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I'm working on a pet project of mine, creating a coffee roaster, where I'm using a heat gun as the heat source. To improve my control of the heat output of the gun, I've set up an Arduino to control a solid state relay (SSR), that in turn (if I get it to work) should control the heating element of the heat gun, by switching it on and off in intervals (high power => longer periods of on, low power => longer periods of off).

Now.. For this I bought a very simple heat gun with two settings - high (high temp, high fan speed), and low (lower temp, lower fan speed). I opened up the heat gun, and it seems that the motor circuit (AC->DC rectifier + motor) is using the heating element as a resistor(!). This means that I'm not able to isolate the circuit for switching heat on/off from the motor.

I've tried to make a circuit diagram (I'm not an electrical engineer, hopefully it's understandable anyway). I THINK I've managed to get it fairly correct.

In this diagram, the whole motor circuit including rectifier is the "M". The heating coils is divided into three. I've attempted to figure out the values as well by measuring and some calculations:

  • R_a is the largest and is used both on setting 1 and 2. I believe this to be 22 Ohms.
  • R_b is only used on setting 2 and is a small section of the same coil as R_a. Think this is 27 Ohms.
  • R_c is a thinner, smaller coil used on only setting 1 (lowest). I believe this is 26 Ohms.

(PS: Take the Ohm values with a grain of salt - but they seem to check out)

Diagram of heat gun circuit

My question is: Is there any hope for me to connect my solid state relay to control ONLY the heating coils, without disturbing the motor? I don't really need the lowest setting - I'm going to use the fan on highest, and simply control the power to the heat coils using my SSR.

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  • \$\begingroup\$ I think the science is flawed. The air temperature gradient depends on flow rate , heat loss and heat conduction to the moving beans. The time duration is also important. I suggest if you have excess heat, to regulate the temperature with an additional blower source to cool it. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 12 at 14:29
  • \$\begingroup\$ Well the roaster is a heat gun + bread machine, so the heat applied is mostly convection, and the beans are heated directly from the heat gun, not as much through the temperature inside the roasting "chamber" itself. The goal is to be able to control the heat output more precisely. Using my Arduino for this allows me to also control this from the roasting software I'm using (Artisan). \$\endgroup\$ – Hallgeir Oct 12 at 16:15
  • \$\begingroup\$ Best roaster I've seen was a dry cast iron skillet under a fume hood or BBQ outside. youtube.com/watch?v=cEbMNpKqWL0 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 12 at 16:19
  • \$\begingroup\$ or popcorn maker youtube.com/watch?v=skZk0WHHDvM \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 12 at 16:25
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Your analysis is inconsistent with your diagram. The motor is in parallel with Rb and in series with Ra, so both of these resistors are used in both settings. These two elements function as a voltage divider for the fan, so no, there's no way to separate its operation from the heating function without providing it with a separate power supply altogether.

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  • \$\begingroup\$ Hm, yes, you are absolutely correct. Thanks for pointing that out! Then it's likely that the resistor values are off as well.. Anyhow, I'll try to find a different power source for the motor. Thanks! \$\endgroup\$ – Hallgeir Oct 12 at 16:04
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You would have to do some experimentation as to what it takes to power the motor sufficiently but it may be possible to re-arrange the heater coil connections like as shown below to just have the motor run at one speed separate from the main heater connections.

enter image description here

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  • \$\begingroup\$ It has crossed my mind. However, my worry here is that either the motor or Rc will get a considerably higher voltage than they are rated for, since Rc is always in serial with Ra when it's in use. I will try to do some more calculations to make sure however.. though I am leaning towards just using a different power supply for the fan altogether... \$\endgroup\$ – Hallgeir Oct 12 at 16:07
  • \$\begingroup\$ @Hallgeir - This is why I said you would have to do some experimentation. What I show may very well make the motor run much faster than you want. As for voltages...check the labeling on the motor to see what it is rated for. If 230-240VAC the motor should be fine and it would depend on what speed you are willing to deal with. \$\endgroup\$ – Michael Karas Oct 12 at 16:13

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