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How Does a Washing Machine Pressure Switch Work?

How Does a Washing Machine Pressure Switch Work?

Typically, washing machine pressure switches have a large sensing diaphragm that’s about 60mm or so in diameter and three quick-connect male terminals. One connection is common, while the others are for normally open (NO) and normally closed (NC) connections. A set-point adjustment mechanism is built in (it’s directly controlled by the ‘water level’ knob) and the switch opens and closes with audible clicks.

(Note that if the washing machine has digital controls, rather than a mechanical switch, it almost certainly uses an electronic variable pressure sensor to detect the water level. These three-wire sensors are easy to interface, but we’ll leave them for another time).

Washing machine pressure switches directly control the hot and cold water inlet solenoids and so are rated for quite high currents – 15A at 230V AC is typical. So for low-voltage DC applications, the switches can certainly cope with (say) 5A. This means that, for most loads, a relay won’t be needed.

Dishwashers use pressure switches that are similar to those used in washing machines. However, instead of having one switching point, they have multiple levels. For example, a two-position switch may switch at 10cm and 15cm of water, while a three-position switch may trip at 15cm, 25cm and 35cm of water.

These switches also look a lot like washing machine pressure switches, except they have multiple electrical terminals (six or even nine) and don’t have external level adjustment. However, some have screwdriver adjustment for both the trip levels and the hysteresis.

When extracting the pressure switch from a washing machine, be careful that you don’t also inadvertently

remove the adjustment mechanism it’s often part of the bracket holding the switch in place! As for identifying the pressure switch, that’s easy. Nine times out of 10, it’s directly behind the ‘water level’ adjustment knob in the top control panel of the machine. If the machine’s upside down or partly destroyed, follow the sensing tube from the base of the wash tub. And while you’re at it, it’s also usually worth scrounging the tube, which is often a high-quality plastic hose. You never know when it might come in handy.By contrast, dishwasher pressure switches are normally buried beneath the stainless steel drum. Because they’re not externally adjustable, they don’t need to be located close to the control panel.

Depending on your application, you might want your salvaged pressure switch to operate at pressures that are different to its standard range. That’s not hard to do if you have a washing machine pressure switch, as they are quite easy to modify.In standard form, turning the adjustment control typically allows the trip pressure to be set to detect water depths anywhere from 9cm to 20cm. But if you remove the adjustment

bracket, you can access the internal spring which sets the sensitivity.For example, by using a very light spring (ie, one that provides just enough force to return the diaphragm to its un-triggered position when the

pressure is removed), it’s possible to get a switch to trigger at just 5cm of water (~0.5kPa or 0.08psi). The external adjustment would then typically give a range of about 5cm to 7.5cm of water, but of course, this will vary depending on the unit and the spring used. Put in a stiffer spring and the adjustment

range becomes larger. While we haven’t tried it, you could probably stop the switch from closing until you had 15 to 20kPa of pressure. Note,however, that the rubber diaphragm isn’t designed to withstand these

pressure levels, so there may be some long-term reliability problems.

So what uses can be made of these switches? That depends on your imagination, but here are some suggestions:

(1). Controlling an electric water pump

– eg, to keep a container full of water. Just as in a washing machine, the depth of the water can be sensed from a hose connected to the base of the container.The advantage over a float switch is that the level is easily adjustable.

(2). Providing a low water level warning eg, the switch could be used to activate a buzzer or light if the liquid level in a tank drops below an adjustable point.

(3). Providing water level indication eg, by using a dishwasher pressure switch to activate LEDs or lights to indicate the water level in a tank. By using two dishwasher switches and adjusting their individual set-points, it’s easy to have six levels indicated. However, you’ll need a lot of wires to connect the switches to your display.

(4). Detecting vehicle movement using a washing machine pressure switch. The switch would be triggered by air pressure when the vehicle crosses a hose. Just remember to plug the end of the hose that’s not connected to the switch! So there are four applications but there are lots more. It’s certainly worth salvaging these very sensitive switches!