How Does a Dumb Moccamaster Coffee Machine Detect Scale and Light a Lamp?  - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: October 07, 2023, 11:16:55 am
content: My best friend brought over her Moccamaster coffee machine for me to fix. She's had this machine for over 7-yrs and it finally went belly-up and refuses to even heat. Since The Moccamaster is sold for over USD 200, I can understand how it might be worth fixing.I found the problem, a quick-connect wire connector was making a bad contact on the mating connector of the copper-heating coil thermostat and caused the female quick-connector to overheat and finally the wire connector became brittle and corroded from heat cycling and broke off the female connector's wire.So, now it works!I've taken it apart and see it has at least two thermostats but it does have a custom toggle power switch that has a built-in 5-min timer(it is custom switch and so labeled). This push toggle switches has four wires attached.My Questions:The one thing that confounds my mind is how does this thing light a neon scale light that stays lit when there is too much scale but does this before the out-of-water thermostat trips and turns off the main copper coil heater?There are twp neon lamps One built into a custom power switch(with an internal IC timer) and the other one is by itself on the control panel that indicate a scale problem.Both neon lamps appear to be connected to thermostats. One is on the heating coil and one on the metal base of the water boiler.Here's how I see it:When started, the main heater heats the water to boiling slowly with full AC power. The temperature of the copper coil heater then stays near water boiling temperature because water always boils at a near constant temperature and this cools the heater element.When the water reservoir becomes empty, the water heating coil rises quickly above 100 DegC because the heating is not regulated by water being boiled. This triggers heater power turnoff that stays latched until the machine is turned of for a cooling down while.I can see two thermostats on the main water heating asm.Question: Given that the main heating coil does not rise above a certain temperature, the excess scale buildup light latches and stays on with too much scale, yet it is not triggered/lit due to the heating element running out of water and turning off at a higher temperature in normal operation. How does this work?I am puzzled. A 5-min delay before turning off the main heater would probably cause a meltdown?

author: mzzj
author_rank: Super Contributor
timestamp: October 07, 2023, 04:30:02 pm
content: bit of guessing here: out of water is actually steam-operated something like in water kettles with the difference that flowing water cools or blocks the steam from the thermostat.Scale indicator activates if thermostat mounted right on heating coil trips before steam thermostat in the input pipe. I'm guessing in that case totally empty coffee machine would also give scale indication.(Ever wonder how electric kettle "thermostat" works? Temperature based thermostat would boil dry in Colorado since boiling point is lower at higher altitudes. Instead of relying on temperature it is triggered by the steam from the boiling kettle piped down to the thermostat switch)

author: SuzyC
author_rank: Frequent Contributor
timestamp: October 08, 2023, 08:55:07 am
content: Thanks mzzj, but an idea of Moccamaster operation is still a little uncooked in my mind, though it is still percolating in my mind towards a water-clear conclusion.Hmmm, I see the water in the reservoir filling a copper tube to the base of the evaporator. There is some sort of one-way valve that prevents the boiling vapor from pushing the water back up the water feeding tube that heats water to a percolating vapor.The water heating tube is surrounded by the coiled heating element and a thermostat mounted on it would not trip and latch while water is still being let in and percolating steam up the tube that pushes water up the tube by a slow stream of pretty bubbles. This bubbling boiling water travels over a rectangular hollow connecting bridge to reach a point where there are 9 drip holes and these holes are positioned right over the center of an inverted funnel shaped ground coffee chamber that sits above the carafe.My theory. as scale builds up, the temperature of the heating element rises above the normal temperature needed to vaporize water to percolate. If this abnormal heating coil temperature rise reaches a critical high enough temperature(but below the main heater cutoff temperature), the scale light thermostat turns off all heat and lights the scale neon bulb because the bulb is connected (with a series resistor, of course) directly across the scale thermostat which has been tripped. This open-circuit puts full line voltage across the thermostat and lights the neon scale lamp.The main cutoff thermostat is not ever actuated until the reservoir goes dry and then the heating coil rises very quickly to the trip point, else heating always ends if the 5-min brewing time limit is reached.What makes this excessive scale scheme work is that the scale thermostat is connected in series with the main heating coil-mounted thermostat, but has a much slower over-temperature tripping time than the main heating coil thermostat, so the scale thermostat mounted on the base of the water heating tube asm takes some time at an abnormally high water temperature to trip.So, now I can see that this whole brewing circus is time limited. The 5-min IC timer built into the power switch limits any brewing session to 5-mins without any scale problems, regardless of the number of cups of water that fills the reservoir over the heating tube.Am I right? I donno. I don't drink coffee and the secrets of operation of this magic machine is news to me!