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 (V_TM 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.

[T3sl4co1l]

Yeh... what voltage?

If this is 240V mains as I would guess from your icon, you'll be hard pressed to find one that doesn't!  Even the MOC3xxx optos are rated for that (albeit, peak only; they're never rated for continuous operation).

If isolation is part of the deal, I would recommend an AC-type SSR.  Otherwise, yeah, TO-92 is about as small as it gets, or SOT-89 if you can fit it on a board, and heatsink that somewhat.

Tim

[floobydust]

I remember Weller adding a 0.01uF capacitor across the Magnastat switch to stop them from arcing, but not enough for 220VAC I guess.
Duty cycle is not 100%, so a small triac would be good.
In the W60 there is room for even an insulated TO-220 triac and some small parts. About 1/2" dia. hole and an inch or two.
edit: added W60 fanboy pic

[David Hess]

Size will be limited by the package so there is no reason not to use the highest current TRIAC in a given package.

TO-92 TRIACs are mostly limited to 1 amp.  That could work but I would want more leeway given the poor thermal environment.

A TO-225 packaged TRIAC gets you to 4 amps which is plenty and the package is not much larger than a TO-92 but it has an exposed pad.  This would be my first choice unless an over-mold TO-220 package will fit.

If larger is acceptable, then a plastic over-mold TO-220 is self insulating which might actually save some space.

[CJay]

I remember Weller adding a 0.01uF capacitor across the Magnastat switch to stop them from arcing, but not enough for 220VAC I guess.
Duty cycle is not 100%, so a small triac would be good.
In the W60 there is room for even an insulated TO-220 triac and some small parts. About 1/2" dia. hole and an inch or two.
edit: added W60 fanboy pic

I think, having taken an iron apart this morning, there will be room for a larger package triac so I've ordered a small selection of TO-220F plastic insulated parts (I'd prefer those, no metal to move around and short things out) so I now need to wait for the post.

I'm also considering a thermal fuse but that could be tricky as they don't have high enough operating temperature ratings so I'd need to work out if I can mount it on the metal plate at the bottom of the element.

[David Hess]

I had the 120 volt version of that iron until I lent it to someone whose iron had burnt out at a work-site.  He promptly burned it out by plugging it into the outlet which was miswired for 240 volts and had burned out his iron the day before.

[CJay]

I went through my bag of 'faulty' irons last night and I've now got four working ones, tested them all and I've got the parts to make a fifth one so I'm happy.

one has a brand new element which 'sings', an odd noise but it could be because the element has absorbed some moisture?

Another, the oldest one of the bunch, worked perfectly out of the bag for about half an hour and then went 'pop'.

Damn Rifa cap.