lm317 maximum input voltage

[thebadtall]

can someone tell me the absolute maximum input voltage of lm317?

i plan to make a regulated psu utilising a 30v 100va transformer, wich will provide around 42 volt at idle load.

i will parallel 6 lm317 to share the load, but can they stand 42v?

thanks

 

[D.A.(Tony)Stewart]

LM317's have a max. differential spec of 40V so it your input is 42V then the output can be adjusted down to 2V worst case. Of course any load current * voltage drop= watts, causes another problem and that is, how are you going to remove the heat loss?

 

[alexan_e]

For LM315 there is no max absolute voltage but a max in-out difference which is 42v (absolute max).
If this condition is met then you can use it for higher voltages but you will probably need a protection that limits the input/output difference on power on/off.

Note that there is a LM317HVT version which has a limit of 60v

To parallel LM317 you have to use resistors in the output to force them share the current.

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It may be worth implementing a pre-regulator, a transistor with a zener that drops some of the input voltage so that the LM317 can operate with a lower stress.

 

[D.A.(Tony)Stewart]

I forgot to mention current sharing yes, alexan is correct. the question is how big.

looking at the load regulation curve at 100'C just before the device goes into thermal shutdown, the V drop is -0.2V @ 1.5A and -0.08V @ 0.5A where ∂V/∂I= ESR = 1.2Ω. I would recommend 1~2Ω rated for 2~5W each for stability, but your heatsink for all devices is also critical to this choice. Better heatsink, can use lower values and prevent power hogging and shutdown of an individual regulator. Poor heatsink and elevated IC temperatures will require a larger value maybe 3Ω.

 

[thebadtall]

according to your replies, is it possible to use 100vin for 80vout ?

also, what about the idle voltage of the transformer? will it harm the ic's because of overvoltage?

as about heatsinking do not worry at all. I have alum. and copper sheets of all shapes and sizes!

 

[D.A.(Tony)Stewart]

Since the LM317 has no ground connection it only cares about differential voltage.... The ADJ pin is connected to an internal 1.250 V reference voltage w.r.t output voltage and thus a feedback divider network controls the amplification of this internal reference voltage.. i.e. if you tap off 1.25V from your output. ( e.g. 100V out 98.75 Vtap to ADJ pin It will regulate as long as the input never exceeds 140Vdc. So it could be as 10KV-40V on output.


Of course if you over drive the current and the voltage drops.. you need to clamp the input to protect this voltage drop condition with a zener or SCR. So there are practical limits and component safety requirements.

Therefore if component ratings are not exceeded, the LM317 will regulate the output with respect to ground but doing so from a ratio drop from the output via R divider.

You may find some use power Zeners and diodes to protect the device from reverse shutdown and overvoltage on differential.

 

[thebadtall]

i didnt knew that! very useful information.
now I can choose a greater voltage transformer.

can you help me with idle transformer/ rectifier voltage?

when we say a transformer is 500va 30v , by 30v we mean the Vpk , Vrms or Vavg?

because from the equations i read, is Vpk=1.414 * Vrms
and Dc out after rectification is 1,41 * Vpk -1.4

thank you

 

[D.A.(Tony)Stewart]

Usually it is RMS AC voltage at rated current so OCV will be the peak voltage. (no load) and average DC voltage using average rectified AC voltage without a filter. Using an LC filter can raise this.

 

[thebadtall]

thank you again.
...electronics, they look so simple on the schematic, but they come along with a truck of parameters!

 

[D.A.(Tony)Stewart]

* for clarity, secondary winding resistance at rated output current drops the voltage to the rated voltage., using an LC filter if large enough can reduce the ripple and gain more output towards the peak. THere are nomographs and java wizards to help calc. all these variables..somewhere..I've seen on the web.

 

[thebadtall]

ok for my current purpose I want the voltage as close to the rated voltage, otherwise, with low load Im overvoltaged

 

[D.A.(Tony)Stewart]

Then your solution is a series pre-regulator to provide minimum load and maximum input voltage.
This can be done in many ways. Clamp circuit or open circuit...

a) TVS diode with PTC protection in case of mains overvoltage
b) TVS with halogen bulb in series ( which acts as PTC)

TVS or zener or active clamp (comparator cct) to any load such as halogen bulb or power resistor or power transistor to dissipate < 10% of rated power but clipping peaks if no load or going open circuit to block flow of charge current to LM317 regulator with input capacitor.

Many open circuit options with say PNP front end for additional OVP.