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I am taking apart an electric pop corn popper in hopes of isolating the fan motor from the heating element. I'm wondering how to re-configure this so that the following would be true.

If the heating element is on, the fan is on The fan can be on even when the heating element is off

Ultimately I want to wire in two relays to control the two elements separately. Also, I would potentially like to control the fan speed, what would be the best way to do this .... A rheostat?

enter image description here enter image description here enter image description here enter image description here

I've taken a closer look at this, and I'm having some trouble understanding how it works, I know that it's been said that the motor and the secondary heating coil are in series, but from my tracing of the wire it looks to me like they are in parallel, pleas see image below ... does that look like a series connection to you?

enter image description here

UPDATE

Ok, So I got another $15 popcorn popper, one that is very common and it seems to be wired in a similar but slightly different fashion, but It's still not quite clear whats going on please see bellow for new photos and diagram.

From what I can tell, it seems like both of the coils and the fan would switch off when the thermostat reaches temp ... this is not how it functions however, the fan stays on the whole time.

The bridge rect and motor Layout of the feeds Two Coils (Both connected to same spot on one end ) The Resistor? seem to be in parallel with thermostat Thermostat and Thermal fuse Diagram

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  • \$\begingroup\$ A rheostat would keep things simple, but if possible you can use PWM to vary fan speed. By varying duty cycle, fan speed can be varied. But for this, do you have some sort of controller? \$\endgroup\$ – Harshad D Aug 25 '16 at 16:57
  • \$\begingroup\$ You could just get an AC/DC transformer and wire that directly to the motor, then have one relay switching the DC to the motor on the transformer and the other on the rest of the popcorn maker electrics. Ensure only one relay is every switched on. You then have the safety of always having the motor running with the heating elements, but you have the option to run the motor on its own directly from the transformer. \$\endgroup\$ – Will Calderwood Nov 6 '18 at 21:42
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There appears to be two heating elements, one is in series with the fan motor. The fan motor is most likely a permanent magnet brushed DC motor which runs on a much lower voltage than the line voltage. I also see no switch so I assume the plug is the only means of power control.

Schematic I see:

schematic

simulate this circuit – Schematic created using CircuitLab

It's not a perfect schematic but you get the idea of how the motor is connected. Pay no attention to the values in my schematic. In the above schematic, the motor is L1, BR1 are the four diodes soldered to it, and R1 and R2 are the heating elements. The heater has three wires, red, black and white. Black is the common lead which appears to connect to the incoming hot lead. The red lead is wire in my schematic between R1 and BR1. We'll call that the series heater. We'll call the remaining heater attached to the white lead the main heater. The main heater white lead connects to the line neutral along with the other side of the motor.

The motor power supply is about as quick and dirty as you can get. Instead of a proper DC supply, they built a bridge rectifier out of diodes directly soldered to the motor and put it in series with the series heater. It acts as a dropping resistor to reduce the voltage across the motor. Since the dropper is part of the heater, the waste heat is used to help heat the popcorn.

First, get a volt meter across the wires attached to the diodes and not the motor terminals. Plug it is and measure the AC voltage across them. That measurement will be the RMS AC voltage across the motor which also happens to translate directly into the DC value you will need to run the motor. Why the direct conversion of values and why not multiply the AC by sqrt(2) to get the DC value? Since there is no DC filtering or smoothing, the raw rectified AC is fed into the motor. This crude DC voltage constantly varies from 0 to peak and back to 0, 120 times per second. The RMS value of that unfiltered DC is the same as its incoming AC value. So the unfiltered DC RMS voltage does the same amount of "work" as the equivalent filtered DC voltage. E.g. if you measure 12 volts AC across that motor, 12 volts from a proper DC supply or battery will give the same amount of fan power.

Next, subtract the motor AC voltage from the line voltage (120V or 230V depending on where you live) which will give you the series heater voltage.

Now measure the resistance of just the series heater by disconnecting it from the motor and putting an ohm meter across the red and white wires (make sure it's unplugged first!). Now you can find out the rest of the values using ohms law. The current of the string well call Imotheat. So Imotheat = (Vline-Vmotor)/Rheater

For you to control the three devices (fan and two heaters) separately, you need to figure out how to power the motor on DC and then use PWM control for speed. Then you need to figure out how to power that second heater at a reduced voltage since it doesn't have the motor attached anymore. Either a bucking transformer setup (auto-transformer) or resistor will work for the heater but you will have to deal with the resistor waste heat which can be many watts. The third heater which receives full voltage is the easiest and can be controlled by relay. This is not hard but you need a lot of external components to get the voltages where they need to be.

A very simple way to do this would be to put a relay or switch on the single heater and a triac dimmer circuit in series with the motor-heater circuit. You can vary the motor speed this way but the series heater will still give off some heat depending on the dimmer setting. So you don't have full heater control but it's much easier to build.

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  • \$\begingroup\$ I think this makes sense, lets forget for a minute about fan control, if I power the fan using a 12v DC power supply, and disable the smaller heating coil, how would I then be able to control the main heating element. I was thinking that the best way would just be to toggle the coil on and off to maintain a given temperature, but I'm not sure if this would prematurely cause fatigue in the heating element, I think this is how my modern electric stove works. \$\endgroup\$ – user379468 Aug 30 '16 at 13:22
  • \$\begingroup\$ This is a tricky thing to do. The popper is simple yet cleverly designed to work at a single setting. The heat doesn't build up in the element because the fan runs continuously. The fans airflow and heater output have an established equilibrium which maintain a specific temperature which is part of the design. I doubt you will get fatigue in the element as long as you don't let the element heat up well past the equilibrium temperature. You may want to measure that with a good thermocouple or non-contact IR probe. Then use an on-off controller with hysteresis or a PID loop controller for temp. \$\endgroup\$ – Mister Tea Aug 31 '16 at 19:18
  • \$\begingroup\$ Yes, a PID was what I was thinking, I also wonder if adding some sort of thermal mass to the cup would be potentially be a good or bad thing ... \$\endgroup\$ – user379468 Sep 6 '16 at 18:45
  • \$\begingroup\$ I looked at the wiring and I'm not so sure the motor is in series ... please see added photo \$\endgroup\$ – user379468 Sep 6 '16 at 20:50
  • \$\begingroup\$ The plot thickens! Where is that thermostat and thermal fuse in the pictures? I think we need to see where the red, white and black wires go. So we need at least three more things: picture of where those three wires go, picture of the coils and thermal fuses, cutouts, and finally, label your schematic's wire colors. \$\endgroup\$ – Mister Tea Sep 14 '16 at 13:37
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Ultimately I want to wire in two relays to control the two elements separately. Also, I would potentially like to control the fan speed, what would be the best way to do this .... A rheostat?

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Cascaded relay contacts ensure the heater can't run without fan power.

I would be cautious about your plan. The popper may be designed to rely on the fan to keep the casing cool. Reducing fan speed may cause overheating. For that reason I suggest ensuring that the fan power is at least on before the heater can be powered.

Since the motor is bridge rectified DC you could get a very simple voltage reduction by breaking one leg of the bridge with a switch. This would make it a half-wave rectifier. Lift one end of any of the diodes and connect it back to the terminal via a switch to select full or half-power.

schematic

simulate this circuit

Figure 2. Breaking the full-wave bridge at any point will make it a half-wave rectifier.

enter image description here

Figure 3. AC, half-wave rectified DC and full wave rectified DC. Source: Wikibooks.


UPDATE

Mister Tea reckons one of the elements is in series with the motor as a voltage dropper. (This is horrible.) Charles points out that my Figure 2 won't work as it will reduce the power in the element.

schematic

simulate this circuit

Figure 4. Another horrible circuit. This time we optionally short out the motor on one half-cycle with a diode. The motor will now get the voltage shown in Figure 3. The heater will get a slightly higher current (due to full voltage being applied to it) on that half-cycle.

I am not recommending any of these circuits!

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  • \$\begingroup\$ Good point about letting the fan run continuously. Without the fan the heaters will build up heat until the unit melts or catches fire. \$\endgroup\$ – Mister Tea Aug 25 '16 at 19:18
  • \$\begingroup\$ If @Mister Tea is correct (and it appears that he is) about the fan motor being connected is series with one of two heater elements, this scheme won't work. \$\endgroup\$ – Charles Cowie Aug 26 '16 at 21:15
  • \$\begingroup\$ Correct, thanks. I hadn't spotted the series connection although it had occurred to me that the motor didn't look like it was suited to even 120 V. See the update. Still an awful design. \$\endgroup\$ – Transistor Aug 26 '16 at 21:48
  • \$\begingroup\$ what about running the fan at full speed, but changing the power to the primary heating coil ... actually was thinking about turning it on and off to get a semi constant temperature, rather than trying to attenuate the power going to it. I'm just wondering if it will cause the heating coil to fail .... \$\endgroup\$ – user379468 Aug 31 '16 at 18:20
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I am taking apart an electric pop corn popper in hopes of isolating the fan motor from the heating element.

It appears that there must be two heater elements, one of which is wired in series with the motor so that an inexpensive, low-voltage DC motor can be used instead of a more expensive universal motor. If that is the case, the motor can not run isolated from that heater element. The photos seem to indicate that is the way the wiring is connected and the motor does not appear to be one that can run on 120 or 240 volts AC.

I had previously posted a circuit showing how a motor that can be connected directly from the AC supply voltage can be run isolated from the heater. I am now removing that.

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  • \$\begingroup\$ This wont work because the fan is in series with one of the heating elements which means the fan operates at a reduced voltage. \$\endgroup\$ – Mister Tea Aug 25 '16 at 19:16
  • \$\begingroup\$ It is astounding that the motor is in series with one of the heating elements, but it certainly appears to be true. \$\endgroup\$ – Charles Cowie Aug 25 '16 at 20:26
  • \$\begingroup\$ Sometimes it's simple shortcuts like this that actually make me smile. This scheme would be laughed at but the waste heat from the motor dropper is used to help heat the popcorn. It's so simple yet practical and easy. \$\endgroup\$ – Mister Tea Aug 26 '16 at 21:27
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    \$\begingroup\$ Designers of consumer products learn to be pretty clever about cutting cost. Think about what was saved using a 12V PMDC motor and 4 diodes vs a 120 volt universal motor. \$\endgroup\$ – Charles Cowie Aug 26 '16 at 21:42
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    \$\begingroup\$ This is basically the design of all hair dryers I've seen too. Heating element, bridge rectifier and DC motor. They've even gone backward- they used to use packaged bridges (eg. W04) but discrete diodes are slightly cheaper. \$\endgroup\$ – Spehro Pefhany Sep 19 '16 at 21:48
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Operating the existing fan separately without extra circuitry is almost certainly "undoable" as proposed.

The fan is in parallel with the little coil and the combination is in series with the big coil.

It is likely that the little coil handles most of the current from the big coil and that the resistances are scaled so that slightly more voltage than is required by the fan motor appears across the little coil if the fan is removed.
Then adding the fan across the little coil diverts some current via the fan and lowers the little coil voltage slightly so that fan voltage is now 'about right'.

As the fan is designed to run from the low voltage provided by the above system it cannot be operated directly from mains. If you really had to separate fan and heater operation with the existing fan you could

  • Add a phase controlled TRIAC circuit (similar to a typical lamp dimmer) to the fan circuit to achieve a suitably low voltage.

  • Possibly use a series Y rated capacitor in series with the fan + bridge circuit to supply required current to the fan. The size of the capacitor depends on current required and mains voltage. Ask if interested.

An easy option may be to replace the fan with a mains rated unit.


Operation of the fan when the element is powered can be achieved by feeding the fan circuit via eg TRIAC powered when the main circuit is powered. This is the solid state equivalent of adding a relay.
Or a diode bridge could be used to produce rectified half sinusoids at mains amplitude and an SCR phase angle control used for fan voltage provision.
Some extra thought needed for fan or fan + main powering but the above should be workable.

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