Heat Gun to reflow station

I have been working on converting my Heat Gun to be part of my reflow station. I will be using the heat gun in the same manor that the "Reflow Oven" works. The problem all started when I took apart the heat gun to see how the high/low switch worked. Turns out it uses a diode as a half wave rectifier.
My idea is to use the diode on the fan only so it will stay running and use an SCR to control the heater. The SCRs Gate will be wired to a 4N25 optocouple and let the Arduino Uno run PID to control the heater. Just like that of a reflow oven. The reason I want to use an SCR is it will let me run the heater at ~1/2 power, (300 deg C not 550 deg C).

I have been looking on the internet to find out how to hook up an SCR to the 120VAC mains. I have not as of yet found what I need.
So here I am asking for help.
How should I bias the gate of the SCR to the 4N25 optocouple?
L(ine) will go to the SCR Anode.
SCR Cathode to the heater on the heat gun.
N(etural) to the heater return side.
Now for the gate...
From what I can tell, I need to use a resistor from the anode to the collector of the 4N25 to limit the current.
From the emitter of the 4N25 is where I get messed up. First I need to run it threw a diode >| to the gate. I think.
Then do I need anything else as the resistor only limits the current and not the voltage.

Could someone show me how to hook up a 15 amp SCR to an 4N25 optocouple so I can control the heater with the microcontroller?

Thanks in advance.
Phil

Can you draw a schematic ?

I have a hand sketch of what I want to do but am having trouble getting the picture to my computer. Will post it as soon as I can get this device connected.

Having problems getting a picture. My Black berry is not working for me.

Ok I think I will just use a transistor from D3 to turn on the SCR. I will just stick in a diode and resistor to the collector of the transistor and stick the emitter on the gate of the SCR. Hope there is no magic smoke.

Later I may come back and let you all know how it went.

Is it an AC or DC heater ?

The air velocity in a heat gun is way too high, you'll simply blow components away.

The air-rework station I know uses a small diaphragm pump for the air and a thermocouple
right at the tip of the heater element for its temperature control, which presumably is a PID
loop. The pump rate is controllable as the smaller nozzles otherwise blow components about.

Reflow ovens use IR heaters , not hot air. I don't think you design is going to work .
As far as the SCR, they are used for switching DC. If you you put two of them back to back you have a TRIAC. Which is used for switching AC with a zero crossing detector (your opto). If you're switching AC you should be using a TRIAC.

raschemmel:
Reflow ovens use IR heaters , not hot air. I don't think you design is going to work .
As far as the SCR, they are used for switching DC. If you you put two of them back to back you have a TRIAC. Which is used for switching AC with a zero crossing detector (your opto). If you're switching AC you should be using a TRIAC.
ecircuitslab.com

I think ovens were only mentioned to introduce PID control, the OP is making
a rework station I think.

What is the SCR for?

raschemmel:
What is the SCR for?

run the heater on DC

Thanks for the replies.
The heat gun runs on AC. When it is on LOW setting the AC (hot side) runs threw a diode. The heater and fan part are now running at 1/2 wave.
So by using an SCR it will do the same thing. @ MarkT, Yes SCRs will control AC but unlike a triac it only conducts for 1/2 cycle.
Like a lot of curling irons, some use a SCR and others, (higher priced ones), use a triac and NTC.
As far as controlling the fan goes, I can always hook up the fan to a triac and wire it up like a motor speed control. This will give me full control over the fan.
Where I haven't worked with SCRs much is where I am having problems interfacing it to the arduino. I just need help controlling the heater part of my heat gun. I did more web searching and I think I have what I need.
The thermocouple will be setting on the PCB that I will be working on. The heater will be controlled via a PID controlled micro controller much like a reflow oven.
I have been in electronic for some 40 years now. I just got into micro controllers this year. I am just a hobbyist an most of my time is spent repairing things. I have used a heat gun to reflow GPUs, CPUs on different PCBs but have also destroyed them as well. With this set up I hope to make it easier for me to automate this. I have built a reflow oven and like what it can do. I am just testing the heat gun idea for my own use. I will also share this with everyone as soon as I get it set up and tested.

Thanks again for your feed back. I need all the help I can get.

Just wanted to clear things up.
The heat gun will be used to reflow a GPU on an XBox 360 mother board. Or a PS3 mother board. Or even a laptop that has over heated one too many times and the GPU solder balls need to be reflowed. I am not going to use this thing as a reflow oven!
My set up:
Preheat the mother board to 100 deg. The GPU will be already to go, (flux, shielding around the GPU to keep other parts from getting too hot). The heat gun will be placed over the GPU on a stand.
When everything is in place I push a button on the micro controller to start the process.
Stage 1: preheat the PCB to 100 deg F.
Stage 2: The heat gun fires up, turning the fan on at a low speed. (this will be preset via a motor speed control and turned on via the arduino. The heat that comes out of these 1500 watt heat guns is way too hot and hard to control so I want to control it at 1/2 the power. (The way it is done with a normal heat gun is via a diode). So I guess you could say it will be running at 60 volts DC.
To be able to control the temperature coming out of the heat gun will be with the same idea as a reflow oven does it. via PID.
Now comes the hard part for me.
The SCR.
This little guy acks just like a diode. On a DC circuit once you trigger it via the gate, it will stay conducting. But with AC you are switching the current back and forth so you have to keep bias voltage on the gate to keep it turned on. Now if I use a triac, I would be getting the full 120 VAC going to my heater part of the heat gun. This is just no good. Where the SCR only conducts one way and blocks the other so I would be seeing 60 VDC on my heater. This I can use. When I remove the bias voltage from the gate the SCR will stop conducting at zero voltage and would stay off until it gets voltage to to gate again. This way I can turn on and off the heater to control the amount of heat I need to reflow the GPU.
The Arduino will control everything so I can just push the "GO" button and walk away. It will preheat the pcb, turn on the fan and heater, ramp up to 150 deg F and go into the next stage.
Stage 3: On a reflow oven it is called "Soak". So I will do the same. The heater will slowly bring the GPU temperature up to 200 deg F. This allows the flux to do its work. Once at 200 deg then it will go into stage 4. Reflow...
Then into "Cool". Stage 4 will shut off the preheat and the heater in the heat gun. Once things have cooled to 100 deg F the fan will shut off as well. The whole PCB can now cool to room temp and the buzzer will sound telling me it is cooked.
I would then remove the board from the stand and prep it with new compound and heatsink. Test and hope everything went well.

Like I said before I repair things. Designing is not my thing. I am new to micro controllers but am learning fast. I don't ask a lot of questions because I am sure someone has asked before. I just can't find the info I need to interface the arduino to a bloody SCR.
I have some ideas but if anyone has done this before could you point me in the right direction please.
Thanks for your time.

Hi Phil55, I'm new here and don't have much in the way of electronic knowledge but I am trying to do the same as you with a hot air gun. I also do reflows of GPU chips but would rather do reballs as a longer term fix and that is why I like reading what you've done so far. This will probably show my ignorance but I was thinking of buying a heat station handpiece ($25 on ebay). The station I already use for reflows (Atten 858D) has a 240VAC to a dual 24V/10VDC transformer. So the handpiece must work on the 24 and 10VDC. I guess the heater element wants the 24V. I am thinking of a transformer that would output these voltages and controlling the handpiece with a PWM/Arduino (my Arduino starter kit arrived today and I can't make head nor tail of any of it so I've a long way to go). This way I think I would avoid all the triac/scr problems you're having. I was going to use a microphone stand (also cheap on Ebay) for the holder and a Puhuit bottom pre-heater (again on Ebay). I'd be grateful for your opinion on this idea. Your post is helping me get started too. I am very interested in your progress so please keep your ideas coming.

raschemmel:
This may work most probably
5 Best Heat Gun Reviews - Mini/Cordless Heat Shrink Gun - All Trusted Tools

@malikkhhan, you're 5 years too late.

Check the dates on topics when replying.

As pointed out, that link was from 5 years from another post.

For reflowing smd ic's you need something like these:

STILL NO SCHEMATIC ?

Sparker1 wants to repair BGA chips and circuit board. A fools quest. IF you can ever get the new balls to stick in the flux and get the thing to stick on the circuit board, how are you going to X-ray the board to see all the problems under the chip?

Paul

Sparker1 wants to repair BGA chips and circuit board. A fools quest. IF you can ever get the new balls to stick in the flux and get the thing to stick on the circuit board, how are you going to X-ray the board to see all the problems under the chip?

I knew there was a reason I have avoided tech jobs involving BGA's these past 40 years...