I got a computer running using a small surplus UPS (110V). You could fool it into turning on without even being plugged in by switching the power switch on and off twice quickly. Then I used a standard PC power supply switched to 110V input. The UPS battery was replaced with leads to the car battery. To efficiently get 12V from above or below 12V without winding transformers, you can use 'SEPIC' topology converters. These are non-isolated and only use an extra inductor. You can use a standard converter chip, lots of these can do SEPIC. ----- Original Message ----- From: "Byron A Jeff" To: Sent: Thursday, January 10, 2002 8:58 AM Subject: Re: [EE]: Car power supply challenges > On Wed, Jan 09, 2002 at 02:59:40PM -0600, Don Hyde wrote: > > This might be time to examine your original specification. > > Ok. Let's. > > > > > Do you need to be able to replace the power supply in any old generic PC and > > make it work in a car? In this case, you need to really provide plenty of > > +12 over the car's full voltage excursions -- which means working from +8V > > or less during cranking the engine or when the battery's almost dead, to > > 13.8V, which is most of the time when the alternator is working, but not > > keeling over dead when the 40V spikes hit as the power windows and door > > locks are used. > > The UPS battery takes care of the undervoltage. The overvoltage issues still > apply. > > > > > If you need to get SOME PC to work in a car, then examine PC's. These days > > most of them don't use much +12. You might find (or configure) one that > > doesn't need any at all -- it's usually stuff like sound cards that still > > use +12. I'm pretty sure there are even disk drives that use only +5. > > Laptop drives. However it's my preference to use ordinary 3.5in drives because > of their phenominal prices. > > Also I plan to interface a 100 MB ZIP drive to the unit which requires 12V. > > > If > > you can get it down to a few milliamps, then you can sacrifice some > > efficiency for simplicity and use an off-the-shelf and > > not-extremely-expensive DC-DC converter to make the needed +12 from the +5, > > which as you say, is much easier to get. > > > > The general solution is probably going to need a pretty sophisticated > > switching boost-buck converter that can take whatever it gets and make nice > > clean +12 out of it, and that's going to be expensive and complicated. > > I think (mind you think) that the UPS battery will preclude the need for a > boost situation. The battery has quite good about providing 12V for moderate > periods of time. > > The issue at hand is how to do LDO regulation with 0.6V of headroom. A PNP > or PMOS pass transistor can do it, but how does one control it? > > BAJ > > > > > -----Original Message----- > > > From: Byron A Jeff [mailto:byron@CC.GATECH.EDU] > > > Sent: Wednesday, January 09, 2002 1:44 PM > > > To: PICLIST@MITVMA.MIT.EDU > > > Subject: [EE]: Car power supply challenges > > > > > > > > > I think I've done my homework on this one folks. I googled. I checked > > > the archives for this list. I checked the news archives. > > > > > > Here's the situation: > > > > > > I want to power a PC in the car. 5V has multiple solutions so it isn't > > > of concern here. I'm particularly interested in the 12V supply. > > > > > > The supply will have a 12V 7Ah gel cell to act as a mini UPS. So the > > > input voltage will be from 12 to 14 volts. > > > > > > I want to get a regulated supply of 12V +- 5% @ 4A. > > > > > > Lastly and most importantly to me: It needs to be a discrete > > > solution with > > > components that can be purchased at the local parts store. > > > Think Radio Shack > > > or Radio Spares. This requirement has a twofold purpose: I > > > want to learn > > > the process, and I want to be able to have readily available > > > replacement > > > components. > > > > > > From the specs the problem is obvious: How in the heck do you > > > get a regulated > > > 12V output from a possible 12V input? It's impossible. > > > However with the > > > error term regulation down to 11.4V is acceptable. > > > > > > So 12V in and 11.4V output screams low dropout regulator. So I started > > > researching them. NPN and NMOS are immediately depricated. > > > PMOS and PNP > > > are the way to go with the series pass element. This leaves > > > the problem of the > > > control/error element. The obvious, a regular linear regulator, is out > > > because the are amost universally NPN devices, which means > > > high dropout > > > voltages. > > > > > > The next choice is a zener. Simple, clean, and reasonable > > > regulation. But > > > each and every circuit I've seen that uses a zener control > > > has an NPN pass > > > element. Again high dropout voltages. > > > > > > So I was wondering if anyone had any ideas on how a zener can > > > be coupled with > > > a PNP/PMOS pass element so that reasonable regulation and low > > > dropout is > > > still possible. > > > > > > On a parallel path I did take a look at the buck switching > > > regulators that > > > have been discussed in the last year. However they seemed to > > > need more voltage > > > headroom than I was able to provide. > > > > > > One last issue to raise again was the input voltage spike > > > protection. Most > > > of the discussions that have been proposed in the past have > > > been rather low > > > current. I was wondering if a low valued resistor with a > > > power shunt before > > > the supply could work. For a shunt I was thinking along the > > > lines of Fr. > > > Thomas' power shunt here: > > > > > > http://www.infosite.com/~jkeyzer/piclist/2001/Jan/2382.html > > > > > > (login and password are the name of this list in lowercase) > > > > > > or a simple crowbar using a high valued zener driving an NPN > > > shunted to > > > ground. This would be helpful because I really wanted to > > > regulate the UPS > > > batteries input voltage to 13.8V anyway. > > > > > > Well this is what I've thought up so far. I'd really be > > > interested in getting > > > some input on how to solve these issues. > > > > > > BAJ > > > > > > -- > > > http://www.piclist.com hint: The PICList is archived three different > > > ways. See http://www.piclist.com/#archives for details. > > > > > > > > > > -- > > http://www.piclist.com hint: The PICList is archived three different > > ways. See http://www.piclist.com/#archives for details. > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.