Introduction: Low Cost Circuit Board Pre-Heat Workstation for Solder Re-work

By Joe Pitz Technology Blog
About: Software Developer, like to work with electronics, embedded systems, robots etc.

Circuit board pre-heat workstations are very expensive, $350.00 - $2500.00.

The goal of this instructable is to demonstrate how to make a circuit board pre-heat workstation for around $50.00 with no special tools and the majority of materials from Home Depot and Harbor Freight Tools.

Special Warning - Note This project involves working with AC line voltage and high current.
If you have little experience working with household line current please get someone to help you with this project.

I will try to assist you with areas of caution, but you must be aware that AC current can be very dangerous.

By reading this Instructable you must be aware that I am not responsible for any injury or damage you may cause to yourself or your surroundings. (With the normal warnings out of the way, lets have some fun)

Step 1: Bill of Materials

Get your pad and pencil we are going shopping:

Some 1/4" and 1/2" plywood from Home Depot or Lowe's)
(Future upgrade, I want to build a metal case for the heat blower and circuit.)

1 $9.99 Heat Gun from Harbor Freight Tools

1 Three prong (grounded ac power cord)
(Special Warning - DO NOT USE THE UNGROUNDED POWER CORD THAT CAME WITH THE HEAT GUN)

1 1 1/2" copper elbow pipe fitting

1/4" square tubing - or whatever size you can get

4 machine screws, nuts and washers

Power cord wire hold down clamp - Electrical department

16 gage wire (assorted colors)

16 gage wire nut and wire terminal.

Special Warning! (You must use at least 16 gage wire for the high current side of the circuit)

Smaller gage wire for fan and low circuit side of the circuit

Roll of steel strap (electrical department, Home Depot)

1 3/4" or 2" hole saw

Electrical Components

Q4015LT Triac/Diac (Mouser, Digi-Key or similar)

Heat Sink (nut and bolt to fasten heat sink)
(I used Digi-Key part number HS107-HD)

Special Warning (YOU MUST USE THE HEAT SINK)

.1 uF Capacitor (Ceramic at least 50V) (Mouser, Digi-Key, Radio Shack)

10K Resistor (at least 1/2 watt)

100K potentiometer (variable resistor) (at least 1/2 watt)

4 wire Terminal connector strip (Mouser, Digi-Key, Radio Shack)

5 wire Terminal strip (Mouser, Digi-Key, Radio Shack)

12V DC Fan 60 cm X 60 cm X 25 cm (Mouser, Digi-Key, Radio Shack)

Knob to fit potentiometer shaft (Mouser, Digi-Key, Radio Shack)

Step 2: Prepare Heat Gun

Get you pad and pencil, Always take notes when you take apart things so you know how to re-assemble things. In this case we are interested how the blower and switch are wired.

Take apart the blower and note the wire connectors and switch connections.

We need this information in order to wire the heat controller in to the circuit and to replace the two wire power cord with the three wire (grounded) power cord.

Special Warning (YES I SAID THIS BEFORE, YOU MUST USE A THREE PRONG GROUNDED
POWER CORD)

Step 3: Naked Heat Gun

This is the item we are interested in. Recycle the plastic parts or keep them for plastic welding projects you may want to do in the future.

We are going to use the low side of the heat gun, The red wire connected to the lower terminal of the switch. You can remove the black wire from the high terminal side of the switch.

Note the white terminal connector, one side has a red wire and the other side has a red and white conector. The single red is considered the line or hot wire. The white wire is the neutral side.

The Heat gun does not come with a ground wire because the plastic case is considered insulated from the outside world. For safety reasons we are going to ground the blower in our project.

Step 4: Mount the Heat Blower

I used a 6" X 12" X 1/2" piece of plywood to construct the base of the pre-heater.

Notice how the holes for the blower are drilled to one side of the base board. We need room for the heat controller circuit and the fan.

Bend the strapping around the heat blower to trial fit the proper length. You can cut the strap to size after you test bolt the blower to the base.

Cut the 1/4" square tubing so the blower can sit on top of the tubing

Paint the base if you want.

Assemble the blower to the base board.

Step 5: Mounted Blower

Notice the copper pipe elbow is mounted to the end of the heat gun. The elbow is just press to fit.

Do not over tighten the strapping nuts. You want the assembly to have a snug fit but not too tight.

Step 6: Heat Controller Circuit Part 1

The heat controller circuit can be found out on the web at:

http://home.cogeco.ca/~rpaisley4/x1200WControl.html

Thanks to Rob Paisley who made this very simple circuit. I did a lot of searching and came across many heat controller circuits. This one is very simple to build and works great as a heat controller.

Special Warning. The max wattage for this controller is 1200 watts. If you use the high side of the blower you will exceed the watt rating of this design.

Not to mention that you will cook your circuit board with way too much heat.

I decided not to design a printed circuit board. This design is old school, but is simple to construct and will safely handle the 8 amps of current that the blower requires.

Notice that we are routing the blower to the top of the terminal strip (neutral and then red wires)
The AC power cord is routed to the lower terminal strip (hot, (black) neutral (white)

Special Warning (NOTICE the green power cord ground wire is connected to the aft mounting screw with the blue wire nut and blue terminal connector.)

(YOU MUST CONNECT THIS GROUND WIRE)

Step 7: Heat Controller Circuit Part 2

Compare the wiring diagram on the web page and the wiring on the image.

The voltage drop across the 10K resistor at max blower setting is 14 volts and 1.4 ma.

This is why I can use only 1/2 watt components on the low current side of the circuit. You are using low gate current to control high current across MT1 and MT2.

The 10K resistor, 100K R1, the gate and C1 make of the low side of the circuit.

You can use smaller gage wire in this part of the circuit.

I wired the potentiometer across pins 2 and three. This gives us high resistance with the pot turned all the way to the left, low resistance with the pot turned all the way to the right.

This want we can increase the heat and blower speed as you turn the pot from left to right.







Step 8: Fan Circuit Part 1

Look at the end of the fan motor. You will see 4 diodes. These diodes consist of a full wave bridge rectifier. This is the circuit that converts AC to DC. With the blower running you will have 14 volts DC to power your fan at max setting of the pot.

As you adjust the heat the voltage lowers which lowers the fan speed.

As a future enhancement I plan to wire in a full wave bridge rectifier that is separate from the heat blower so I can maintain full fan speed for cooling.

Notice the red and black wires coming from the end of the blower motor.

Wire your wires from the two solder points across from the line power wires. (not to the same point that the line power are wired to, (LINE POWER WILL FRY YOUR 12 VOLT FAN MOTOR) You want to make sure that you position the cooling fan in the right direction. You want the fan to blow across the Diac/Triac.



Step 9: Fan Circuit Part 2

On two free terminal strips, wire in your cooling fan wires to the blower wires.

I originally wired in two resistors, as indicated in the below image. I wanted to keep the max voltage of the cooling fan to 12 volts. But I later removed them to keep the fan speed up at the lower blower heat setting.

As I indicated before, a later enhancement will be to add a separate full wave rectificer that provides full fan speed.

Again make sure that your fan blows across the diac/triac. Notice in this image that you can see the heat sink bolted to the diac/triac.

Step 10: Build Box Cover

I used 1/4" plywood to build the box cover for the preheat workstation.

Make sure that you cut slots or install vented covers at the ends of your box. You need plenty of airflow to cool the electronics as well as the heated blower.

Use a hold saw to cut the hole for the blower exhaust.

You can add angle brackets to hold the box cover in place.

Step 11: Testing and Clean Up

Plug the Preheat workstation into a power strip with an on/off switch.

Turn the blower switch on before plugging on the preheat workstation in.

Turn on the preheat workstation.

When power is applied to the preheat workstation DO NOT TOUCH ANYTHING ON THE PREHEAT WORKSTATION. Check connections and voltage drops across connections. 120 volts across line current sections.

There should be 8 - 14 volts across the low current side of the circuit depending on the position of R1.

Before you make any adjustment, to R1 turn the power strip off and unplug the preheat workstation.

Adjust R1 and repeat power on and measurements.

After operational check, disconnect all power and tidy up the components.

Notice how I have added zip ties to hold the wires in place.

You do not want any wire coming in contact with the blower.

I used Velcro to connect the fan to the base plate. After final prototype I will fasten the fan to the fan box cover.

Step 12: Using the Preheat Workstation

Start out by setting R1 to the lowest setting. Place your circuit board at least 1.5 to 2" away from the blower exhaust. Let the temperature of the circuit board stabilize. Make minor adjustments to R1.

You want the temperature to be just below solder melt temperature. Then from above the circuit board you apply your solder iron or re-flow nozzle to apply just enough heat to remove the component.

You can make a circuit board rack to sit over the preheat workstation or use a circuit board vise.

Enhancements

Metal Case
Full Wave bridge rectificer for faster cooling fan speed.
Circuit board surface temperature thermocouple sensor.

Enjoy

Thanks

Joe Pitz