How Does A Washing Machine Pressure Switch Work?

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Typically, washing machine pressure switches have a big sensing diaphragm that is 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 inbuilt (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 an additional time).

Washing machine pressure switches directly control the new 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. Which means, for many loads, a relay will not be needed.

Dishwashers use pressure switches which are similar to those utilized in washing machines. However, instead of having one switching point, they have multiple levels. For instance, 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 so much like washing machine pressure switches, except they’ve multiple electrical terminals (six or even nine) and haven’t got 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 is often a 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 the wrong way up or partly destroyed, follow the sensing tube from the bottom of the wash tub. And while you’re at it, it’s also usually worth scrounging the tube, which is usually a high-quality plastic hose. You never know when it would are available handy.By contrast, dishwasher pressure switches are normally buried beneath the stainless steel drum. Because they don’t seem to be externally adjustable, they do not must be located close to the control panel.

Depending on your application, you might want your salvaged pressure switch to operate at pressures which 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 when you remove the adjustment

bracket, you’ll be able to access the interior spring which sets the sensitivity.For example, by utilizing a really light spring (ie, one that provides just enough force to return the diaphragm to its un-triggered position when the

pressure is removed), it is 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 spread of about 5cm to 7.5cm of water, but after all, it will vary depending on the unit and the spring used. Put in a stiffer spring and the adjustment

range becomes larger. While we have not tried it, you may probably stop the switch from closing until you had 15 to 20kPa of pressure. Note,however, that the rubber diaphragm is not designed to withstand these

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

So what uses may be made of those switches? That depends on your imagination, but listed below are some suggestions:

(1). Controlling an electric water pump

– eg, to keep a container stuffed with water. Just as in a washing machine, the depth of the water might be sensed from a hose connected to the bottom of the container.The advantage over a float switch is that the extent is well adjustable.

(2). Providing a low water level warning eg, the switch may very well 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 point the water level in a tank. Through the use of two dishwasher switches and adjusting their individual set-points, it is easy to have six levels indicated. However, you may need loads of wires to attach the switches to your display.

(4). Detecting vehicle movement using a washing machine pressure switch. The switch could be triggered by air pressure when the vehicle crosses a hose. Just remember to plug the top 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!


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