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Advise needed > lightning vs 7824

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Bloch:
Hi All

This is my first post here so be kind to me  ;D

At work I have this circut "24V power supply" see drawing.

It have been built i many units with no problems.

It powers some wind sensors there is very likely to get stiket by a lightning.

My problem is that i would like to change it to a LM7824.

I need a current limiter and  need to make a new PCB and price / speed is always important.   

Pro.

* Cheaper time and parts
* Curent liniter
* Smaller space
Con.

* Will it survive the lightning as well as the old tried circut
And no i don't have the possibility to get it tested at a lab.


tecman:
Your existing design has the merits of being robust and inexpensive.  The 7824 will give you tighter regulation and current limiting (sort of).  It is not true current limiting, but it limits based on thermal, so you will reach some steady state current on an overload.  The other caution is that the 7824 have a max Vin of 40 volts, which could be an issue with surges that might occur with lightning.  You could add parts to mitigate the surge sensitivity, but the cost will go up.

I would be tempted to stay with what you have and just ass a polyfuse (PTC fuse) to the DC line feeding the transistor.  Again thermal protection that will limit current.  Simple and robust.

paul
 

Simon:
surely R1 needs to come from the positive supply not negative, like that the transistor will just not pass current

oPossum:
Current limiting could be easily added to the existing design. A TVS (transient voltage suppressor) may improve robustness.


Bloch:

--- Quote from: Simon ---surely R1 needs to come from the positive supply not negative, like that the transistor will just not pass current
--- End quote ---
Thanks. Yes i did miss i a wire.


--- Quote from: tecman ---Your existing design has the merits of being robust and inexpensive.  The 7824 will give you tighter regulation and current limiting (sort of).  It is not true current limiting, but it limits based on thermal, so you will reach some steady state current on an overload.  The other caution is that the 7824 have a max Vin of 40 volts, which could be an issue with surges that might occur with lightning.  You could add parts to mitigate the surge sensitivity, but the cost will go up.

I would be tempted to stay with what you have and just ass a polyfuse (PTC fuse) to the DC line feeding the transistor.  Again thermal protection that will limit current.  Simple and robust.
--- End quote ---

Thanks. It is also my gut feeling that the 7824 is not robust enough.

I have tried the polyfuse solution.

The circuit powers 4 wind sensors. 

On installation they did an error (shorted) one sensor. Then they did see that they did it wrong. The polyfuse was so hot that the consumption from the 4 sensors did keep the polyfuse hot  >:( and newer got reset before they changed the PCB.

So the polyfuse is not a fix here.


--- Quote from: oPossum ---Current limiting could be easily added to the existing design.
--- End quote ---

Thanks.

Why did i not think on that ? I am surely getting a little rusty.
I was thinking of adding a current limiter. But thinking in the direction of an extra power transistor+current sensor.

Thanks this must be  the answer to my "little" problem. Only 3 cheap/easy components more.

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