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LM317 Variable power supply


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They are test standards of the parameters used by the chip manufacturer. Your application might very well be different. If your application falls within the generic specs, then sure. If you are pushing the edge, then you have to look closer. You can still achieve the manufacturer's specs if you use proper designing techniques.

For example, when a data sheet states that the device will work between -40C and +150C, they usually list a min max and typ to show their results. What they might not tell you is that 50% dropped out at +142C, and thus they can still compete with their competitors and advertise the +150C top end tolerance for temperature. But if you are manufacturing this device to be sent to the equator, you will want to provide more testing before you package it. This would be considered an extreme for any company. If you are using the device in your lab, you are pretty safe with the min tolerances given in the data sheet.

So, in answer to your question, yes, you can rely on the specs. If your application is extreme, you might have to add something to the circuit to continue to achieve this.

MP

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Hi Jandro,
I have never seen anyone use this application. Perhaps the simplest way to do it would be to have the current change the intensity of light which activates a light sensitive resistor where the light sensitive resistor takes the place of the pot. You could make your own custom package if needed.
I am sure there are many other ways to do it as well.

MP

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I bought the lm317t from radio shack. It shows on the package that 2 is the out or the middle pin. As the picture goes I think with the pins pointing down looking at the thing with the heat sink part laying on the table pin 3 is on the left and then pin 2 is center and pin one is right. Here is the schematic of my power supply, only I have a 25 v 450 ma transformer. I had it working for a minute and put a meter on the output and the fuse blew. I tried again after checking it and it blew as soon as i turned it on.

post-2882-14279141653821_thumb.gif

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Jeff, glad to see you got it working. You are right, there is really nothing on this device which indicates the pins are numbered from which end. You will want to memorize this since it is standard on all devices with this package type. This will surely come in handy on another project in the future. Nice looking project!

MP

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  • 2 months later...

Pinky...i am planing to build this project, a regulated PSU with the LM317, i was searching on the net and i found a very similar regualtor the same voltage butinstead of 1.5 A, it can manage 3 A.......but bell I have some questions......for the LM317 PSU:

1 - if Iwant a 30VDC at maximun output, with 1.5A (very demandin), which is the transformer I need, because I have a 12 - 12 V 75VA?, or I have to use a 30 V one?...

2 - I am very intereting on the idea that you told at the beginig of the forum, that we can use a 12+12 transformer and a circuit to conect the 24V when i need more than 12.....but I can't find a circuit.... or something symilar, to have an idea.....Have you got a circuit or soemthing like that, or a croquis...???

3 - another question I have is about the ripple.....how can I improve the filter section in the ciruit to make better the ripple rejection?.....is a good idea use a choke input filter?......please help me...

4 - may be the LM317 could be replaced by the lm350K.......

Thank you very much!!!....and sorry for my bad english.......

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Some notes on ripple:
If you have power supply ripple getting past the regulator, you should increase the "input" side cap, not the output side.

The output side of a reg should already be a very low impedance, and adding caps to it beyond that needed to suppress HF noise should not be necessary, and can actually drive a reg into oscillation under some conditions.

On the input side, you must simply ensure that at the max load current, the bottoms of the AC ripple are above the min dropout voltage of the regulator. More cap = less ripple, until you get above the dropout voltage.
On this regulator, you can also use a bypass cap on the adjustment ternminal and get excellent ripple rejection. Try this if you find you still have a problem after using a good sized bypass cap on the input pin.

In regards to the voltage needed, I think you need a minimum of 3 volts more "in" than what you are trying to get "out" of it. You can check the data sheet to verify. This refers to the voltage after rectification.

MP

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Hi Guys, some info: ;D
The LM317 is guaranteed to have an output current of 1.5A with 15V or less across it. In order to protect itself from second breakdown, it cuts back on its output current with more than 15V across it. With a high input voltage and low output voltage, it is guaranteed to supply only 150mA.
The 3A LM350 cuts back its output current with only 10V or more across it.

These regulators shut-down when they get too hot. The TO-220 "T" package has lousy thermal conduction to its heatsink, the TO-3 "K" package is much better.

The ripple rejection is typically (no guarantee) 65dB, which is one/two-thousandth. With a 10uF cap on the adjustment terminal to ground, the ripple rejection is typically 80dB, which is one/ten-thousandth. With the cap, the ripple rejection is guaranteed to be 66dB, which is one/two-thousandth. Pretty darn good.
If you don't use a "star" separate-ground-wires wiring technique, its excellent voltage regulation and ripple rejection are ruined, due to the voltage drop across a common ground wire at high output current.

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Hi Ante,
Why do we talk about making things cheap? We sound like baby chickens! We aren't mass producing these things. Let's make it good.

There is a guy who is selling "the ultimate" audio amplifier for only $3,300US. Inside, it has only 2 National Semi's power amp ICs. You know how cheap those ICs are. Just think about how much profit that guy is making! The magazine that reviewed it said that it sounded harsh at first, then mellowed after a 6 month break-in period. The first one that the magazine tested blew-up! Then its replacement blew-up too! The designer had to take over there a 3rd amp and a replacement power supply (it was separate).

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To dispell any myths, I use the LM317 a lot. I am refering to the TO-220 package. I have not had problems overcoming ripple or heat. Only in extreme designs would you even need a heat sink. If you are having problems with this regulator in a design, look at the data sheet and see if you have the wrong application. Next look at how you have designed the board. Most problems I have seen from others with this regulator were actually due to crappy design or layout. This regulator has a good history and is used in many consumer products. I would never purchase a more expensive package from fear of these things listed. It is a total waste of money. Why build things cheap if you are not a manufacturer? Because what you do not spend in parts can buy a lot of beer and pizza!

MP

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I use the LM317 a lot. I am refering to the TO-220 package. I have not had problems overcoming ripple or heat. Only in extreme designs would you even need a heat sink, ... beer and pizza!

MP



No heatsink?
Oh yeah?

Let's make a simple calculation without using extremes:
1) The LM317 is rated for 1.5A, but let's choose only 1A.
2) It is spec'd with a minimum of 3V across it, but let's choose a reasonable 5V.
3) Some parts of this planet reach 40 degrees C, but let's choose only 25 degrees C.
4) The TO-220 "T" package has a Thermal Resistance for Junction-to-Ambient of 50 degrees C per Watt.
Its Max Allowable Temp is only 125 degrees C, but this non-extreme sample's junction will try to be at 275 degrees C! It will shut-down very quickly.
Try it with only 1/2A, and it will also shut-down. It needs a heatsink.
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Hi Ante,
For years I have been using the low-dropout LM2931 for all my 9V alkaline and 7.2V Ni-Cad battery low current projects. It sqeezes all the life out of the battery. The 7805 and LM317 get into trouble when the battery is stll new and their idle current is 10 times or more what my circuits draw. The LM2931 has a very low idle current and has all the protection (and more) of the others. I use the small TO-92 size.

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