voltage regulator question adjustable voltage needed

On a sunny day (Mon, 22 Dec 2008 11:28:28 -0800 (PST)) it happened panfilero wrote in :

That is basically a 78XX, 5V, 12V ,and 15V output. The output of teh 5V can be boosted to 12V by lifting ground reference with resistors. If you only need 12V, use a 7812!!!!!!!!!!!

If you need a programmable voltage or current, why not use a LM317? Easier to get. The LM317 goes all the way down to about 1.22 V. Diagram is the same.

You looked up the data sheet so are probably smart enough to learn basic electronics without too much difficulty. You need a much better grasp of circuit theory and instruction on asking intelligent questions before you can expect the professional users of groups other than sci.electronics.basics to be friendly.

You can start learning by answering the questions (in the group sci.electronics.basics *ONLY*):

What is your load and what is its minimum and maximum current requirements?

What is your supply voltage and what is it from?

N.B LM340-12 is absolutely the *wrong* choice unless you only want to adjust from 12V to lets say 15V.

That's my good deed for the day done, Merry Christmas all.

ero

th resistors.

Thanks for the reply.... I'm confused now, I thought the LM340-xx was a fixed output voltage regulator... and that if I order LM340-12 I will be getting a 12V voltage regulator (and if I order LM340-15 it would be the 15v regulator... and the LM340-05...etc etc). So you are saying that if I get a LM340-05 (5V regulator) and lift the ground (I'm not sure what lift the ground means) that I can turn it into a

I do only need a 12V regulator so I will look into the 7812... looks like I was looking at the wrong stuff... much thanks, appreciate your help!

ero

th resistors.

ok.... you recommended the 7812 over the LM340-12.... but I can't tell the difference (besides the LM340 series sounding a little more robust) check out these two descriptions for each series:

The LM78XX series of three terminal positive regulators are available in the TO-220 package and with several fixed output voltages, making them useful in a wide range of applications. Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible. If adequate heat sinking is provided, they can deliver over 1A output current. Although designed primarily as fixed voltage regulators, these devices can be used with external components to obtain adjustable voltages and currents.

The LM140/LM340A/LM340/LM78XXC monolithic 3-terminal positive voltage regulators employ internal current-limiting, thermal shutdown and safe- area compensation, making them essentially indestructible. If adequate heat sinking is provided, they can deliver over 1.0A output current. They are intended as fixed voltage regulators in a wide range of applications including local (on-card) regulation for elimination of noise and distribution problems associated with single-point regulation. In addition to use as fixed voltage regulators, these devices can be used with external components to obtain adjustable output voltages and currents.

they kinda sound basically the same to me... is the 7812 really a better more obvious choice for a fixed voltage regulator?

On a sunny day (Mon, 22 Dec 2008 13:30:55 -0800 (PST)) it happened panfilero wrote in :

Yes, sometimes if a bit more voltage is required then say 5V, they add a diode in series with the ground connection, then, for a Si diode, you get 5.7V. I am not sure if it will work very well to do make a 7805 output 12V.

You must have see the little formula on page one of the pdf datasheet, where its says what Vout is in relation to those 2 resistors, and its effect on load regulation. The load regulation clearly gets worse.

You can put a 6.8V zener in the ground circuit and get around 11.8V out. The current out of the GND pin is fairly constant.

This sometimes comes in handy if you need +/- supplies, can float the "ground". You do have to make sure not to draw too much current from the "minus" supply to ground (but to the + supply is okay).

But in general, if you need a 12V regulator, just buy one.

Use the example in the datasheet.

The 'guts' of that thing works by passing current through from the supply until the output pin rises to 12 V above the ground pin.

So by connecting the ground pin to the center of the voltage divider, you trick it into passing current from the input to the output pin until just that part of the pot has 12V across it. If the pot is set to the mid-point, that means 12V across half the pot's resistance, or 24V across the total pot from end to end. So the output pin is 24V above the return line and just

Does that help?

daestrom

resistors.

The 7812 costs less, and may be more widely available.

Ed

Yes, a huge advantage. The output pin can put out an amp of current, but used that way, it only produces output voltages greater than the nominal output. So you can turn a 12V regulator into a 13V or 15V regulator if you want.

What you seem to want is a lower voltage regulator. The best way to get that is to use a lower voltage regulator.

Starting from +12V and dividing down to a lower level won't give you a regulated voltage. It'll give you the equivalent of a regulated voltage with a large source impedance.

Seems to me his best route is to get himself an LM317

resistors.

the LM340T-12 is a 7812. The price will be the same because it is exactly the same part.

Hmmmm.... well since I don't need 1A of current... I can't see the huge advantage... and when you say it will give me the equivalent of a regulated voltage with a large source impedance.... is that a bad thing? The equivalent of a regualted votlage sounds good... and so does a large source impedance... I'm sorry, I'm not seeing the drawback here... I'm probally just not understanding what you are trying to say there....

thanks!

The large source impedance means that the output voltage will drop (a lot) as the load current increases. So, even though the source voltage feeding the divider is well regulated, the output voltage from the divider chain won't be.

This may, or may not, be a problem. If your load is extremely light, then its effect on the voltage divider will be small.

If your load is extremely constant, a simple series resistor (to drop the excess voltage) may be all that is needed - the combination of load and series resistor will form the voltage divider.

However, using a design that produces a regulated output voltage with a low source impedance has the advantage that it can be used with a wide variety of loads, without the output voltage being affected.

-- Sue

Just about the only sane way to use this particular voltage regulator is as a fixed-voltage regulator at the voltage it's designed for (12 V), or possibly slightly higher. If you want to adjust the voltage, there are much better options - e.g. the LM317.

Your approach will work fine if the voltage out of the pot has essentially zero load on it - e.g. if it's connected to one input of a DMM. But if it's supplying any significant amount of current to anything, then the "adjusted" voltage will be pulled down by the extra load that's effectively in parallel with the lower half of the pot. Worse, if the load varies, the voltage will vary too. You can reduce the amount of variation by using a lower-resistance pot, but then it wastes more power in the pot itself.

Plus you need to watch the maximum rating of the pot. A 1/4 W pot means it can dissipate 1/4 W over the entire resistance element. But if you have the shaft set to the 90% position, all the load current is passing through only 10% of the pot element, and it can handle only 25 mW. In practice, this means there's a maximum current it can handle no matter the shaft position. For example, a 1/4 W pot across 12 V can handle about 144 mA - and that's the sum of the load current *plus* the current dumped through the pot itself.

In contrast, if you use a LM317, you can also use a pot to set the output voltage, but the pot wiper is connected to the regulator's adjustment terminal, not the load. The pot handles a few mA of current in the voltage divider, so there's no heating to worry about. You can easily adjust the output voltage from about 1.25 V up to 30 V or so, and the voltage will remain constant under a wide range of load current. And that's what you want a regulated power supply to do!

Lookup up the LM317. It requires at most a few more parts around it than a fixed regulator, but it's much more flexible.

Dave

resistors.

Wrong. Same part, *different* price. Check Digikey, Mouser, Newark. I did the work for you. For a TO-220 package qty1:

• = " Cross Reference Match For: LM340T12" and the cross-reference pn given is 511-L7812CV

The part numbers found by the search and used in the table above:

In each case, the 7812 costs less.

Ed