Whats more efficant?

[banditpaw1]

What more efficant? A 7805 linear regulator or a 5 volt dc-dc car adapter? Help me out
Also 7808= 7.5 volt regulator right?

[snowpenguin]

7805- 5v regulator

7808- 8v regulator

You have to open up your car adapter to see what kind of regulator it uses, we cannot help you if you do not give us details. For all we know, the car thing could use a 7805 as well.

The most efficent option to get 5v would be a TI card, as it is a switching regulator and that type generates no heat and draws significantly less current away from the power.

[banditpaw1]

7805- 5v regulator

7808- 8v regulator

You have to open up your car adapter to see what kind of regulator it uses, we cannot help you if you do not give us details. For all we know, the car thing could use a 7805 as well.

The most efficent option to get 5v would be a TI card, as it is a switching regulator and that type generates no heat and draws significantly less current away from the power.

Its not a 7805 its a chip with caps and resistors? Can a 7808 be haked to ooutput 7-7.5v?

[Steamdnt]

Running 8V through something that is made to take 7.5V will not harm anything, And no, you cannot hack a 7808 in any way.

[snowpenguin]

DNT, technically, yes you can, with some resistors as you can make a 7808 with a 7805 and some resistors.

Ih wait, can you go down on the regulated voltage though? Hmmmmm...

[robm]

Its not a 7805 its a chip with caps and resistors? Can a 7808 be haked to ooutput 7-7.5v?

I would assume you're talking about a PCB with capacitors and resistors on the board, along with ... some sort of switching regulator? Or maybe it's linear, who knows. Take a picture so we know what exactly we're dealing with. Otherwise, it's a hootchermabob that hooks to the watchamajigger.

In any case, a linear regulator is going to be the least efficient, but easiest to use, of the regulators out there. They generate a lot of heat, and have problems with oscillation when they are off of a load. If you don't want to have to deal with reading a datasheet to learn how to set up a switching regulator, they are your best bet.

No, a 7808 cannot be hacked to output 7v.

The real question is... what are you trying to do? Do you need 5v, or do you need 7v? I'm confused.

Running 8V through something that is made to take 7.5V will not harm anything, And no, you cannot hack a 7808 in any way.

Yeah, what he said.

[banditpaw1]

DNT, technically, yes you can, with some resistors as you can make a 7808 with a 7805 and some resistors.

Ih wait, can you go down on the regulated voltage though? Hmmmmm...

well my sega needs one voltage my screen needs another. i new a 7805 could do 3.3 but not sure how to make it go up. what do i need to make a 7805 a 7808?

[robm]

A 7808.

[banditpaw1]

A 7808.

uhhhh ya i dont have one sooooo ya...

[robm]

uhhhh ya i dont have one sooooo ya...

You cannot make the 7805 output higher than its regulated voltage. If it could do this, they would not call it a 7805, they would call it an adjustable linear regulator, which it is not. A 7808 can't be anymore than 5 bucks down at your local Radioshack, if even that much. (Haven't bought a regulator new for quite some time, can't remember how much they are anymore.)

If you want an adjustable linear regulator, get the LM317. Here's the datasheet:
http://www.national.com/ds/LM/LM117.pdf" onclick="window.open(this.href);return false;

Here's where you can buy one:
http://search.digikey.com/scripts/DkSea ... LM317AT-ND" onclick="window.open(this.href);return false;

That same site, you can purchase whatever SET output voltage regulators you could want.

[superdeformed]

Running 8V through something that is made to take 7.5V will not harm anything

Maybe not for whatever he's working with, but in general this is not a good assumption to make. There are plenty of parts out there with fairly tight supply voltage tolerances, and there's always the chance that whoever designed the circuit is already running it near the upper limit.

You cannot make the 7805 output higher than its regulated voltage. If it could do this, they would not call it a 7805, they would call it an adjustable linear regulator, which it is not.

Actually, you can make the output higher. Linear regulators work by trying to develop a specific voltage between the output and a reference pin. In the case of the 7805, it wants to see 5V, so if you simply ground the reference pin you get a 5V regulator with no additional parts. If you connect the reference pin to something higher than ground, such as the middle of a resistor divider, you can increase the output voltage of the regulator. Page 31 of ST Micro's 7805 datasheet shows you two ways of doing this.

The first one (Figure 19) just uses a resistor divider, but in doing this you also have to consider the reference pin current, which is significantly higher than the reference current of an LM317 (4 to 8 mA versus 100uA). Their solution is to simply use small enough resistors so that the current going through the voltage divider is high enough that the current of the reference pin doesn't have much of an effect on the voltage divider. The problem with this is that you're wasting a good amount of current through the voltage divider, which is bad if you're trying to run off of batteries. Their second solution (Figure 20) uses an op-amp as a buffer, which eliminates the need to worry about the reference current effecting the output voltage, but will still draw a good amount of current and uses even more parts.

If you don't care about the current loss and have a 7805 on hand, this will work just fine, otherwise an LM317 is probably a better choice as far as linear regulators go. Also, a 7805 cannot output 3.3V (at least, not with a single supply), and in most cases, these regulators cannot output a voltage lower than their ideal value.

As far as the original question goes, as a few other people have already said you really need to say what your input voltage is and what you actually want for an output voltage.

[banditpaw1]

Also, a 7805 cannot output 3.3V (at least, not with a single supply), and in most cases, these regulators cannot output a voltage lower than their ideal value.

With a 10k on grond my 7805 output 3.35v