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Repairing hot air soldering station

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cowana

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I've got a chinese solder reflow sation, and the element in the hot air gun blew.

When the element blew, it took also blew the fuse and triac.

I've ordered replacements for the fuse and optocoupler, although I'd rather not have to desolder the optocoupler if it might not be blown.

So;
What do you think of the attached schematic? It seems pretty standard to me.
The triac blew due to a large current surge - do you think the optocoupler will need replacing?

The important parts:
Triac:

BTA08-600C (rated for 8A).
Dead short between A1 and A2, ~65 ohms between qate and the other pins.
Optocoupler:
MOC3023-M triac driving optocoupler

Schematic: attached.

Many thanks!

Andrew

Edit - I just realised this might be more appropriate in the repairs section. I originally posted it here as it was more a question on how this triac circuit works - if you want to move it, mods, feel free!

Edit 2 - that 210R resistor is actually a 270R resistor - the purple band is rather faded!
 

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The circuit is very odd. It MIGHT not drawn right, but let's go on the assumption it is. I don't like it. For one, you effectively have paralleled Triacs, but you do get a voltage divider between the heater anf N. It's odd that the heater is part of the divider.

In contrast, take a look at this https://www.electro-tech-online.com/custompdfs/2011/05/fk423e-1.pdf circuit, I'd replace it with this one. For one, there is a capacitace divider and there is the potential to have an RC snubber. Both are good things. Metal oxide resistors are a good type to use, but they will destroy themselves completely and act like a fuse.

The lack of a snubber in your circuit is bad. It allows dv/dt of the mains to turn on the triac independent of the drive signal.

Note some of the suggested driver sheets in the datasheet: http://www.datasheetcatalog.org/datasheet/fairchild/MOC3023-M.pdf

I have a microwave oven I need to attack. No schematic. Those small resistors blew up and I am expecting that the opto did too.

A heater that opens can cause the triac to close because dv.dt is exceeded. A surge will do the same thing. A short can to. The voltage collapsed too fast.

At work we had special SCR units operating into transformers. They used to replace $30 fuses every once in a while. After I set them up to use current limit properly, used overrated (25 A) SCR's instead of 10A and added a $0.25 3AG glass type fuse cost savings went through the roof. Later, with much fuss from management, I eliminated them all together and changed to a DC power supply. This gave metering, short circuit protection, 120/240 operation in a 1RU rack rather than a 2U rack.

There were lots of configurations, but a 4U rack held two controllers, then a 2U powersupply, then another 2U power supply and another 4U for 2 controllers. In the final set-ip two evaporative sources had top and bottom tantalum heaters and usually the top heater was there to prevent clogs. It wasn't temperature controlled.

So. moral. An oversized triac with a fuse sized to the load makes a very reliabe cost-effective solution.
 
The optocoupler is showing the same resistance across it's pins as one that I know is working (>40M), so it's either blown open or it's still working.

I'm going to say it's working - if my new triac doesn't trigger, then I'll replace it.

Andrew
 
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