Most people who are dependent on caffeine have breakfast with an old friend -- the coffee maker! Every morning you scoop in the coffee, add the water and turn it on, but have you ever wondered what is happening inside? How does the water get from the little compartment where you pour it in to the top of the coffee maker, and what is that gurgling sound?

Now we will look inside a typical drip coffee maker so you can understand exactly what is happening when you make coffee. We will also look at the possible problems that might cause your coffee maker to stop working. By the end of this article, you may look at your old friend in a completely new way!

How Simple!
A modern drip coffee maker is a surprisingly simple device. Manufacturers have had 25 years to hone their designs, so these coffee makers are pretty straightforward once you open them up. Here is the coffee maker that we will be examining today:


The view from the front


The view from the bottom


The warming plate


The place where the hot water drips into the ground coffee

Now let's take a look inside.

What's in There?
If you take off the top of the coffee maker, you find three things:


The view inside the top of the coffee maker

Looking at this picture, you get your first impression that this isn't a high-tech device. If you take the bottom off the coffee maker, here is what you will find:


The view inside the bottom of the coffee maker

The depression on the right-hand side of this figure is the bottom of the bucket. The orange tube in the center is the hot-water tube (it connects to the black tube that we saw in the previous picture). The other orange tube picks up cold water from the hole in the bucket. You can also see the power cord coming in as well as the switch that turns the coffee maker on and off.


The on-off switch

The heating element is an aluminum extrusion with two parts: a resistive heating element and a tube for water to flow through, like this:


The resistive heating element and the aluminum tube to heat the water

Turn the Heat On!
The resistive heating element is simply a coiled wire, very similar to the filament of a
light bulb or the element in an electric toaster, that gets hot when you run electricity through it. In a resistive element like this, the coil is embedded in a plaster to make it more rugged. The heating element has two jobs:

  1. When you first put the water in the coffee maker, the heating element boils it.
  2. Once the coffee is made, the heating element keeps the coffee warm.
In the following pictures, you can see the connection between the heating element and the warming plate. The heating element presses directly against the underside of the warming plate, and a white, heat-conductive grease makes sure that the heat transfers efficiently to the plate. This grease, by the way, is extremely messy (very hard to get off of your fingers!). You find this grease in all sorts of devices, including stereo amplifiers, power supplies -- pretty much anything that has to dissipate heat.

The coffee maker's switch turns power to the heating element on and off. To keep the heating element from overheating, there are three solid-state temperature sensors, as shown here:


Simple solid-state temperature sensors cut off the current when things get too hot.

The device attached directly to the coil is the primary temperature sensor. When it detects the coil getting too hot, it cuts off the current. Then, when it cools down, it turns the coil back on. By cycling on and off like this, it keeps the coil at an even temperature. The other two devices appear to be thermal fuses. They simply cut power if they sense too high a temperature. They are there for safety reasons, in the event that the main sensor fails.

One part that is not visible in this coffee maker is a one-way valve. This valve is either in the hole in the bucket or in the aluminum heating pipe. If there were no one-way valve, then the boiling water would be just as likely to flow back into the bucket as to rise up the black tube. The one-way valve lets cold water into the aluminum tube, but forces the bubbles of boiling water to flow up the black tube. If you blow on the tube leading into this value, the valve should be open. If you inhale through the tube, the one-way valve should block any air.

A Cup of Joe
You can see that a coffee maker is about as simple as an appliance can get! Here's how it works:

  1. When you pour in cold water, it flows from the bucket through the hole in the bottom of the bucket and into the orange tube.
  2. The water then flows through the one-way valve into the aluminum tube in the heating element, and then partially up through the black tube. This all happens naturally because of gravity.
  3. When you turn on the switch, the heating element starts heating the aluminum tube, and eventually the water in the tube boils.
  4. When the water boils, the bubbles rise up in the black tube. What happens next is exactly what happens in a typical aquarium filter: The tube is small enough and the bubbles are big enough that a column of water can ride upward on top of the bubble.
  5. The water flows out the end of the black tube to drip into the coffee.
This boiling-water pump, by the way, is the same mechanism that drives a percolator-type coffee machine.

As you can see, there is no mechanical pump of any type and really no moving parts (except for the moving portion of the one-way valve). This makes coffee machines extremely reliable....

A Cup of Woe
... But things can still go wrong. Here are some of the most common problems that can cause your drip coffee maker to stop working:

There are two problems that are nearly impossible to fix:
  • Failure of one of the heat-sensitive switches
  • Failure of the heating coil
Because it's nearly impossible to get replacement parts, you probably have to buy a new coffee maker if one of these problems disables your machine.

Now, when you make coffee tomorrow morning, you can do so with a new appreciation for exactly what's happening inside!

For more information on coffee makers, coffee and related topics, check out the links on the next page!

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