At 11:26 PM 12/22/2003 -0600, you wrote: >Along this line - can anyone tell me, a electronics hobbyist, how to do >calculations as far as how big a heatsink is needed for a SSR or triac? > >For instance I found the power dissipation curves for a triac. From that >and my system knowledge I know how many watts of power need to be >dissipated. What I don't understand is what the C/W numbers are used for. There's a maximum temperature at the die (see the data sheet, you'll want a bit of safety margin). At a given dissipation you know how much hotter the die will run than the case. There will be another =B0C/W across the thermally conductive grease (and MUCH more if you need to put a Silpad or mica insulator in there) and another =B0C/W for the heatsink itself. You're not really done yet. You may specify the maximum ambient temperature, but the heatsink might be in a confined space where the temperature is higher than ambient. As is more or less usual in engineering, these things add up in the least desirable way. ;-) The point is that each thermal "resistance" at a given power has a temperature drop analogous to the voltage drop across a resistor. Most heatsinks have a nonlinear drop with power (for example, above a certain power level more natural convection will occur). >In one application I am kicking around in my head I am thinking of just >bolting an SSR to a big hunk of steel. What I didn't know was how to >figure out if that would be sufficient or if I should use the piece of >scrap aluminum I have here instead. I'm sure I could post it here and have >half the people yell at me for being stupid and saying it will burn up and >the other half saying no problem - but I'd like to be able to do the math >myself and have some level of confidence that it will work. > >Please be gentle as I'm a newbie in this area. Even a couple of good >references to books or web sites would be great. The old GE and RCA and Motorola SCR/Thyristor manuals have good information on this, unfortunately they are long out of print (mine are dated from the late 1960s and early 1970s). Heat conduction from a semiconductor die to the heat sink is easy, you just add up all the r-thetas and multiply by the watts, just like Ohm's law, the thermal resistances are all in series. The heat sinks themselves are quite another matter- convection and flow does not follow a simple proportionality. You can calculate rough values for simple shapes like a flat plate (we used the front plate of an instrument, about 5" x 5" as a heat sink for a VERY conservative 5W (they can be packed cheek-by-jowl and the ambient can be very hot). For commercial heat sinks, go to the web sites and download the data including curves showing the =B0C/W under various conditions. In this case there is around 10-12W of dissipation and if you want it to work to 50=B0C and with a maximum temperature at the plate of 100=B0C you need a heat sink that has a maximum of about 4=B0/W. Here are a couple of major US commercial heatsink manufacturers, with a wealth of data on their sites: http://www.r-theta.com/indexf.html http://www.wakefield.com/ These people are more interested in you buying their heatsinks than you using a scrap piece of aluminum (steel and stainless steel are bad because of their poor thermal conductivity), but you can get an idea from using= their tools and looking at their data. To do detailed thermal analysis of a general part including spreading= thermal resistance and transient thermal analysis, AFAIK, requires a very expensive Finite-element-analysis program (the COSMOS add-in for Solidworks can do it, for example). Unless you are in a big company with deep pockets it's easier to just prototype it and measure it, given some judgment to start with. Hope this helps, Best regards, Spehro Pefhany --"it's the network..." "The Journey is the= reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads