Fog Machines are Dumb
Also, here’s how they work
The typical fog machine is an extremely simple device. It consists of a big aluminum block (that acts as a reservoir for heat) with a heater attached and a metal tube running through it, and a pump for a water / glycol mixture. When the aluminum block is heated to around 220˚C, the pump can be activated, passing glycol mixture into the hot tube and shattering it into tiny droplets. What comes out the front of the fog maker is tiny droplets of glycol and water vapor.
However the typical design (which has not changed in about 25 years) has an extremely annoying flaw, all in service of saving about $2.
Why they’re dumb
To understand this flaw we need to first look at the simplified electrical schematic.
Shown above is a heater followed by two thermostats (only the first is used in typical operation; the second is for safety and is not connected to the aluminum block but rather to the chassis). The pump that controls the glycol flow is in a circuit in parallel with the thermostats.
When the machine is turned on, the thermostat switches are closed (because they are not yet heated to their turn-off temperature) and so current flows through the heater and back/forth to neutral. Note that the pump cannot be activated, because its circuit is in parallel with two switches that are both closed and will hog all the current.
When the aluminum block reaches temperature, the first thermostat opens. At this point the pump can be activated because it is no longer shorted by the thermostats. What might be surprising is that when the pump is activated, current is still flowing through the heater! However the heater’s resistance is so low that the pump doesn’t really notice, and the heater doesn’t emit significant heat because the current is limited by the pump’s relatively high resistance.
So what’s the problem?
This is kind of an elegant, inexpensive design in that it achieves thermostatic control of the heater, and also manages to inhibit the pump when the temperature is too low (can’t have a jet of cold fog fluid shooting out the front) with only a single required thermostat component (ignoring the safety).
But look at what happens to the temperature over time. When the off setpoint is reached, the temperature starts declining, and when the on setpoint is reached the temperature increase again. However because of how the circuit is designed as we just discussed, the pump cannot be activated when the heater is on.
The ramifications as a user are that
When you press the fog button, you have no idea whether it’s going to work (because you might have caught the machine during it’s heat cycle [the non-yellow portion of the curve])
The duty cycle is terrible (because if you press the fog button for a short duration, thus accelerating the cooling, the machine will not immediately start to heat back up because it first needs to reach the on setpoint)
The solution
The solution is for the companies making these foggers to add an additional thermostat that is used solely to inhibit the pump, solving both the unpredictability and duty cycle problems. They already have control circuit boards (which they did not 20 years ago) so the change is literally only the cost of a $2 max thermostat component. There do exist professional foggers (~$500) that have solved this problem, but $500…
I have also hacked my own fogger to solve the problem. Details soon!