Lm 317

[plasma]

hi
I need to have an output of 6V at a maximum current of 1.8A from a 2s lipo source. I am thinking of using 2 x LM317 in parallel to regulate the voltage to 6V and using another lm317 to limit the current to 1.8A.
Is my design correct and efficent?

[civic3106]

hi
I need to have an output of 6V at a maximum current of 1.8A from a 2s lipo source. I am thinking of using 2 x LM317 in parallel to regulate the voltage to 6V and using another lm317 to limit the current to 1.8A.
Is my design correct and efficent?

Not quite sure whether can parallel the output of linear regualtors just like switching regulator. My feeling is no, but will be interesting if it it yes.

If you could lower down your requirement to 1.5A max, then you can try it with just one LM317K plus some discrete components for current limiting.

[Super-Hornet]

For LM78xx, the normal type is 1A max before thermal shutdown and for LM78xx, u can connect in parallel to double the current output. LM78xx is a linear voltage regulator.

For LM317, the voltage o/p varies according to resistor so not sure whether can connect in parallel but I believe it can. If it cannot due to internal o/p resistance affecting the resistor bridge, what u can do is connect a diode as a separator before u connect 2 o/p together.

Note I did saw a linear voltage regulator (in old motherboard) that it can supply up to 7A o/p. It is like those LM78xx or LM317 type of chip.

Super-Hornet

[plasma]

can i just use one lm317 and draw 1.7 A from it at a regulated voltage, since the max current it can provide is 1.5A?

[Super-Hornet]

Hi Plasma... I think it will go thermal shutdown first before u can draw 1.7A.

Two ways for your requirement. One way is to use Power Transistor to boost up the current. There is a why to change the design to supply higher than 1.5A by using Transistor. Check out the sample on the LM317 characteristic manual.

Another way is to use 2 or 3 pieces of LM7806 (Or 12 or whatever o/p voltage u prefer) and connect them in parallel.

Super-Hornet

[plasma]

hi
What do you specifically mean by thermal shutdown?
does it means that it will only supply a limiting current of 1.5A when it reached a certain temperature?

I read the lm317 char and it says that it is capable of supply an excess of 1.5A. Does this imply that a current of more than 1.5A can be squeezed out from lm317?

[Super-Hornet]

Hi plasma

What I saying about Thermal Shutdown is that u need good heatsink to dissipate the heat. What I saying is just a guessing whereby I suspect the LM317 will get very hot and then go thermal shutdown even when u able to achieve 1.7A.

U can disagree with me but I think that if u want high current like more than 1.5A, u should not use LM317 at all because it can only provide u up to 1.5A.

If u read the Characteristic manual, it did mention that:
----------------------------------------------------------
For applications requiring greater output current, see LM150
series (3A) and LM138 series (5A) data sheets. For the
negative complement, see LM137 series data sheet.
Features
--Guaranteed 1% output voltage tolerance (LM317A)
--Guaranteed max. 0.01%/V line regulation (LM317A)
--Guaranteed max. 0.3% load regulation (LM117)
--Guaranteed 1.5A output current
--Adjustable output down to 1.2V
--Current limit constant with temperature
--P+ Product Enhancement tested
--80 dB ripple rejection
--Output is short-circuit protected
-----------------------------------------------------

Also, there is a sample circuit of how to use Power Transistor to Boost the o/p current.

If u want simple circuit that can provide u more current, u might want to check/look for other alternative.

Super-Hornet

[plasma]

hi
i did an experiement today.
I connected 8.4V as the input, and tuned the pot so i get 6 V at the output at no load.
When i connect 4 ohms at the output, my current is around 1.5A, voltage is around 6V
When i connect 3 ohm at the output, the current shot up to around 1.6A, voltage drops to 5V.

i guessed tat the power output at the regulator is constant. if i were to shortcircuit the output, i would get a very high current with near zero voltage.
however i did not observe any thermal shutdown though. I quickly turned off the power supply when i smell something from my regulator.

[Super-Hornet]

Hi Plasma... your voltage drop... I not sure whether it is like that or it is because your external or load resistance interfering the bridge resistor that cause it. For LM78xx, it should not have such voltage drop.

I did try on LM78xx whereby it goes too hot that it goes thermal shutdown. After it cool down, the LM78xx is working again.

Super-Hornet

[plasma]

will putting a heatsink helps to slowdown thermal shutdown?
right now, i need a regulator that supply 6V at less than 1.6A to my loads.

[Super-Hornet]

Yes... HeatSink will help. For me, any linear regulator I always put a heatsink.

Super-Hornet

[civic3106]

hi
i did an experiement today.
I connected 8.4V as the input, and tuned the pot so i get 6 V at the output at no load.
When i connect 4 ohms at the output, my current is around 1.5A, voltage is around 6V
When i connect 3 ohm at the output, the current shot up to around 1.6A, voltage drops to 5V.

i guessed tat the power output at the regulator is constant. if i were to shortcircuit the output, i would get a very high current with near zero voltage.
however i did not observe any thermal shutdown though. I quickly turned off the power supply when i smell something from my regulator.

Check your input source, what is the amp rating.

One more issue at 1.5A your LM317 will be dissipating 3W plus of heat, you do need heatsink.

If you really need above 1A output better choose other regulator. Don't work around the spec limit.