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# DC-DC Converter

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#### Brian

##### New Member
At the mp3car.com forums, we have found a \$35 55W DC-DC power supply to power a small motherboard. It's similar to a laptop's power supply in that the power brick provides a regulated 12VDC to the PS. The PS then creates the necessary 5VDC, but simply feeds the 12VDC straight through. That's where we need some help.

This will be used in cars, so the input voltage will be anything from 9-14.4VDC (with the car on and off). That means that if we hooked it directly to the alternator, and it was getting the full 14.4VDC, it would send that unregulated 14.4 voltage to the motherboard and hard drives. Not good.

So what we need is a way to provide 12VDC@5A to the PS *or* regulate the output to 12VDC@2A. It needs to have an input variance of 8-16VDC, and pretty durable. Anyone have a simple (the cheaper the better) circuit to do this?

Could this be modified to do what we need?
https://www.onsemi.com/pub/Collateral/AN954-D.PDF

Thanks.

You can use 7805: 3-Pin 5V Regulator IC in High Current Configuration. The circuit along with required formulae is given in its datasheet. If you don't find it let me know I'll send it to you.

Hi,
The 78xx series can maintain its specified output voltage provided that the input is atleast 2V - 3V greater than the specified output voltage. That means that if you are using 7805 the input should be anything between 8V to 32V and if you are using 7812 (12V regulator) the the input voltage should be between 15V to 32V. No the high current circuit is no given in the National.com's datasheet. Give me your email address so that I can mail the circuit diagram to you.

*removed*

Datasheet is on its way!

Hi

The circuit is on page number 17. Figure 18

The link doesn't work. Try hostmb.com or web1000.com. Both are free.

I appreciate you helping me out.

Series Regulator or Switched mode?

Have you overlooked something here ...
The supply to the power supply can vary below the required output, the 7805/7812 type devices are 'Series Regulators', they are only capable of regulation by dropping volts across themselves, they are incapable of 'step up'.

The 5v rail is easy with a series regulator (you imply something around 3A for this), but for the 12v rail ...
Some form of regulator capable of outputting more than its supply is required if the input is to drop below the 12v out.

Consider a switched mode power supply, either producing the 12v directly or just stepping up the input (adding 5v or so?) followed by a 2Amp series regulator?. These devices are not particularly cheap or easy to build (but not impossible).

Oh yes Mechie, I think it was quite late in the night when I read his question for the first time and went on replying based on what I understood that night (half asleep eh..).
Brian actually needs 12V/5A supply and not 5V/5A supply. I am sorry Brian but the 7812 won't work out in your case. Let me think of some other solution based on the circuit you gave. :roll:

Is his goal to have 12 volts at up to five amps to powere an item in his car? If that is the case a cars battery should never drop below 12 volts unless there is something wrong. Arent car batteries around 13.2 volts or so and up to 14 + when engine in running.
If you are lookin for some highe current voltage regulatrs i know off a source. I have gotten adjustable one with a 7.5 amp output form https://www.linear.com/prod/datasheet.html?datasheet=620

No its not true that car batter always has 12V at its terminals. When the car is not use for quite long time the battery voltage starts dropping and may reach to 7 to 8 Volts. So thats the reason why he needs a regulator which can also step-up the voltage in case the battery supply drops.

The regulator that you have shown is a series regulator which is similar to 7812 but with high current handling capacity. Here we need a shunt regulator.

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