Electronics > Beginners
Triac current rating headroom
CJay:
Really simple but I'm not having much luck finding anything definitive (maybe my google-fu is weak today)
What headroom would you want in the current rating for a triac controlling a 60W resistive AC load?
Space is tight so size matters.
Circlotron:
If the load is an incandescent light globe, the inrush current can be 7-10 times the normal run current for a brief moment. Measure the cold resistance to see. If it is a plain resistive load like a heating element then there is no inrush as such, but it is nicer all round if you switch the triac on at the voltage zero crossing. That way, if the triac is running close to its maximum rating there will be less di/dt stress on the die so presumably less margin would be needed, however much that may be.
CJay:
It's the heating element in a soldering iron, specifically the Weller W60 as I'm fed up with Magnastats dying and costing a small fortune to replace, someone else here suggested using a triac triggered by the Magnastat with the low voltage irons but I want to try with the Mains irons.
Ideally I'd like to be able to use a TO-92 package device which I could easily squeeze into the handle and insulate but I could possibly find space for a small PCB and SMD package.
Looking at
https://docs-emea.rs-online.com/webdocs/1294/0900766b81294114.pdf
and
https://docs-emea.rs-online.com/webdocs/152f/0900766b8152fe5f.pdf
Ian.M:
The problem is probably going to be the temperature in the handle, and thus how much derating the TRIAC needs. You'll need to keep the TRIAC as far as possible from the element and 'suck it and see'. It may be worth snaking a thermocouple in there and using the iron for a typical session to see what you are up against.
MagicSmoker:
I just looked at the first datasheet and off the cuff I'd say it will work fine, though note that the TO-92 package will have the worst thermal resistance of the three (and therefore lowest actual RMS current rating). 60W at 220VAC is 0.27A and with a worst case voltage drop of 1.6V (VTM in the datasheet) that will result in 0.44W of dissipation. Multiplied by the 60C/W thermal resistance gives a temperature rise of 26C, which seems eminently reasonable.
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