circuit, to power tablet, in car?

[settra]

hello guys. i want to make a "power supply" for my tablet, that i will use in my car. my tablets AC power supply, says output: 5v 2A.
what i want to do, is take the 12v unregulated from the battery, then use a voltage regulator of 5v , and connect that output, to the tablet??

what additional circuit, would i need to add before and after the regulator, to protect the whole thing, from the car's harsh environment (spikes etc)?

P.S ( I prefer linear regulator, than a switcher, because efficiency is not neccecery, and i don't want the switchers noise in the system... )

 

[3v0]

A running cars voltage is about 14.5V volts rather then 5V. You are going to need a lot of heat sinking!

Also car voltage is far from clean. Look to see if you can find a 2A voltage regulator designed for automotive use instead of trying to build one.

 

[cowboybob]

How about one of these: https://www.amazon.com/Lighter-Socket-Charger-Adapter-White/dp/B004OQFYOI

Don't know if they'll ship to Greece, but this type of 5VDC (USBtype) supply is very common and, as you can see, very cheap. I've used similar devices in my car and have had zero "noise" problems.

 

[settra]

nope... what i want must be permanent, and a car charger is too ugly plus, i dont want to use the usb, but the DC input

 

[Ian Rogers]

If you build one, I would suggest taking 3V0's advice about the noise problems... All will be okay until you start the engine and the noise will affect the screen refresh and audio elements...

You'll need to filter the alternator noise right out...

 

[settra]

well. what i have in mind by now is something like this:

i diode, in before the Vregs input, to protect from negative spikes.
a zener diode, from Vreg input to diode, to protect from 15V + Spikes,
a fuse ofc

you think i should add a low pass filter as well?? (RC, since voltage drop wont be a really big problem , since the vreg would need 6+ v to work )

any advice's on that??

 

[MrAl]

Hi,

One idea would be to use a linear, but before that a switcher. So you'd be using a pre-regulator which is a switcher and a post regulator which is a linear. So you'd get good efficiency plus clean output.

Using a linear alone means dropping about 10 volts, and at 2 amps that's 20 watts, so you'd need a big big heat sink and that's a lot of heat anyway you dont want to have to deal with.

Even with a switcher that is set for 10v output, you'd only drop 5 volts and at 2 amps that's only 10 watts. With a switcher that puts out 7 volts, that's 2 volts times 2 amps so it's only 4 watts of heat now.

However, the output of switchers can be improved quite a bit without using a linear regulator. The idea is to use a post LC filter that filters out more of the normal ripple that we usually see out of the switcher. A very small inductor even hand wound plus a decent low ESR capacitor and you should see a pretty clean output.

Filtering the supply is a must for automotive applications.

Also take note that the wall warts (wall adapters) that comes with these tablets for charging are usually not linear but the switcher type too. That means some ripple is acceptable anyway. You could perhaps look at the output of one of these to see how much ripple they are allowing at the higher current and lower current levels.

 

[settra]

cool. thanks allot man!

 

[MrAl]

Hi,

Sure, you're welcome. Keep us posted if you can about what you decide to do.
I was thinking of a battery operated version too, one for when the power goes out around here. We talked about it quite extensively in another thread on this forum. It was going to be a switcher. Then i realized i have another switcher i use for something else i could modify to use for this too. It's based on one of National's Simple Switchers (which is now part of TI's device lineup).

 

[settra]

well. actually i decided i will use a ready switching buck converter.
i do this, becouse, after all , i will have use arduino, to controll the power to the switcher (with a relay). the arudino will be running a sketc, that every 4 days, it will close the relay for 3 hours, to charge up the tablet, so efficiency, is a bit important after all... (since i dont want to get my secondary battery drained)