LM317AHV - and 48V DC DC convertor
Posted by davegermain at
Hi all sorry about this bit i have an issue with the above project, and i am not very good at fault finding yet.
ok the story goes... i have a 48V DC input and need to drop the voltage to around 15v (12V to 25V) the power is comming from a forklift truck and powering a Mobile computer (teklogic 8255)
i have two possible paths i can get a new power suplly rated for the new forklift trucks and much expence or make a little cct to mount inside the computer (in line with the computers power supply).
EG. Power in (48v) > to my cct > mobile computer power supply (12v to 30v it has a big range)...
So i did my research and then made the above prototype. I selected the LM317AHV as this seems to have a higher input voltage tolerance then the normal LM317T.
The above cct seemed to work for a few seconds then fail so i guess its the LM317 that has gone. But why? that is the puzzle.
i have also provided the cct schmatic that i have designed. (very typical just the Resisters have been changed from the referance design in the datasheet).
Do i need the output Cap when testing the cct?
Does the cct need to have a Load to make it work correclty?
How do i correect the design so that the LM317 does not fail?
I know this is a big ask as with out the pcb its very hard just a few pointers or anything i have done which is obviouls faulty.
Replies
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lots of people viewing no one brave a responce :D Laughing is not considered a responce...
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Hi, I haven't checked the 317 spec, but you are probably overheating it and burning out the chip before it even has time to warm up the case.
What current do you need to supply? Again check 317 rating. Lets assume 1Amp, thats 1Watt for every volt dropped! You have 48V dropping to 15V so at 1Amp 33Watts, thats a lot of heat.
Also a lead acid 48V battery is probably 4 x 12V lead acid batteries, so when charging or just been charged the volts will be much higher than the nominal 48Volts!
I suggest finding out the current rating required and selecting an efficient DC to DC converter for this job.
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Hi davegermain,
As you probably have 4 x 12V or 2 x 24V lead acid batteries, you should be able to connect your pc at an intermediate tap @ 24V? No more LM needed (this circuit is self-protected against overloading, and able to deliver max 1.5A ... thus most probably entering protected mode when you try to use it).
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Hi dave....
I have looked at the datasheet of your Teklogic 8255 and found the needed current is about 3A (as referred by the chargers). The LM137 being limited to 1.5A, use a LM138 instead: the maximum allowable current is now 5+A.
But the solution to find a point with 24V is not good, as the unloading of one half of the batteries will be rather high, and the reload probably problematic.
The regulator must then be fed with 48V, and another problem arise: the maximum input to output voltage is too high in start-up and short-circuit conditions, as the LM138 supports only up to 40V.
But here is the solution.
Look at the datasheet, at http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS009060.PDF. Go to page 10, you will find a Tracking Preregulator Scheme. As described, the preregulator (at the left-hand side), maintains a 6V voltage drop between the input and output of the second LM138, due to the R1 and R2 values.
To limit the power dissipation, it should be wise to choose an output voltage of 24V. To equally spread this power between the two LM138, the voltage drop across each regulator should be 12V, assuming the input voltage is 48V. This can be done with R1 = R3 = 120 W and R2 = Radjust = 1032 W. Set these resistors at 1200 W, foreseeing a higher input voltage when the batteries are fully loaded (53V). A bleeder of 1 KW must be added across the output terminals, to absorb the quiescent current of the preregulator when no load is connected and this way allow the whole to function properly.
The power dissipation of each LM138 will be roughly 40W @ 3A load current; a sufficient heatsink must be provided. To limit the junction temperature at 100°C when ambient temperature is more or less 30°C, the total thermal resistance must be lower than 70/40 or 1.75 °C/W. As the thermal resistance between junction and case of the LM138 is 1°C/W, the thermal resistance of the heatsink must be lower than 0.55°C/W, with a security margin of 0.2°C/W for the gap between case and heatsink. Film coating/aluminium oxide of the heatsink must be gently removed, and thermoconductive paste added.
I suggest something like this one: http://be01.rs-online.com/web/p/heat-sinks/0158556/
Hope this will help you...
Kind regards,
B.B.
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Hi, I haven't checked all the ratings but what about a higher efficiency solution?
Traco do a range of modules, e.g. RS445-012 at £55.25 is 18 to 75Volts input and 15V 2A output at a nominal 90% efficiency. That's if you can get away with less than the charger rating!
445-078 comes closer regarding current at 2.5A and 12Vdc out so a bit closer to your minimum voltage spec.
Going up in spec (and price) RS 706-6394 at £111 gives 15V at 5A output.
There are many other models available, but the above were found with a quick search for your consideration.
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Hi,
If Mr davegermain is still alive, I suggest him to go to “profile” > “my alerts” > “alerts settings” and check the little circle “each four hours”...
I have a more elegant (switching) solution for his problem: nearly finished updating an old and similar design I retrieved in my folders. But I don't know how to send a drawing or something like that via the “Reply” of DS. Never seen it; I'm using Open Office... If somebody can help?
simply have to choose between buying or making...
Kind Regards,
B.B.