I happen to have about 500 NOS (New Old Stock) LM317T (The larger package like TO220).
If anyone will (even eventually) use any of these to teach something to somebody, I'll send you 6 of them if you send me a SASE (Self Addressed Stamped Envelope)..
I am using a 24 to 48V DC Power source and attempting to use a LM317HVT to reduce the voltage down to 5V.
Do you think this is a good idea? 300mA through a linear regulator from 48 to 5V
dissipates 13W, if you use a DC-DC convert you will dissipate perhaps 0.3W instead and draw
perhaps only 50mA from the supply.
The LM317HVT can't necessarily handle the power anyway - if you look at the second graph
in this datasheet you'll see the max output current for the HVT with 43V difference is about 300mA http://www.farnell.com/datasheets/22656.pdf,
I wouldn't want to run that close to the shutdown limit!
Go for an 18--72V input DC-DC converter, they are pretty standard devices and cover your input
voltage range with a plenty of headroom.
MarkT:
Do you think this is a good idea? 300mA through a linear regulator from 48 to 5V
dissipates 13W, if you use a DC-DC convert you will dissipate perhaps 0.3W instead and draw
perhaps only 50mA from the supply.
The LM317HVT can't necessarily handle the power anyway - if you look at the second graph
in this datasheet you'll see the max output current for the HVT with 43V difference is about 300mA http://www.farnell.com/datasheets/22656.pdf,
I wouldn't want to run that close to the shutdown limit!
Go for an 18--72V input DC-DC converter, they are pretty standard devices and cover your input
voltage range with a plenty of headroom.
Hi MarkT
I thought about using a DC-DC converter but was unable to find anything locally that would suite the conditions it could be exposed to. (Living in a 3rd World Country where LM317HVT's and DC -DC drivers are difficult to come by - South Africa. ) The input could drop as low as 12V....
The board at it's peak would be drawing a total of way less than 100mA. One ATMEGA 328P and 3 X 3mm LED's at 20mA each. I used the reference of 300mA as absolute worst case.
My calculations of the LM317HVT dissipation was approx 4.3 W which is based on 100mA Iout.
I'll keep an eye on the Heatsink and see how it handles.
I thought about using a DC-DC converter but was unable to find anything locally that would suite the conditions it could be exposed to. (Living in a 3rd World Country where LM317HVT's and DC -DC drivers are difficult to come by - South Africa. ) The input could drop as low as 12V....
The board at it's peak would be drawing a total of way less than 100mA. One ATMEGA 328P and 3 X 3mm LED's at 20mA each. I used the reference of 300mA as absolute worst case.
My calculations of the LM317HVT dissipation was approx 4.3 W which is based on 100mA Iout.
I'll keep an eye on the Heatsink and see how it handles.
Thanks
So the drawing "way less than 100mA" would mean the absolute worst case was also less then 100mA, not
300mA.
Xproximity:
I'll keep an eye on the Heatsink and see how it handles.
Thanks
Keep a finger on the heatsink. You eye won't see anything until it's red hot and slumping into a blob of molten aluminum... at which time it will be way too late.
(actually, the LM-317 will shut itself off long before it gets that hot, but that kinda ruins the point of what I was trying to say).
MarkT:
So the drawing “way less than 100mA” would mean the absolute worst case was also less then 100mA, not
300mA.
Apologies for the incorrect data of 300mA. That was an assumption based on the use of high output LED’s which I have now replaced with lower power ones…
I ran the board for a couple of hours last night and it seems to be holding.
I’m going to be checking for another heat sink since the one I have is getting warm to hot. I was able to keep my hand on the heatsink for minutes without it burning me.
What is the safest temp I can let it run at? I’m guessing that it’s currently at 50 to 60 *C
In general cooler is better, but semiconductors can take pretty high temperatures, like 100C on the die,
but its everything around it that suffers (PCBs can degrade over long periods if cooked, for instance).
Reliability reduces as running temperatures go up too.
If you can put your hand on it for minutes it's probably less than 50...
Xproximity:
Apologies for the incorrect data of 300mA. That was an assumption based on the use of high output LED’s which I have now replaced with lower power ones…
I ran the board for a couple of hours last night and it seems to be holding.
I’m going to be checking for another heat sink since the one I have is getting warm to hot. I was able to keep my hand on the heatsink for minutes without it burning me.
What is the safest temp I can let it run at? I’m guessing that it’s currently at 50 to 60 *C
Thanks
You know, there’s a simple way to boost the 317 (or 7805 or any other 3 terminal regulator).
MarkT:
In general cooler is better, but semiconductors can take pretty high temperatures, like 100C on the die,
but its everything around it that suffers (PCBs can degrade over long periods if cooked, for instance).
Reliability reduces as running temperatures go up too.
If you can put your hand on it for minutes it's probably less than 50...
Data sheets usually state 100 C or 105 C for a max temperature. That's crazy. My rule of thumb is "Too hot to keep my finger on == too hot".
