I disassembled the roaster today and took many photos, so I will guide you through the photos, show you the humble circuit we made and then show you the schematic for it (which is nothing but the example application circuit mentioned in the data sheet of the LM150 component). The amount of detail someone is interested in will necessarily vary depending on the person, but I tried to cover as much detail as possible, so that more people can benefit from this post. For the length of time I have been going through roaster modifications made by others, I always wished if those people could provide as much detail as possible, so I guess this is what I am trying to do here.
Front and back views of the machine before disassembly.
This is the fan control circuit, with a heatsink attached to the LM150 component.
Next to the fan control circuit is the 1000W dimmer switch, with a heatsink added to it as well (will be shown clearly later when I take the dimmer out).
The cooling fan, which sucks air right off between the plates of the dimmer's heatsink, and also aids in the cooling of the fan control circuit on the other side (the cooling fan, the dimmer and the fan control circuit are all on the same line).
Note that the type of terminals used for motor wires can still be inserted successfully even though there is a capacitor welded into the terminals of the DC motor (this 100nF capacitor was advised in the data sheet for the linear voltage control circuit).
The original heating assembly (not shown, enclosed inside) is arranged into two heating elements rather than one, which both serve as a simple voltage divider that can be used to produce an unregulated 12V supply. To those who don't know what a voltage divider is, there are two heaters inside the pop-corn popper, one much smaller than the other. By passing electricity through the path composed of the big and the small heaters, the manufacturers of this cheap appliance could extract a low voltage from in-between the heaters, which can be used to operate the low-voltage (12VDC) motor. In my case, I deal with the two heaters as one big heater, pass the overall current through them both, and neglect the middle wire which was originally used to extract the low voltage from in-between the two heaters (this is the wire insulated with red tape).
These are the terminals of the AC panel current meter. Given that the current to be measured is alternating (AC), there is no point of trying to connect the terminals in a crossed fashion like this. I was too tired at the time of assembling the roaster so I did this stupid mistake. This simple mistake lead into one of the wires getting above the other, which in turn caused it to touch the heating chamber. When I examined the machine, the thermal sleeve protecting the wire had a hole at the point of contact with the heating chamber, and the wire was about to start deteriorating at that point. However, I was lucky enough to open the machine up for this photography session and to replace the thermal sleeve/jacket before this happens.
The crowded interior of the machine.
There is also a black dot at the thermal sleeve that protects the wires of the fan speed potentiometer. This also has been a point of contact with the perimeter of the heating chamber.
Oops, too crowded, about to touch the insulation of the switching power supply (switching power supplies are noisy units which is why they are sometimes covered with metal sheets to shield electromagnetic radiation).
The dimmer switch taken out (front panel and original knob removed of course).
We had also to bend the end of the dimmer's metal plate (which serves as a heatsink) to make the dimmer switch fit in the roaster.
I sawed the side of the plastic casing, namely the side where the power component is located (which is the component enclosed in a TO-92 package and screwed to a metal plate that supposedly acts as a heatsink). After sawing the side to expose the back of the power component, I attached an additional heatsink as shown next.
This is the type of heatsink I used. I used a paper clip to mount it and a high density thermal compound to enhance the thermal conductivity between the back of the power component and the added heatsink. Note that the cooling fan is directed to this added heatsink. The continuous stream of air will continuously change the air between the plates of the added heatsink, achieving a very good cooling capability.
The fan control circuit taken out of the roaster, along with the terminals I used to connect the motor to the case of the LM150 component (in the case of the TO-3 package shown, the package case serves as the output voltage terminal which necessitates a reliable connection such as the one shown here).
The interior of the roaster without the dimmer switch.
Due to the fact that my DC power supply provides 15.5 volts whereas the cooling fan requires 12 volts, I used a resistor in series with the fan to pass only 11.5 volts to it. The resistor is covered by a heat shrink tube and thus cannot be seen in this photo. We determined the value of the resistor by connecting resistors of different values and measuring the voltage across the fan using a multi-meter.
Needless to say, someone can use any knob he pleases for his own invention, which I believe is one of the most amusing choices in the process. I originally used the classic knob shown at the back but decided recently to pick the one made of aluminium. This knob is much better because it is larger and thus allows me to control the heat more easily (due to the use of a cheap dimmer switch, very small moves can sometimes lead into big differences in the current).
The new knobs, much easier to handle though the roaster looks now like a stereo.
All inside again, can't believe it.
The application circuit I used, which is very simple (only the LM150/LM350 component is needed along with a heatsink and two resistors). Note that to obtain the voltage range someone wants (such as the range from 4 to 12 volts for example), some simple calculations needs to be performed to induce the values needed for the resistors. In general, R1 will be 240 ohms whereas R2 will be broken into two parts: a fixed-value resistor in series with a potentiometer (a variable resistor). The values for the fixed resistance and variable resistance will necessarily depend on the minimum and maximum voltages someone wishes to obtain. Note that this circuit doesn't allow you to shut the device you are controlling completely (i.e. it doesn't allow you to go below 1.25VDC), which has not been a problem in the case of the fan control circuit because I needed a considerable minimum voltage to secure the heating element from damage. Beyond this, however, the circuit is very linear and hence is a pleasure to deal with. The LM150/LM350 components can pass up three amps of current, but there are similar components that can pass a guaranteed five. Note that in either case, there are also some calculations that need to be done to determine the type/size of the heatsink that needs to be used. In my case, the component was dissipating 4W at the worst case which means that a small heatsink such as the one shown could do, however, this could change based on the dissipation.
I am very sleepy, so I think I will go to sleep. In case someone needs any help with this circuit, I will be more than happy to help him with the calculations.