There have been many times when I have wished that I had a hot air heat source that could be controlled. The rework station has a controlled heat source but it will not work below 100 C. For this project I have decided to build a modified hot air gun that will have controlled temperature output between room temperature and 350 C. This should be very useful for testing and calibrating devices using thermistors. I will begin by hacking my inexpensive heat gun from Spark Fun.
https://www.sparkfun.com/products/10326
I began by opening the gun and removing the connection of the motor from the heater taps. The common practice in these devices is to put the mains across the heater element and then take a tap off the heater element around the 15 volt level, rectify it and use it to power the fan motor. For this project I wanted the fan to be tied to a fixed 12 volt source and independent from the variable AC voltage I will put on the heater element. Once the motor was cut loose from the heater element taps I removed the diodes from the back of the motor housing and installed a length of high strand flexible wire. Later I would pull a silicone sheath over the heater and motor wires to make the cord from the control to the gun functional. You will see this cord in later pictures.
The heart of this project is a small variac, about the size of a soft ball, that I salvaged from an old Ritter Dental X-ray.
Besides the full 0 to 110 volt sweep contact this unit also had hard taps at 8, 17, and 55 volts. My plan is to rectify the 17 volt tap and use it to drive a 7812 regulator which in turn will provide the power for the fan in the gun. My test of the fan motor showed that it drew 0.25 Amps at 12 Volts and ran with enough RPM to serve my purpose. The 7812 can handle 1 amp when heat sunk so with a small radiator it should be able to dissipate the watt or so it will be required to shed.
It is not my intention to build a precision device but I do want to have some indication of what my input settings are so I can estimate the output temperature. Therefore an AC voltmeter will be used to monitor the voltage that is powering the heater element. I will combine the voltage with an empirical listing of approximate output temperatures on the dial of the variac to get my output temperature as close as practical to my target temperature. There are many variables that will contribute to the output temperature of the air. I have attempted to limit and control some of the variables such as the fan speed and the voltage to the element but ambient temperature will also be able to influence the output. I will be using a small 0 to 55 volt 4 digit 3 wire digital voltmeter module to display the AC volts.
The challenges in using this module are that it is DC, doesn't read directly as high as I want to go, and requires an isolated power supply from the variac to do what I want it to do. I experimented with using the 8 volt and 17 volt taps on the variac to power it but I was not able to get it to zero. Finally I took an old linear 7 volt wall wart, broke it apart and used it to supply the power for the LEDs and logic. This allowed me to get accurate readings on the sense line. In the final design I used a 100K potentiometer as a voltage divider to to bring the voltage to the meter down one magnitude and then manually moved the decimal to properly reflect the real voltage. With a 4 digit display and no need for that much accuracy it left me with a good solution and the meter now displays 0 to 110 volts while actually sensing 0 to 11 volts.
Before I began the actual build I wanted to do some bench testing and to run some empirical measurements of current versus output temperature. For those who feel my bench is always too organized here is proof to the contrary.
While I am working with some mains voltages in these tests I have taken special precautions to use isolated sources and good test procedures. Unfortunately sometimes there is just no way to make it look nice and get results in a reasonable time frame. Here is a graph of the Temperature versus Current characteristics of the heat gun. I have a very inexpensive two channel thermocouple based thermometer. I tested the thermocouple for accuracy using boiling water and ice in water. It actually tested extremely accurate considering it only cost $13.00. I placed one of the thermocouples 2 cm in front of the outlet of the heat gun and taped it in place. I wanted my readings to be consistent. Finally I fired the test up and took readings at 10 volt increments from 10 to 100 volts. I also used a second also inexpensive thermometer and you can see the difference between the black and red graph lines.
This will give me a rough estimate of my temperature output based on the voltage shown on the meter. Once I was convinced that things would work as envisioned I began the build. I had to make the small circuit for powering the fan motor and also one to adapt the AC voltage from the variac sweeper to what would be acceptable for the meter. The small isolated power supply for the meter's electronic was already built. Here is the motor board in progress.
and the salvaged wall wart:
Here is the schematic for the entire unit:
The construction preceded with the customary glitches. I had a bout of dyslexia and hooked the variac up backwards on the first test. I put way too much voltage on the fan motor for a split second and had to make a repair to the motor board. By the time I got things straightened out it was 3:30 AM. Once I knew it was working I went to bed and planned to put the finishing touches on it in the morning which begins about 10:00 AM for me. Here is the finished unit. The little thermocouple thermometer is still attached in the picture as I was running final tests and I wanted to place temperature labels on the face of the unit so I wouldn't have to use the graph to convert voltage to temperature. Here is the finished unit.
Finally I took a short video of how it works.
John
Top Comments