulator's output into the laptop's charger input or directly to the battery-compartment contacts. I'd suggest that you check your insurance and warranty, as well ... it'd be a shame if a spike from the ignition fried your laptop. If you were thinking of running your laptop from the car's battery, with the ignition off: be advised that most auto batteries are not intended for deep discharge. They're designed to provide large amounts of current in short bursts, and to be recharged immediately and kept "topped up" by the alternator. A "low maintenance" or "no maintenance" auto battery can be ruined by only a few deep-discharge/recharge cycles ... it will lose its ability to hold a charge. If you really want to run your high-amperage laptop from an external battery, you'll probably want to get something along the lines of a deep-discharge lead/acid battery, such as are often sold for marine or RV applications. These batteries are designed to be able to be discharged most of the way to zero, and then recharged fully, a large number of times (the tradeoff being that they don't have the same peak-current output as a lead/acid battery designed for automotive use). You could charge this battery from an auto's alternator, or with a standalone battery charger. - Dave Platt VOICE: (415) 813-8917 Domain: dplatt@ntg.com UUCP: ...apple!ntg!dplatt USNAIL: New Technologies Group Inc. 2468 Embarcardero Way, Palo Alto CA 94303