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Why do DC to DC converters cost so much?

post #1 of 11
6/8/05 at 10:38pm
Thread Starter 
I'm trying to find an alternative to spending an extra $100 dollars on a battery. I was thinking of using a small lead acid battery and some sort of converter to get the neccesary voltage. It seems the converters are very expensive so i'll probably just get an extra battery if i need it. I need 19.5 volts and 90W. The cheapest i've seen for 82w (close enough) is $50. Any one seen any different or know why they're so expensive?
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post #2 of 11
6/9/05 at 12:26am
They price them at what the market will bear. Less than $40 bucks will get you a dc to ac inveter, which can provide power to your laptop's power brick fine. For a 90 watt laptop brick, get a 120 to 200 watt inverter. A smaller one might not handle it and a larger one will eat your external battery too quick. Bear in mind a lead acid battery doesn't have nearly the energy density of a Lithium Ion or Polymer battery such as the one in your laptop. A 10 Amp Hour 12 volt battery will only last about an hour or so.
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post #3 of 11
6/9/05 at 12:34am
Hopefully, someone more knowledgeable than me can also answer this--but trying to remember my basic physics/electrical engineering . . . transformers only work with alternating current. So, I am thinking that maybe the DC first needs to be converted to AC, then run thru an AC transformer, then converted back to DC.

Sorry if I am wrong.
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post #4 of 11
6/9/05 at 11:04pm
You Got it !!!!!
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post #5 of 11
6/13/05 at 12:35am
Thread Starter 
you mean anthean's got it? Can anyone verify that? I believe it but I've never heard it before. if that were true then it would make alot of sense to just get a dc to ac converter. I was worried that you would add unessicary steps and waste power.

"A 10 Amp Hour 12 volt battery will only last about an hour or so"-olyteddy

how do you get that figure? A 10 Amp Hour 12 volt battery would be a 120Whr battery right? This would give much more then 1hr on most laptops.
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post #6 of 11
6/13/05 at 2:13am
Quote:
Originally Posted by macman1
"A 10 Amp Hour 12 volt battery will only last about an hour or so"-olyteddy

how do you get that figure? A 10 Amp Hour 12 volt battery would be a 120Whr battery right? This would give much more then 1hr on most laptops.
Figuring about 80% conversion efficiency, and the fact that a lead acid battery's voltage drops during discharge (to the point that the inverter will shut down before the battery is drained) I'd say an hour or so. The "or so" might be as much as another half hour. NiMh would probably be a better battery choice (voltage remains constant during discharge).
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post #7 of 11
6/13/05 at 3:32am
i am a junior in college for electrical engineering and for a long time you could only use transformers on ac but they have developed a transformer that will do dc without converting to ac. The big thing though is that it is very expensive. I actually read somewhere that one plant in california actually has dc going to it from the power company.
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post #8 of 11
6/13/05 at 9:27am
Quote:
Originally Posted by oachalon
i am a junior in college for electrical engineering and for a long time you could only use transformers on ac but they have developed a transformer that will do dc without converting to ac. The big thing though is that it is very expensive. I actually read somewhere that one plant in california actually has dc going to it from the power company.
Good luck as an engineer. Thomas Edison of Edison Electric Works was the early adopter of Direct Current distribution. Broadway (in NYC) was all wired for DC. That's why early light dimmers were HUGE Rheostats. Very inefficient. Good old Thomas Alva believed that George Westinghouse (along with that damned Russkie, Nichola Tesla) were evil and murderous for believing in Alternating current. As to the modern distribution by DC, you need look no further than Central California, and the Northwest Intertie, operating at I believe 500,000 Volts DC. Directly adjacent to it is a 750,000 Volt AC Line. Go figger.
And yes, a tranformer can be used on DC. How do you think the Ignition Coil in your Gasoline Automobile works? (Pulses of DC).
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post #9 of 11
6/13/05 at 11:51am
Well, if carbon nanotubes become commonplace, they are nice for carrying DC current
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post #10 of 11
6/13/05 at 1:33pm
Quote:
Originally Posted by olyteddy
And yes, a tranformer can be used on DC. How do you think the Ignition Coil in your Gasoline Automobile works? (Pulses of DC).
I don't know much about electricity but I *think to remember* that transformers work by induction from one coil to another. And a fluctuating current (AC) on one coil will induct another current in the other one.

If what I write above is 75% true, then pulses of DC will serve a similar purpose of a "fluctuating" or changing current. So I guess it's a clever way to do it with DC.

Disclaimer: I'm talking out of my ass
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post #11 of 11
6/13/05 at 1:49pm
Quote:
Originally Posted by oachalon
i am a junior in college for electrical engineering and for a long time you could only use transformers on ac but they have developed a transformer that will do dc without converting to ac. The big thing though is that it is very expensive. I actually read somewhere that one plant in california actually has dc going to it from the power company.
Good luck as an engineer. Thomas Edison of Edison Electric Works was the early adopter of Direct Current distribution. Broadway (in NYC) was all wired for DC. That's why early light dimmers were HUGE Rheostats. Very inefficient. Good old Thomas Alva believed that George Westinghouse (along with that damned Russkie, Nichola Tesla) were evil and murderous for believing in Alternating current. As to the modern distribution by DC, you need look no further than Central California, and the Northwest Intertie, operating at I believe 500,000 Volts DC. Directly adjacent to it is a 750,000 Volt AC Line. Go figger.
And yes, a tranformer can be used on DC. How do you think the Ignition Coil in your Gasoline Automobile works? (Pulses of DC).
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