How The Pressure Switch On Washing Machines Work



Typically, the pressure switches on washing machines have a big sensing diaphragm with a diameter of about 60mm and three male terminals that allows quick connection.  A single connection is not unusual, but the other two are for normally closed or NC and normally open or NO connections. A mechanism for set-point adjustment is built in (it is controlled directly by the knob labeled ‘water level’) and the switch closes and opens with audible clicks.

It is important to keep in mind that if the clothes washer doesn’t have a mechanical switch but comes with digital controls, it is highly likely that the level of water will be detected using a pressure sensor with electronic variable. It is simple to interface these sensors, but that will be explained another time.

The pressure switches on a washing machine control the cold and hot water inlet solenoids directly. As a result, they are usually rated for currents that are quite high - a typical one is 15A with 230V AC. This means that for DC applications with low voltage, the switches should definitely cope with like 5A or so. It also means that there won't be a need for a relay, for most loads.

The pressure switches used in residential washing machines are really similar to those dishwashers use. One of the main differences is that they come with multiple levels, rather than having a single switching point. For instance, a switch with two positions may trip at 15cm and 20cm of water, while one with three positions may switch at 10cm, 20cm and 30cm of water.

It is important to pay attention as these switches look like the pressure switches of a washing machine a lot. However, they have several electrical terminals (it can be 6 or even 9) and don't come with adjustments for external level, although some can be adjusted with screwdriver for both the hysteresis and the trip levels.

You should be careful when you are extracting the switch from a machine, so that you won't accidentally remove the mechanism for adjustment as it is usually part of what is holding the pressure switch in place. It is pretty easy to identify the switch though. Generally, it is usually behind the adjustment knob for 'water level' in the machine's top control panel. However, if the washing machine is partly destroyed or upside down, trace the sensing tube protruding from the wash tub's base.

While you're doing that, scrounging the tube is also generally worth it. The tube is often a plastic hose with high quality. It might be very useful when you least expect. The pressure switches of dishwashers on the other hand are usually found beneath the steel drum. There is no need to position them close to the machine's control panel because they can't be adjusted externally.

Depending on the application you have, you might decide to make the pressure switch you salvaged operate at different pressures that are not in its standard range. If your washing machine has a pressure switch then it is easy to do, as they are not difficult to modify. When the adjustment control is turned normally, it typically lets the trip pressure be set, so that the depths of water can be detected anywhere from 8cm - 20cm.

However, when the adjustment bracket is removed, the internal spring that sets the sensitivity can be accessed. For example, if you use a spring that is very light (one that provides a force that is just enough to return the diaphragm successfully to its normal position when it removes the pressure), it is possible to make a switch trigger as low as at 5cm of water (0.08psi or ~0.5kPa).

Then, the external adjustment should typically show a range of around 5cm - 8cm of water. However, this may differ based on the spring and the unit used. If a stiffer spring is put in, the range of adjustment will become larger. Although I haven't attempted it yet, it is possible to prevent the pressure switch from closing; till you had the particular pressure you want. However, note that the diaphragm isn't made to withstand these high levels of pressure, so in the long term, you may have some reliability problems.
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Some Common Suggestions:

Many people don't really know what these switches can be used for. In truth, your imagination is the main tool. However, below are some common suggestions:

(a) Controlling A Water Pump (An Electric One)
For instance, keeping a container filled with water. Similar to a washing machine or clothes washer, the water depth can be sensed from the hose that is connected to the container. It has some advantages over a standard float switch and one of them is that you can easily adjust the level.
(b) Providing A Warning For Low Level Of Water
For example, you can use the switch to activate a light or buzzer if the level of the liquid in a tank falls below a certain adjustable point.
(c) Providing Indication For Water Level
For instance, you can use the pressure switch of a dishwasher to activate lights or LEDs to show the level of water in a tank. If you use two switches, you can adjust the set-point of each one and easily have a six-level indication. However, the wires that will be needed to connect the display and the switches will be much