Mosaic
Well-Known Member
This is useful enough to bear repeating.
If u have a design and discover your v.reg is running too hot due to a large voltage drop, offloading the heaitng with a couple resistors in parallel can save the day.
I had a design that spec'd the D2 PAK size LM317 surface mount part. But due to limited PCB space it was downgraded to the smaller D-PAK case.
Eventually I discovered the unit was running at about a 72°C delta.
No space for a heatsink...what to do?
The Vreg was dropping 8.4V @ 120mA.
I squeezed in a couple 39Ω , 0805 resistors in parallel and placed them 1mm away from the 1/4" spade terminal Vcc input for heat dissipation. I added a soldermask stop to permit thick solder on the 1mm PCB trace length. The resistors drop about 2.4V and have a 50°C delta. The LM317 also has a 50°C delta now. Without the 1/4" spade terminal the resistors have a 75°C delta.
The 50°C delta is useful as the module being designed might have to sit in subzero temps and not form condensation on the PCB. Sealed Enclosure Air volume is only 0.02L
If u have a design and discover your v.reg is running too hot due to a large voltage drop, offloading the heaitng with a couple resistors in parallel can save the day.
I had a design that spec'd the D2 PAK size LM317 surface mount part. But due to limited PCB space it was downgraded to the smaller D-PAK case.
Eventually I discovered the unit was running at about a 72°C delta.
No space for a heatsink...what to do?
The Vreg was dropping 8.4V @ 120mA.
I squeezed in a couple 39Ω , 0805 resistors in parallel and placed them 1mm away from the 1/4" spade terminal Vcc input for heat dissipation. I added a soldermask stop to permit thick solder on the 1mm PCB trace length. The resistors drop about 2.4V and have a 50°C delta. The LM317 also has a 50°C delta now. Without the 1/4" spade terminal the resistors have a 75°C delta.
The 50°C delta is useful as the module being designed might have to sit in subzero temps and not form condensation on the PCB. Sealed Enclosure Air volume is only 0.02L