Thread: Newbie - AC/DC or DC/DC ?

1. Most inverters cut out at 11 volts. So with most inverters, yes the computer will reboot if you have the computer on before the engine. The ATX DC-DC Power supply I built will supply its full rated current all the way down to 8 volts. Some other DC-DC power supplies will go that low too. My car's battery drops as low as 8 volts during cranking. Most new batteries drop as low as 9 or 10 volts.


2. Most AC-DC or AC-AC power supplies are about 80 percent efficient, most inverters are 85-90 percent efficient. If you use both, you get the idea...

3. I'll sell you one, but cheep to me is about 80 bucks, and that is how much all the parts alone cost!!

4. If you use a celeron, 566-800, a dvd, gps, standard 5400rpm or 7200rpm hard drive, and an all in one motherboard, 70 watts should be just fine.

Jeff, thanks for your info.

But due my not as good english, I think you did not understand my second question
-sorry-

What I am wondering is that I can use my PC (DVD, MP3, webcam as a security device) when the engine is not running. And does the usage of either a inverter or DC-DC psu affect the time I can make use of these PC features?

But as a reply of the 3th and 4th points:

3. If you will build a DC/DC 12V ATX psu for me, how much will this cost for me? And will this psu be as good (considering securities, fuses etc.) as a psu made by a manufacturer on the net? (e.g. www.keypower.com)

4. As I look for DC/DC 12V ATX psu on the net from manufacturers, they all are about 250W and above, and you say that 70W will be enough, but why do these manufacturers on the net sell such high watt psu's if not needed?

You won't be able to run your computer off of a car battery for more that a few hours, regardless of if you do inverter or DC-DC power supply.

The inverter method will be the cheapest method, and many people are using it without complaint. You might have some problems with the computer rebooting while cranking, and with an inverter you won't be able to use the computer for very long at all while the car is off (DC-DC you can use it for longer, but still not more than a few hours).

Car batteries are designed for high-current short-time output (i.e. turning over your engine). I tried to work out how to power a laptop + stereo off of a car battery, and got about 2 hours before the setup died.

You can get around the cut-off for the inverter when you start your car by using some rather large capacitors. I guess it depends on how much you want to use your computer while the car is turned off.

As for a "webcam for security", I don't think that's plausable or even really useful. If someone steals the car, what's the webcam going to help you with?

As for power, 250W is required if you want to run a normal computer with a PIII/1GHz and 2 hard drives, a cdrom, a burner, amplified sound, a high end video card, and using 100% CPU all day long.

70W is a little on the low end, but as long as you don't run the CPU at 100% with complex video, you should be able to get away with it if you only have 1 hard drive and a Celeron 566-800MHz processor. Personally I'd be hesitant to get a 70W power supply just because one of the major reasons for component failure is due to either crappy or insufficient power. Also having your computer crash because you couldn't give it enough juice isn't a nice thing either. If you want to decode DVD by using the CPU, and plug in an LCD screen powered off the computer, I think 70W will be cutting it *very* close.

Why do you want to be able to watch DVD in your car? Just curious.

In any case, you're going to have to spend at least $110USD for a decent ATX PS that you can use from your car. I'm working on the design specs for a sweet 300W ATX 12V power supply that you can build yourself, and apparently there is news that we can get the keypower PSU for $110USD, and there was a group buy already done for $155USD per (I think... whoops!).

As for Jeff's power supply, I'd imagine that it'd be perfectly fine for what you need. I wouldn't expect to get a warranty or anything with it, but I'm sure it would work fine However as Jeff said, at the cost of $80 for parts, and if we can get the keypower 250W for $110, I don't think that he would want to sell you one of his for $100, thus it'd probably be better to go with keypower.

Anyways, good luck.

> Why do you want to be able to watch DVD in your car? Just curious.

When making long trips it's a good way to keep other passagers quiet ! And it's very nice to watch a DVD during a traffic jam you know 8)

I think I go for the keypower psu, but do they also ship international? Does anyone got experience with this?

yeah just search here for keypower and you'll find a lot of posts about it.

as for the DVD, I would suggest that using it while you are behind the wheel, no matter when, is a BAD idea. Even in a jam

However it does keep the kids quiet

If you're not in a huge rush, wait up a week to figure out which is the best commercial power supply, and what can be built yourself. You can always just go inverter, they're not expensive and then you can replace the PSU with a DC-DC when you have time later.

And BTW, the Inverters that I was looking at have low voltage alarm at 10.5V and cutoff at 10.0V, which isn't too bad. If you do have problems while starting the car, a few caps will fix ya right up.