Pressure switches for washing machines typically have a large sensing diaphragm approximately 60mm 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 incorporated (controlled directly by the “water level” knob) and the switch opens and closes with audible clicks.
(Note that if your washer 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 interconnect, but we’ll leave them for another time). time).
Washer pressure switches directly control the hot and cold water inlet solenoids and are therefore rated for fairly high currents: 15A at 230VAC is typical. So for low voltage DC applications the switches can certainly cope with (say) 5A. This means that for most loads, a relay will not be needed.
Dishwashers use pressure switches that are similar to those used in washing machines. However, instead of having one switch point, they have multiple levels. For example, a two-position switch can switch to 10 cm and 15 cm of water, while a three-position switch can activate at 15 cm, 25 cm and 35 cm of water.
These switches also look a lot like pressure switches on washing machines, except they have multiple electrical terminals (six or even nine) and no external level adjustment. However, some have screwdriver adjustment for both trigger levels and hysteresis.
When removing the pressure switch from a washing machine, be careful not to
remove the adjustment mechanism, it is often part of the bracket that holds 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 on the top control panel of the machine. If the machine is upside down or partially destroyed, follow the sensor tube from the bottom of the wash tub. And while you’re at it, it’s usually worth looking for the tube as well, which is usually a high-quality plastic hose. You never know when it might come in handy, instead dishwasher pressure switches are usually buried under the stainless steel drum. Since they are not externally adjustable, it is not necessary to locate them near the control panel.
Depending on your application, you may want your salvaged pressure switch to operate at pressures other than its standard range. That’s not hard to do if you have a washer pressure switch, as they’re pretty easy to modify. But if you remove the setting
bracket, you can access the internal spring that sets the sensitivity. For example, by using a very light spring (i.e., one that provides enough force to return the diaphragm to its unfired position when the
pressure is removed), it is possible to make a switch trip in just 5 cm of water (~0.5 kPa or 0.08 psi). External adjustment would normally give a range of about 5cm to 7.5cm of water, but of course this will vary depending on the unit and spring used. Put a stiffer spring and the adjustment
range becomes larger. While we haven’t tested it, you could probably keep the switch from closing until you have 15-20 kPa of pressure. Please note, however, that the rubber diaphragm is not designed to withstand these
pressure levels, so there may be some long-term reliability issues.
So what uses can be made of these switches? That depends on your imagination, but here are some suggestions:
(1). Control of an electric water pump
– for example, to keep a container full of water. Just like in a washing machine, the depth of the water can be sensed from a hose attached to the base of the bowl.
(2). Providing a low water level warning, for example, the switch could be used to activate a buzzer or light if the liquid level in a tank falls below a set point.
(3). Provide water level indication, for example, 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 will need a lot of cables to connect the switches to your screen.
(4). Detection of vehicle movement by means of a washer pressure switch. The switch would be activated by air pressure as the vehicle crosses a hose. Just remember to cap the end of the hose that is not connected to the switch! So there are four applications, but there are many more. These very sensitive switches are certainly worth saving!
