[ed_denu]

I think this is an interesting discussion, but if you really want to know how hot your ssr is getting, measure its temperature with an IR thermometer or temperature probe, or both. My ssr documentation shows an allowable operating temperature of up to 80 degrees C.

Using both an ir and temperature probe I see temperatures to around 43 degrees C(110F) pulling about 23A thru a 40A ssr . I'm using a heatsink I made out of aluminum sheet.

Carwiz, you wrote that an ssr "looses about 20% for ever 10°C over 40°C (104°F). So by the time you start feeling that light bulb warmth over your body temperature, that 40A SSR has started loosing its load capacity. By 50°C (122°F), its capacity is down to 32A."

I've read a little about ssrs and heatsinks and all the formulas to compute heat dissipation, but I've not seen this drop in capacity documented. If this were true why would ssr manufactures list acceptable operating temperatures up to 80 degrees C? Using your calculation my ssr at 80 degrees C would only support 20% of its stated load. It doesn't seem logical that manufacturers would list an acceptable temperature range to 80 degrees without a disclaimer noting that significant of a drop in switching capacity.

I also think your analogy of the 40 watt bulb is a little misguided. While I certainly agree that you could not keep your hand on a light bulb for any length of time, you certainly could contact something in the 40-50 degree C range without feeling any discomfort.