LM317 Variable power supply

[audioguru2]

I clearly explained about the operation of an LM317 regulator.

Your 36.8V is way too high for a 5V fan, 9V meter and an LM317 regulator.
The fan uses 110mA at 5V when it is running, but might need 1A to start. It probably won't start running when fed through a 300 ohm resistor. Try it.

The voltage is so high that the fan might not start when fed from an LM317 regulator.

 

[Um...Me123]

I guess you misunderstood me.  The supl yto the fan and meter do not go through the LM317.  I know 38 is way to high thats why I was asking about the resistors.  I thought the 38 might drop down when a load was put on the regulated portion.  Anyway...I think I figure it out myself.
Thanks anaways

 

[audioguru2]

As far as I know the maximum current that LM317 can handle is 1.5A. Now I was wondering is it possible to have two LM317s (with their relevant circuitry) in parallel to increase the supplied current?

Two regulators in parallel won't be exactly the same. The one with the slightly highest output voltage will carry as much current as it can, and the one with the slightly lower voltage will carry the remainder (if there is any) of the current. Therefore one will be hot (and not last long) and the other will be cool. When the hot one fails then the other one will be overloaded and will also fail.
They have current-limiting and thermal shutdown for protection. But the factory doesn't recommend overusing the protection due to thermal shock causing fractures.
An LM350 is rated for 3A and an LM338 is rated for 5A with restrictions on the amount of voltage across them.

 

[Staigen1]

Hey, you can use a PNP transistor to increase the current! I belive there is a description in the datasheet, or was it in the application notes for the LM117/217/317. Look!

//Staigen

 

[audioguru2]

Hey, you can use a PNP transistor to increase the current! I belive there is a description in the datasheet, or was it in the application notes for the LM117/217/317. Look!

//Staigen

Then you lose short-circuit protection, unless you add a 2nd transistor.

 

[Staigen1]

Can be done in an another way! Bye using a pair of diodes in series with the input of the LM317, also use emitter resistor for the PNP, the diodes should also be paralelled with a resistor!

//Staigen

 

[audioguru2]

They use 0.1 ohm equalizing resistors with them in the circuit you posted to cancel differences in the gain of the transistors and in the slightly different regulators' voltages. Then the excellent voltage regulation is ruined by the resistors and made good again by the gain of the opamp.

 

[Staigen1]

Also look at my reply's, they are simpler, uses only one IC. For 10 Amps use several transistors, and reduce the current thru LM317 to somewhere around 100 mA before the transistors start to conduct. Restrict the current thru the transistors to 2-3 Amps or so.

//Staigen

 

[Staigen1]

Hi

Which replies??

Replies #124 and #126 in this topic!

//Staigen

 

[Staigen1]

Not exactly, but it is similar, well, maybe not, after a closer look, mine is much simpler, and its safe, and have a good regulation. Audioguru know what i mean, only by the description i gave. I will upload a schematic, as soon i have done one! I will do that at once, but it will take a few minutes or maybee a little moore. See ya!

//Staigen

 

[Staigen1]

Here is the sematic.

//Staigen

View attachment 38406

 

[Staigen1]

Sorry, forgot values! ???

R1= 6.8 ohms
R2=R3= 0.47 to 1 ohms(depends on current throu transistors)
R4= 120 ohms(acording to datasheet, some LM317 can use 240 ohms, acording to Audioguru)
C1=C2= 0.22 uF
C3=C4= 10 uF
Transistors can be TIP36 or similar

//Staigen

 

[audioguru2]

audioguru, how do you compare the quality of regulation in the circuit I posted with the 0-30V stabilized power supply project?

http://www.electronics-lab.com/projects/power/003/index.html

Hi Zeppelin,
Our project uses an unspec'd zener diode voltage reference operating at a current that is too low for good regulation. If a low-current zener diode is used and the resistor that feeds current to it is changed to give it its required current, and the other modifications to the project that I recommend are done, and its wiring is correct, then it is an excellent regulator.

An LM317 has a 4:1 range of regulation. An LM317A has a much better minimum amount of regulation, about the same as our modified project, if it is wired correctly and has a capacitor connected to its ADJ terminal to ground.

The circuit you posted depends on the minimum amount of regulation its LM117 has. It has the added gain of the external opamp which would make its continuous regulation better, but its regulation with quickly changing input voltage or output current would probably have wild swings. 

 

[Staigen1]

Oops! The diodes!

D1=D2= 1N5401

Hope i dont forgot anything moore ;D


//Staigen

 

[audioguru2]

It looks like when the load draws too much current or is shorted, then the extra current through the diodes causes the LM317 to current-limit or shutdown. Good. ;D

 

[Staigen1]

The transistors need a good heatsink! But it is simple and easy, and it work! ;D


//Staigen

 

[Staigen1]

Hi Zeppelin

Yes, i can, but i leave it up to you to figure out and tell us how it work!
Hint, the forward drop over each 1N5401 is about 0.7V at full output.

//Staigen

 

[autir]

Hi Autir,
I also built my LM317 variable supply on stripboard. I had the load connected to the heatsink tab, with a terminal screwed on with the same bolt that attaches it to the heatsink, then connected the current-setting resistor to the output pin without any load current in it. Then the output "star" connection is inside the IC.

I connected the supply's ground, the load's ground and the pot's ground all together in one spot, called a ground "star"connection.

It works perfectly. ;D

Hi
is the circuit of this guy
http://casemods.pointofnoreturn.org/vregtut/tutorial-full.html
a good example of star topology?
any ideas on how to connect the LM317K (TO3 package), mounted on a huge heatsink?

also

Hi Autir,
If you have it wired so that load current travels through a wire or pcb trace to which R2  (the pot) is connected to ground and the resistance of the wire or trace develops a voltage across it, then the regulation is much poorer.
National Semi explanes a similar problem that is caused when load current travels in a wire or pcb trace between the regulator's output and the current setting resistor R1:

I have comprehended the problem with the resistance between the 317's output pin and R1. But not the problem with the resistance between ground and R2 (the pot).

Care to explain both a little bit more?

Thanks.

 

[JuniorHP]

Hi,

I am building a power pack for a laptop... reason being we make recordings on the laptop of sermons, but the power in the building where it is done is very "Noisy"? resulting in terrible recordings.

What we did was using a 6V and 12V car battery in series, connecting the original power to the batteries and getting perfect(clean DC) power into the laptop, which results in beautiful recordings.  This is a temporary solution.  I am planning to use 3x 6V batteries in series, a few diodes and the input and output connectors. I have attached a picture of a drawing i planned, this may be incorrect in which case please let me know.  I am open for any suggestions.

View attachment 39219

 

[audioguru2]

Your batteries are shown wired in parallel, not in series.
There is not a current-limiting resistor from the power input to the batteries. Therefore they will charge with an extremely high current.
The diodes to the output are backwards and are not needed anyway.