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LM317 Output

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Suraj143

Active Member
I need high current from LM317.I also used a pass transistor. I'm getting 1.25V across XY pins.

I have two Problems.

1)I want to shut down the regulator from a digital In. Atleast (1.25V) from the output after shut down.Now its showing 1.95V on X point.

2)The transistor Vout is far than the regulator voltage out :(

How to reduce those?
 

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spec

Well-Known Member
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Hi S143,

2N3055 leakage current is probably causing the problem. Fit a 100 Ohm resistor between the emitter of the 2N3055 and 0V.

It is also possible that the LM317 is oscillating- on your schematic there are no decoupling capacitors, as required by the LM317 data sheet, .

By the way, the circuit you are using does not provide good voltage stabilization. There is an alternative arrangement which will supply high current and, at the same time, give the normal LM317 tight voltage regulation. If you are interested in this approach, just say and I will explain.:)

spec
 
Last edited:

ericgibbs

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hi suraj,
The LM317 is designed to work down to a typical 1.2V, this is because of the LM317 internal 1.2V ref.

What is the maximum output current that you require.?

Eric
A003.gif
 

ericgibbs

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Hi thanks for the view.i need 3A current.

And for digital turn off thing.1.25V will be fine on output.
hi,
I see that R2 resistor is a fixed value, does this mean you want a regulated voltage output of ???? volts upto 3Amp
and when the Off transistor T2 is switched On, the output voltage falls to 1.25V upto 3Amps.??

E
 

Suraj143

Active Member
Hi.actually i am selecting couple of R2 resister values using ULN2003.

what i need is to turn off the lm317 with another digital IN.the diagram shows the shutdown transistor.when it is shut down no need any current.

I know the lm317 cannot go below 1.2V.it is ok when shutdown condition 1.2V still there.no any circuits will operate from 1.2V.

I think i need another resistor to make 1.2V acts like shut down.i am planing to replace both LM 317 & tip3055 and insert a single LM350.

Whats your idea
 

MikeMl

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Suraj143

Active Member
Hi mike thanks for that.

spec
I added a 100R resistor and it worked fne.now i need a suitable load resistor for those output voltages.

2V,3.3V,5V,9V,12V

Does an 100R/2W sufficient?
 

spec

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spec
I added a 100R resistor and it worked fne.now i need a suitable load resistor for those output voltages.

2V,3.3V,5V,9V,12V

Does an 100R/2W sufficient?
Hi S143,

Yes, a 100 Ohms 2W resistor will be more or less OK.

At the worst case, with the output voltage set to 12V, the dissipation would be 12V * 12V)/100 Ohms = 1.44W. But I would go for at least a 3W resistor.

Remember too that the resistor must be in reasonably cool free air (below 70 Deg C), so mount the resistor body at least 7mm away from any surface (printed circuit board for example).

spec
 

Suraj143

Active Member
Hi spec thanks for the info.I will do like it.

One last problem here.
When I use a 24V transformer it will produce DC 34V.when I apply this voltage to the input of the regulator it generates heat and voltages are messed up.It wont maintain 1.25V across R1.so i changed my transformer back to a 12V one.Is there any way to get 24V from this regulator using a transformer?what value transformer it supports maximum?
 

ericgibbs

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hi suraj,
My solution would be as post #7, from Mike.
A power PNP bypass transistor, NOTE: it will require a heatsink for a nominal 24Vdc input to say 5Vout at 3 Amps.

E
EDIT:
Added a circuit.
This is for 24Vin , 5Vout at 3Amps,, note the ~50Watts in the power transistor.
 

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AnalogKid

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With a 24 V transformer, the regulator will need a large heatsink. Without it, it will go into thermal shutdown. What do you mean by the output being messed up?

ak
 

Suraj143

Active Member
Hi it means it wont maintain 1.25 V across R1.so the tuned voltages are now showing a big value.this happens when I use a 24V transformer.I measure the input of the regulator.it shows 34V which is too high I think :-(
 

ericgibbs

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hi,
Typically a 24Vrms transformer will charge, off load, a capacitor to (24V * 1.4) - 2 Diode voltage drops , say 2 * 0.7V

So that's ~32.2Vdc

E
 

spec

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Hi S143,

Hi spec thanks for the info.I will do like it.
One last problem here.
When I use a 24V transformer it will produce DC 34V.when I apply this voltage to the input of the regulator it generates heat and voltages are messed up.It wont maintain 1.25V across R1.so i changed my transformer back to a 12V one.Is there any way to get 24V from this regulator using a transformer?what value transformer it supports maximum?
The LM317 has over temperature protection built in that reduces the current if the LM317 case temperature gets too high. Typically LM317 temperature protection would not operate with an LM317 power dissipation under 1W, if the LM317 were not mounted on a heatsink. Strangely enough, an LM358 would be exactly the same. This means that, without a heat-sink, an LM317 or LM358 will only provide a relatively low current.

The power dissipation in the LM317 is the voltage between the LM317 input and output terminals (Vio) multiplied by the current being sourced by the LM317 (Iout) at its output terminal. So if the input voltage were 30V and the output voltage were 12V, Vio would be 30V-12V = 18V. So, in this instance the LM317 could only supply a maximum current of 1W/18V = 0.056 Amps.

To get a higher current you would need to mount the LM317 on a heatsink, the bigger the better, to keep the LM317 case temperature down. The heatsink does not necessarily need to be a bought heatsink. It can be the chassis of your equipment or even a sheet of aluminum.

Even if you use an external power transistor, the external power transistor would still need to be mounted on a substantial heatsink.

Just a general note. It is not normally practical to get much more than 1A from an LM317, even though the LM317 is rated at 1.5A. And the 1A would only be possible with a fixed output voltage and an optimum transformer, rectifiers and reservoir capacitor.

A far better approach, to meet your requirement, would be to build a power supply from transistors or a high specification power supply using an opamp and transistors.

spec
 
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spec

Well-Known Member
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Further to the above post, you need to take ripple voltage into account, and that is another factor that effectively limits the current that a three terminal regulator can produce.

spec
 

JonSea

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Just a,silly question. You do have big caps on the transformer output after the rectifiers, yes?

Keep in mind the power dissipation or a linear regular is (input voltage - output voltage) × load current, so with 35 volts in and 1.2 volts out, it doesn't take much load current to generate a lot of heat.
 

audioguru

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Why not use the old LM350 that has an output up to 3A or the old LM338 with an output up to 5A? They work the same as an LM317 but are in a TO-3 big metal case. TI still make them but they are not listed anymore on Digikey.
 

Suraj143

Active Member
Hi audioguru.
You are right.i also bought LM350T which is in a TO220 package.The problem is when shutoff the minimum voltage is 1.25V which is still too high for me.the above design drops minimum voltage to 0.6V which is satisfy a bit.
 

AnalogKid

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Is the 24 V transformer rated to deliver 3 A? If not, that might be why the circuit is pulling out of regulation.

ak
 
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