macleod@drivax.UUCP (MacLeod) (05/27/89)
I have been thinking about the praticality of using a computer in a camper where the only power source is 12Vdc. Battery powered laptops which might run off the camper's power are available, but you pay twice as much to get half the functionality with a laptop. Since the internal AT power supply takes 120Vac to produce +5Vdc @20A, -5Vdc @.5A, +12Vdc @8A, and -12Vdc @.5A, shouldn't it be relatively easy to cook up a modification to the power supply (or a replacement for it) which would run directly from 12Vdc power? Michael Sloan MacLeod (amdahl!drivax!macleod)
henry@utzoo.uucp (Henry Spencer) (05/29/89)
In article <4682@drivax.UUCP> macleod@drivax.UUCP (MacLeod) writes: >... Since the internal AT power supply takes >120Vac to produce +5Vdc @20A, -5Vdc @.5A, +12Vdc @8A, and -12Vdc @.5A, >shouldn't it be relatively easy to cook up a modification to the power >supply (or a replacement for it) which would run directly from 12Vdc power? Sorry, not so. For one thing, don't confuse the 12Vdc from your power source with the power supply's +12V output. Your 12V battery will be putting out something probably within a couple of volts (either way!) of 12, and it may be fairly rough if there's anything else on the line. The power supply's +12V output is within a quarter of a volt or so, and is fairly pure DC. Getting the latter from the former is not simple. Getting 5 from 12 is not so bad, since there's some headroom to work with there, but if you want a simple circuit it's going to be very inefficient and fairly bulky (translation, it will need considerably better cooling than your current supply and probably won't fit in the same housing). Your supply is most probably a switching supply which has fairly specific expectations about its input voltage and doesn't use any intermediate low-voltage-unregulated-DC step in the course of producing regulated DC outputs. There isn't likely to be any modification that will work; you'd probably need a total redesign. NB I am not an expert in this and may have missed something subtle. -- Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
phillip@bartal.CRLABS.COM (Phillip M. Vogel) (05/29/89)
In article <1989May28.214142.26384@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >In article <4682@drivax.UUCP> macleod@drivax.UUCP (MacLeod) writes: >>... Since the internal AT power supply takes >>120Vac to produce +5Vdc @20A, -5Vdc @.5A, +12Vdc @8A, and -12Vdc @.5A, >>shouldn't it be relatively easy to cook up a modification to the power >>supply (or a replacement for it) which would run directly from 12Vdc power? > >Sorry, not so. For one thing, don't confuse the 12Vdc from your power [stuff deleted] >but if you want a simple circuit it's going to be very >inefficient and fairly bulky (translation, it will need considerably better >cooling than your current supply and probably won't fit in the same housing). [more stuff deleted] >NB I am not an expert in this and may have missed something subtle. >-- Henry, I'm afraid you're way out of your field (what field is yours, anyway?). While it may be slightly tricky to get +12V regulated out of a car battery (ie. you can't just use a 7812), there's those of us who know how to design supplies with something other than 3 terminal regulators. I'm not saying the job is trivial, but it certainly is not unreasonably difficult. >Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology >deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Van Halen, adj: pertaining to deadly hazards to your ears. Sorry about that, but I'm not too witty this early in the A.M. Really, Henry, your input to the net is highly valued, and many of us are asking the question, "just who is this Henry Spencer, and how does a guy who seems knowledgable on so many subjects end up in the zoology department?", especially when I never have seen you post in sci.bio??? -- Phillip M. Vogel, President | #include <standard_disclaimer.h> Bartal Design Group, Inc. Englewood, NJ | (201)567-1343 FAX:(201)568-2891 UUCP: killer!crlabs!bartal!phillip | Domain: phillip@bartal.crlabs.com
henry@utzoo.uucp (Henry Spencer) (05/30/89)
In article <30@bartal.CRLABS.COM> phillip@bartal.CRLABS.COM (Phillip M. Vogel) writes: >... While it may be slightly tricky to get +12V regulated out of a >car battery (ie. you can't just use a 7812), there's those of us who know >how to design supplies with something other than 3 terminal regulators. >I'm not saying the job is trivial, but it certainly is not unreasonably >difficult. Depends on who's doing it. Unless I'm much mistaken, the original inquirer basically doesn't know anything about power supplies. What's "simple" (note that that's the word I used) for such a person isn't the same as what's simple to somebody who designs switching regulators for fun. :-) Yes, if you're willing to spend a little while learning about power-supply design, it's no big deal -- but that's not the question that was asked. >Really, Henry, your input to the net is highly valued, and many of us are >asking the question, "just who is this Henry Spencer, and how does a guy who >seems knowledgable on so many subjects end up in the zoology department?", >especially when I never have seen you post in sci.bio??? While I have been accused of being an AI program, I deny that vigorously vigorously vigorously [INFINITE LOOP DETECTED, RESET BEING PERFORMED...] [Oops. :-)] Really, I'm just me. :-) As for why I'm here, I'm not a zoologist -- I run the computer facility here. How that came to pass is a much longer story. -- Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
macleod@drivax.UUCP (MacLeod) (05/31/89)
In article <1989May29.235155.26128@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >>I'm not saying the job is trivial, but it certainly is not unreasonably >>difficult. > >Depends on who's doing it. Unless I'm much mistaken, the original inquirer >basically doesn't know anything about power supplies. What's "simple" >(note that that's the word I used) for such a person isn't the same as >what's simple to somebody who designs switching regulators for fun. :-) I once had an FCC 2nd class license, but that was long ago, and I can barely remember the basics. But it occurs to me, and still seems reasonable, that I could afford to hire some enthusiast, spend $200-$300 for such a supply, and still be ahead, considering the cost/benefit skew between 12Vdc laptops and AC systems. The target system I have in mind could be a 12mHz AT clone with one 5 1/4 HD floppy and a 20 meg hard drive and Herc card and monitor and 640K of memory. Such a vanilla desktop AC system should be easily obtained for $1200, while a comparable laptop is (I'm not positive, as I don't monitor laptop prices that closely) probably around $2500. By the way, I am getting mail from excited onlookers who show great interest in such a project. Perhaps this is a narrow but profitable niche market which somebody might be able to service (please). Michael Sloan MacLeod (amdahl!drivax!macleod)
davidc@vlsisj.VLSI.COM (David Chapman) (06/01/89)
In article <1989May29.235155.26128@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >In article <30@bartal.CRLABS.COM> phillip@bartal.CRLABS.COM (Phillip M. Vogel) writes: >>... While it may be slightly tricky to get +12V regulated out of a >>car battery (ie. you can't just use a 7812), there's those of us who know >>how to design supplies with something other than 3 terminal regulators. >>I'm not saying the job is trivial, but it certainly is not unreasonably >>difficult. > >Depends on who's doing it. Unless I'm much mistaken, the original inquirer >basically doesn't know anything about power supplies. What's "simple" >(note that that's the word I used) for such a person isn't the same as >what's simple to somebody who designs switching regulators for fun. :-) >Yes, if you're willing to spend a little while learning about power-supply >design, it's no big deal -- but that's not the question that was asked. Having built a switching power supply or two, I think I'm qualified to say "it's not that easy." Yes, there are easy-to-use chips that step up, step down, and invert. I've used them (TI's TL497 comes to mind). But look again at the original post: he wants 5V at 20 amps (plus others, but you get the point). That is _not_ a trivial power supply, can _not_ be constructed easily using most of the chips available, and probably _will_ require some fancy construction, cooling, etc. The original poster (sorry, forgot your name :-) may be better off with a 12VDC-120VAC inverter. Most of the switching power supplies should be able to handle the usually trashy AC (square waves) that these things output. Anyone know this for sure? (Be careful when you use one of these that you don't drain your battery in an all-nighter. It's hard to push-start a PC :-) As for me, my portable weighs 2 pounds, has 512K RAM and removeable 128K EPROM cartridges, and runs on batteries (4 AA) for most of a day. Of course, I just use it for writing and don't need MS-DOS compatibility on the road (runs a Z-80 with a proprietary operating system). It's called the Z-88 and is made by Cambridge Computers in the UK (Clive Sinclair's latest venture, I believe). Costs about $1K as described. None of this 120 VAC stuff for me (no car batteries on my bike :-). -- David Chapman {known world}!decwrl!vlsisj!fndry!davidc vlsisj!fndry!davidc@decwrl.dec.com
wtm@neoucom.UUCP (Bill Mayhew) (06/02/89)
Since a 12-MHz AT-clone is being discussed as the target system, volumetric efficiency must not be a terribly high priority option in specifying the system. What comes to mind is using a stand-by power pack that would normally be used for AC power failure protection. My questa stand-by power pack already has lugs on the back to accomodate an external 12-14 volt DC source in lieu of the internal battery. The questa is about the width and height and about 2/3 the length of an average shoe box; it puts out 200 watts for 20 min., which is the life of the internal battery. My particular unit has to be derated to 150 watts for continuous duty from external power. 150 watts is more than enough for an AT-clone and monochrome monitor. By the way, questa is a fairly high-priced brand; you can probably do better by shopping around, but you might want to check to see if the unit under consideration already has an external DC input to save you the bother ov having to kludge it yourself. I got my questa at a close-out sale for $100 from a dealer who thought it was only a filter of some kind. A rough guideline for shopping is about US$1 per watt out max output. One other alternative is to go buy the Tandy LT-1400. It has 768K meory two 720K drives, full ports, drives external kbd/monitor, etc. and has been "discounted" to $999 list. With our corporate/educational discount of 15%, I can buy it for $849.95, which makes it difficult to resist. The CPU, is only a 7.16 MHz V20, but that seems in keeping with the bulk of computational demands that are placed on a _portable_ machine. I seldom have need to do complex ray-traced rendering while riding in my car... The pctools program rates the LT-1400 at 1.6 the performance of a standard 4.77 MHz XT, which is about what the subjective feel is in normal operation too. Bill wtm@impulse.UUCP
dueck@dvinci.USask.CA (Harvey C. Dueck) (06/03/89)
In article <4682@drivax.UUCP> macleod@drivax.UUCP (MacLeod) writes: >... Since the internal AT power supply takes >120Vac to produce +5Vdc @20A, -5Vdc @.5A, +12Vdc @8A, and -12Vdc @.5A, >shouldn't it be relatively easy to cook up a modification to the power >supply (or a replacement for it) which would run directly from 12Vdc power? On the table behind me is a CMOS PC clone which our lab uses for field processing. The power supply runs off two car batteries which are used to provide +12V and -12V to a DC to DC converter (PICO KRA5S, according to the label on the converter). A simple circuit using a diode and two miniature relays protects the converter against inadvertently connecting the batteries incorrectly and a voltage regulator is used to improve regulation of the converter output. In all, the supply requires 13 components including a heat sink. I think that the total parts cost was something like $450 Canadian. The converter itself cost ~$350. I don't know if there exist converters which will provide +12V and -12V from a single 12V supply. Ask your local electronics parts dealer. If someone wants schematics for our supply, I could probably be induced to mail them a copy. - harv dueck@dvinci.usask.ca ...!uunet!dvinci.usask.ca!dueck
hughes@math.Berkeley.EDU (Eric Hughes) (06/05/89)
In _Circuit Cellar Ink_, Issue 8 (the most recent), Steve Ciarcia has an article on construction of a switching power converter for a distributed power system. It is a small one (no 20A here); its ratings are +5 V and 1.5 A, +12 V at .5 A, and -12 V at .25 A. He presents both his first design and his better redesign, as well as discussing the basic types of switchers. The final circuit uses a Linear Technology LT1070 integrated switching regulator. It was a very informative article to me, who knew very little beforehand. Access-- Circuit Cellar Ink 4 Park Street, Suite 20 Vernon, CT 06066 203-875-2751 CCI, Subscriptions P.O. Box 2099 Mahopac, NY 10541 203-875-2199 Linear Technology 1630 McCarthy Blvd. Milpitas, CA 95035 408-432-1900 Eric Hughes hughes@math.berkeley.edu ucbvax!math!hughes