LM317 application problem

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mhamzah3

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lm317 application

I am trying to build a step down voltage regulator using the LM317T.

What I have designed, is using 2 LM317T chips. The input voltage is a 70-77V battery.
The first chip will step down the voltage to 35V and the second chip will step it down to 21V.

When I tested the circuit, I could not increase the load to 500 mA. When I increase the the load to draw more than 200 mA, the Voltage drops down. The datasheet says that the maximum output current is 1.5 A but I am unable to achieve the current draw

Attached is the schematic. Please help
 

lm317 zener

The data sheet also says that the maximum voltage is around 40Vdc and you expose these regulators to 70V and more ..
Try to replace the first stage with a high (>100Vdc) voltage Darlington BJT (eg. TIP102) with a 36V Zener diode connected between the base and 0V and a resistor to bias the base .. see picture below ..
More details here:
**broken link removed**
This should nicely drop the voltage from 70 to 35, and then you can use the 317 voltage regulator to regulate the output voltage down to 21Vdc ..
Of course, both the Darlington and the 317T voltage regulator have to be mounted on a heatsink ..

Rgds,
IanP
 
lm317t application

Thanks for the quick reply, I do not experience with the darlington BJT, what values of R should I use?

Added after 16 minutes:

Okay I did look up on the TIP1102, and it is totally different from the schematic that you gave with a diode and 2 extra resistors on the TIP102 schematics.

If I wanted to drop down the Voltage from 70 to 35V, where do I put the 36V Zener diode and the what is the value of the Resistor?

I really appreciate if you could help me out.
 

lm317 application note

Don't worry about R1, R2 and the Diode ..
They will not interfere with what you are trying to do ..

If you look at the Darlington configuration you can see that it consists of two cascaded transistors - identcal to original the circuit from my previos post ..
Connect Zener diode (rated for 2-5W) between the Base and 0V and as R you can use 1kΩ/2W ..

Also, you can consider a "small" dummy load, say 10kΩ, connected between the output of this stage (Emitter) and 0V ..

Rgds,
IanP
 

    M

    mhamzah3

    Points: 2
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lm317 applications

Is there a zener diode which can draw 500 mA?
 

lm317 heatsink


The exact solution of your problem is this circuit, You can use LM317 even at 120V input and have a regulated output. No need to worry about short circ uit etc.

ENJOY!
 

heatsink for lm317

The BZY-series of Zener diodes (20W-rating) can operate with currents of 500mA @33[Z]V, more with lower Zener voltages, or less with higher Zener voltages ..
Here are details:
http://www.littelfuse.co.kr/data/en/Data_Sheets/BZY93.pdf

My personal choice, however, is a smaller [in size] Zenner diode with an external power BJT mounted on a heatsink ..

Rgds,
IanP
 

    M

    mhamzah3

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lm317t applications

IanP

I dont know if the BZY Zener diode would be available anywhere, do you have an alternative to the Zener Diode other than the bzy?

what do you mean when u say a smaller zener diode with an external power bjt?

is that an aditional darlington bjt?
 

lm 317 applications

I was wondering if I could use a TIP120 instead of a TIP102.

Will the TIP120 burn on when I supply a 70V and loading 500 mA to the first stage of the circuit?
 

lm317 problem

The power transistor has to be rated for ≥100V and TIP120 (60V) does not fit into this category ..
Try the following:
TIP122, TIP127, BD649, BDX53C, and so on .. or 2 x BD681 (connected in parallel with two separate base resistors) ..

Finally, you can try combination of standard BJTs such as 2N3055 (TI3055) + MJE340 connected in Darlington configuration ..

Rgds,
IanP
 

    M

    mhamzah3

    Points: 2
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lm317+zener

Hey Ian,

I guess I have to ditch the old design because I could not heat the circuit for heat test. It seems that the heat generated from the circuit is killing the current draw and it stops working when I increase the heat to 90 degC.

Anyway, I have another design that I am thinking about. What do you think on using a HEXFET Power MOSFET IRFB17N50L as buck converter. Here is the schematic that I am planning to use.

Do you think it will not heat as much?
 

satcure focus

Hi mhamzah3,

I really do not think anything good will come from that circuit.
It is certainly not a buck converter.


Cheers,
/Ghydda
 

lm317 not working

The shown circuit is a kind of linear regulator (although not fitting your application with the present dimensioning). Thus the power losses would be basically identical to LM317 or any other linear circuit at a given output current.

As a late comment, to the original question: You simply ignored the LM317 datasheet, that gives clear current limits depending on voltage differential and case temperature. You can see, that even with a good heatsink, the LM317 isn't able to supply the intended current.

Furthermore, at least a LM317HV should be used, otherwise damage to the device, probably resulting in shorting the full input voltage to the 21V output, must be feared.

A discrete transistor as linear pre-regulator with sufficient heatsink or a switching (buck) regulator would be a solution.
 

lm317t 2n3055

why you not upload the image ??
dont upload the file to view .
 

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