Quote Originally Posted by Ross77 View Post
If it only = 2W then why do the res need to be 5W? I thought vref was up to 1v as well? if so then 1/4 = 0.25 hence the two 0.5r, but this would mean max current is 4W? probably got it wrong again :whistling:

Theres not much available in 2 or 5W resistors in that range as they cant be wirewound.......Why is it possible to get a 2W smd res 30p? dosnt make sense.

For the heat sinks I was going to reuse the originals, their about 50mm square and 140mm long with fan at each end. The 6203's mount on both sides with the fins top and bottom.

Does the shunt regulator have to be on each board or could I just put one on the PSU distribution board?
You are right, Vref for the L297 is 1v - I was confusing it with the TBA6560 chip on the other thread... so the sense resistor should be 1v/4A = 0.25ohm non-wirewound, rated at 1v x 4A = 4W you would derate by 25% - 50% so really you need something rated at 5W - 8W

Look at the datasheet for this 0.5ohm SMD resistor from Farnell (you'd need 2 in parallel to give 0.25ohm/4W). At its rated dissipation it needs, and I quote "2 Watts with total solder pad and trace size of 300 mm2" - I doubt you'll be close to that amount of copper, and even if you were the temp rise is 90degC above ambient... thats 105degC+ - ouch!

A better product is this nickel chrome metal resistor from Vishay, also from Farnell. This is rated at 5W and seemingly doesnt need a heatsink, but will hit 120degC at rated load so some airflow would help.