Hot triac

[throbscottle]

How hot are triacs supposed to get?

I'm building a 936 clone soldering station, with the modification that the triac's gate is driven by an opto-triac with zero crossing, which is triggered by a comparator, since these are components I already have. I'm using a BTB04 triac, passing about 2 amps and it's dropping about a volt. So it gets pretty hot. The supply is 24VAC.

Are they supposed to have such a high voltage across them when conducting? Only I don't like that I'm losing 2W on it!

Thanks for looking

 

[fezder]

this is interesting info for me too, sorry i'll just hang in this thread as ''leech''

 

[kubeek]

Looking into the datasheet those values sound about right. Triacs and thyristors are by no means low-loss devices, and this becomes even more aparrent when you use SSR relays, which will drop something like 2V at 10A. You could try running the iron on DC and using a mosfet, but that would mean a completely different circuit.

As for the temperature part, without heatsink it has around 60K/W thermal resistance to ambient, so with 2W continuous dissipation expect around 145°C case temperature, way above any sane limit.

 

[dr pepper]

Theres a rule of thumb that a triac can switch a 400w lamp max without a 'sink at 240v, so thats around 1.5a, so i spose 2ais over that limit a little.
As mentioned a little clip on 'sink ought to do the job.
What are you using for zero crossing, a moc 30xx opto?

 

[throbscottle]

Hi folks
Thanks for the quick responses!
It's on an aluminium h/s with paste, measures 35mm x 17mm x 9mm - think it's adequate!
Only other thing I think of to change is gate drive resistor. Choosing that seems a bit of Dark Art.
Dr Pepper, yes, moc3063 is what i'm using. Board pull from motor control circuit I found...

Hakko themselves actually used a triac with a higher on voltage, the AC05D, and a now obsolete driver chip, the uPC1701C. So I wondered if there is a trick I was missing. Their gate drive resistor is 330R

 

[kubeek]

Can you post the whole circuit? I guess you will get better answers like that.

 

[throbscottle]

Ok here's the schematic. It's basically a copy of the Hakko one, but I used 2 dual op-amps instead of a quad one because that's what I have. Here's a link to the one I used to copy from: https://www.eevblog.com/forum/revie...ter-a1321/?action=dlattach;attach=49735;image

 

[ronv]

If I use my rule of thumb heatsink calculator. Degrees C/watt = 50/√area in cm. I get a temperature of about 65C. To hot to hold, but not enough for a quick blister. Does that feel about right? If so should be ok.

 

[throbscottle]

Hi ron
I can't run it for long enough to see how hot it gets yet because it means having the iron on full power continuously - I turned it off when it went past 400C! But with the h/s on it didn't get finger burningly hot.

 

[kubeek]

Well then call it done and put it in a nice box

 

[dr pepper]

Dont hakko make floor sweepers and things.

 

[throbscottle]

I posted a bad schematic up there. Here's the one that actually works! PCB layout too. I fried the half of the comparator I was using trying to figure out what was wrong, hence the change to the other side.

 

[chemelec]

Wouldn't it be Simpler to just make a 24 Volt, 2 Amp, DC Power Supply with an Adjustable Current Regulator so to give some temperature control?

 

[throbscottle]

Why would I do that? The iron provides temperature feedback via a well understood ptc thermistor in the tip, someone has kindly posted the schematic for the manufacturers analogue controller on another forum, and I would still have to obtain a transformer for the power supply (which I made by rewinding an old one). The only "work" I had to do was figure out the function of the uPC1701 chip which is obsolete, and how to replace it's functionality, and layout a PCB. Not to mention, setting the current (or voltage for that matter, since it's simpler) without feedback will only limit the power available to the iron, not actually keep it at a steady temperature.

Anyways, I can now run the iron with the controller, and going back to the original subject, see how hot the triac gets!