Switches are an integral part of electronics. Almost every electronic device has a switch on it, if not several switches. This is because when devices are being used, there are typically several modes of operation. Obvious modes would be on or off, but often there are settings that can be adjusted. Car windows are controlled by momentary switches that can raise and lower windows. Gaming stations use switches to control the game. The list of switch applications is endless in the electronics industry. There are several different kinds of switches in the market place today and several considerations that are generally taken when picking a switch. Some of the questions that need to be asked are how many poles and throws are needed, does the switch need to be maintained or momentary, and what are the electrical considerations such as maximum current and voltage ratings? Hopefully this article can shed some light into the world of switches.
Before going into each type of switch it would be good to go over the different circuits that are found in each switch. There are two terms that are used to describe the circuit in a switch. These are “pole” and “throw”. A “pole” refers to how many circuits there are in a switch. A single pole switch will only have one active circuit at a time. The term “throw” refers to how many positions the pole can be connected to. Figure one shows a circuit diagram of a single pole single throw switch (SPST).
Figure 1: SPST circuit diagram.
A SPST switch will have one pole; there is only one possible circuit using this configuration. It is either open, or closed. A single pole switch could be normally open, or normally closed. When closed, current can flow through the switch; when open, it cannot. Imagine now that there is only one path for current to flow at a given time, but there are two options for where the current can flow. This would be a single throw double pole (SPDT). Figure 2 illustrates this point.
Figure 2: SPDT circuit diagram.
Using this type of logic makes it easy to understand switches. Another example similar to a SPDT is a double pole double throw (DPDT). It will look just like Figure 2, but with two poles instead of one. Figure 3 illustrates the circuit for a DPDT.
Figure 3: DPDT circuit diagram.
One thing to keep in mind with switches is that several different circuits can work to achieve a similar end. An example would be a simple on/off switch with no momentary function. This can be achieved with a SPST or a SPDT. Take for example the GRS-2011-2019 from CW Industries. This device has two terminals on the bottom of it which is synonymous with a SPST switch. When the switch is in one position these will be connected and current can flow. When the switch is in the other position it will not be connected and current cannot flow. If one was to take a SPDT like the GRS-2013A-2000 instead of a SPST they could achieve the same thing. It would simply involve using only two of the terminals and leaving one open. This is handy for situations where a normally open or normally closed SPST switch is needed. A SPDT can be either of those options. Looking back at a SPDT, it is also important to understand the function of the switch. A SPDT can either have two positions for the actuator or three. To explain this, think of a switch that is ON-OFF-ON. This is common for motor applications where the motor can turn one way, then stop, then turn the other way. A SPDT can do this, but not all SPDT switches can do this. The GRS-2013A-2000 is an ON-OFF-ON switch so it would be possible for that switch to accomplish that action. The D508J12S205QA from C&K would not be able to achieve this; it is an ON-MOM switch. That means that the switch will typically be in the ON position and can momentarily turn off. An application that this could be used in would be the conveyors at the checkout line in the grocery store. By simply hooking the wires up backwards, the motor will not run on the conveyor until the actuator is depressed. When the actuator is depressed there will be momentary contact inside of the switch and current will be able to flow. Once the actuator is released, the switch moves back to its normal off position and current stops flowing.
Electrical characteristics are also important to keep in mind when choosing a switch. If a circuit calls for 50 amps, a switch that is rated at 10 amps will not work. It is always important to look at voltage and current ratings on a switch. There is even a distinction between AC voltages and currents vs. DC. Often manufacturers will give ratings for both of these, but not always. When evaluating a switches voltage rating, look at what the manufacturer is calling out for the type of voltage. If a rating says 125 VAC, then it is calling out an AC voltage. If the rating says 125 V then it is calling out a DC voltage. This vernacular will be called out on the datasheet or drawing for the product. On the Digi-Key website a further distinction can be made. The product PS1024ARED from E-Switch has an AC rating; however, Digi-Key says 125 V online. This is because the column that rating is in is for “Voltage Rating – AC”. Figure 4 shows this from the Digi-Key website and Figure 5 shows the rating on the PS1024ARED.
Figure 4: Product attributes for PS1024ARED.
Figure 5: Drawing for PS1024A electrical specifications.
Something that is not evident on the Digi-Key website, but obvious on the drawing notes is that there are a couple of different ratings for this switch. The first rating is 3 A at 125 VAC and the second rating is 1.5 A at 250 VAC. How is it possible to have two ratings for the same switch? This is because power is a function of voltage and current. Both of these ratings carry the same power rating. If one were to plug the following numbers in for the power rating they would get the same numbers.
Often a switch will only have an AC rating and no DC rating. There is a rule of thumb that can be used to determine the DC rating for an AC switch. Whatever the highest current rating is for the switch, the current will stay the same and the DC voltage will be set to 30 volts. The PS1024A has two ratings, 3 A at 125 VAC and 1.5 A at 250 VAC. This rule means that the switch could take the 3 A rating and use that for up to 30 VDC. This is a general rule taken from Carling Technologies called the “DC Rule of Thumb”. Often switches will have a horsepower rating as well. This is because inductive loads can have extremely high inrush currents. The HP rating will be designed for switches that are used with AC motors. This does not mean that one of these switches can’t be used on other devices, but this is why that rating is put on the switch.
One type of switch is the pushbutton switch. These switches will have an actuator. The actuator could also be referred to as a button using less formal vernacular. These will often be panel mounted or put onto a board. Figure 6 illustrates the KB16CKW01-5F-JF from NKK Switches.
Figure 6: KB16CKW01-5F-JF.
An interesting point about KB16CKW01-5F-JF is how many terminals it has. This is a SPDT which should only have 3 terminals, but this device has 5 terminals. This is because this device is illuminated. Many switches are illuminated and will have a nominal illumination voltage. This particular switch has a 2.1 VDC illumination voltage. The documentation calls out that terminal number 2 is the common terminal and the LED will be powered off of terminal L (+) and L (-) as shown in Figure 7.
Figure 7: Documentation for KB16CKW01-5F-JF.
Often switches will have accessories listed at the bottom of their product page. Looking at the bottom of the product page for the KB16CKW01-5F-JF on the Digi-Key website it is possible to see four accessories that go with the switch.
|Cap Removal Tool
|Bushing Mounting Socket Wrench
These accessories can be found on the bottom of the KB16CKW01-5F-JF page on the Digi-Key website, or through the documentation on the series datasheet. Often manufacturers will create an entire series of switch. This is a group of proprietary products that go together. The KB16CKW01-5F-JF was specifically designed to take certain accessories which are called out in the documentation. The protective guard accessory can be found on page D34 of the series datasheet. Figure 8 demonstrates the protective guard along with its orderable part number of AT494.
Figure 8: Documentation for KB16CKW01-5F-JF.
Often switch manufacturers put an entire series on one datasheet because some of the changes from one part to another can be trivial. Often the different part numbers will only be a change in color. The LED accessory AT635F is a good example of this. In the documentation there will only be the number AT635 listed as an option. The product mentioned earlier has a suffix of “F”. Figure 9 shows the ordering scheme which calls out the “F” suffix as being a green LED. On the left side of the ordering scheme is the base LED number of AT635. On the middle column there are options for Red, Amber, and Green. The box labeled “Green” says 5F under it. This means that for the AT635F, the LED will be green.
Figure 9: Documentation for KB16CKW01-5F-JF.
Tactile switches are similar to pushbutton switches, but they are typically a lot smaller. Imagine a personal home router that has a reset switch. The switch is typically recessed in the plastic so that it cannot be accidentally pushed. Usually the use of a paperclip or toothpick is needed to press that type of switch. These types of switches will typically be tactile switches, momentary switches and will not usually be used as an on or off switch. Often these will be used to reset something, count something, stop something, or any function of that nature. These will typically have very low voltage and current ratings. The MJTP1230 from APEM Inc. is an example of a tactile switch. This switch is a SPST-NO, or single pole single throw normally open circuit. This means that current cannot flow through this switch until the actuator has been pressed. Once the actuator is released, the switch will automatically open up again and current will not be able to flow. Figure 10 is an illustration of the MJTP1230.
Figure 10: MJTP1230.
There are four legs on this switch. This will often be the case for tactile switches. It is not because there are two different circuits that can be connected, but rather because adding two additional legs adds physical support to the switch when it is soldered onto a board. The two legs on one side will be permanently connected regardless of the position of the switch. The other side of the switch will follow the same rule where the legs are always connected. When the actuator is pressed, then there is continuity between both halves of the switch. As far as the circuit is concerned, this switch will follow the circuit diagram from Figure 1.
Dipswitches are switches that are in a “Dual Inline Package”. These will be designed to be able to fit into a breadboard. These switches will be a conglomeration of small SPST switches. An example of where these are used is in DMX control for stage lighting. Every light fixture will have an address that can be assigned to it. In modern DMX controlled lights, this can be programmed by a microcontroller and simply moving a number up or down on a display. On older DMX controlled lights, there would be a dipswitch that could be changed to give the proper binary address for that fixture. These types of switches are often surface- or through hole-mounted, but there are also panel-mounted options. Figure 11 illustrates the 208-4, a dipswitch from CTS Electrocomponents.
Figure 11: 208-4.
Rocker switches are also very popular in the marketplace. This type of switch has a concave actuator that rocks back and forth from each position. Many ON/OFF switches are rockers for consumer electronics. These will typically be panel mounted, but can be mounted directly onto a board as well. Figure 12 depicts the CRE22F2BBRLE from ZF Electronics.
Figure 12: CRE22F2BBRLE.
This switch is a SPST switch so it will only turn on or off. There is a middle terminal on this switch because it is illuminated. SPST switches with illumination will usually have three terminals. The outside terminals will be responsible for allowing current to flow from source to ground. The middle terminal is used for the light. When the switch is open, current cannot flow through the light. When the switch is closed, current will flow from the source, through the light, to ground.
Another thing to look for with a rocker switch is the actual physical size of the terminal. These are often panel mounted so they are often terminated with discrete wires using quick connect style terminals. The CRE22F2BBRLE has terminals that are 0.25” wide. This is called out on the Digi-Key website under the product attributes for “Termination Style”. Common sizes for terminals are 0.110”, 0.187”, and 0.250”. Quick connects for these can be found in the “Terminals – Quick Connects, Quick Disconnect Connectors” page on the Digi-Key website.
Snap action limit switch
Often in manufacturing, there is a need for a switch to count things. Imagine a production line that has to count how many products go through it at a certain point. The product may weigh a certain amount and be able to actuate a switch. This application could call for a switch that just needed to move a little bit to count something. The D2VW-5L2-1HS from Omron is an example of a snap action limit switch that could be used to accomplish something like this. Figure 14 shows how the D2V2-5L2-1HS has an extended arm that could be used for such a process.
Figure 13: D2VW-5L2-1HS.
There are snap action limit switches that maintain their circuit like an ON/OFF switch; most of them will be momentary. It is very common to find SPST-NO or SPST-NC snap action limit switches. A nice trick that had been mentioned a little bit earlier is the use of a SPDT circuit in place of the normally open or normally closed SPST. This is a good trick because if the application calls for normally open or normally closed, the switch will be able to perform the task.
The most common accessory for a snap action limit switch would be the actuator arm. Often there will be rollers on them, sometimes just a flat bar, there are several different types of configurations. Figure 15 illustrates a few of the actuator styles for the D2VW series.
Figure 14: Actuator styles.
Toggle switches are another popular style of switch. These will often be panel mounted like the rocker switches are. These can be used for momentary functions, but are usually used for maintained functions. They typically have an actuator that looks like a little baseball bat and a threaded bushing for panel mounting purposes. Figure 16 shows the 200MSP3T1B1M2QEH from E-Switch.
Figure 15: 200MSP3T1B1M2QEH.
Common accessories for toggle switches are lock washers and hex nuts for mounting. These can typically be found on the bottom of the product page on the Digi-Key website in the “Associated Product” area. The 200MSP3T1B1M2QEH has both of these listed on its product page as shown in Figure 17.
Figure 16: Associated lock washer and hex nut.
Other common accessories for tactile switches are safety covers and boots. Figures 18 and 19 illustrate these accessories respectively. The safety cover is an accessory for the GTS447A101HR from CW Industries and the boot is an accessory for the M2012SS1W01 from NKK Switches.
Figure 17: Safety cover for GTS447A101HR.
Figure 18: Boot for M2012SS1W01.
Often there is a need for a switch that can select between several modes of operation. These applications can sometimes call for a rotary switch. These will have one to several poles, but can often have quite a few throws. The C7D0124N-C from Electroswitch is an example of a rotary switch. This is a SP24T switch. This means there are 24 different options for current to flow through, but current will only flow through one at a time. Common accessories for rotary switches are knobs, dial plates, lock washers, and hex nuts. Like the previous products, these accessories can usually be found in the “Associated Product” tab on the Digi-Key website. Figure 20 illustrates the C7D0124N-C.
Figure 19: C7D0124N-C.
Another distinction that can be made for a rotary switch is whether it is “Make-Before-Break” or “Break-Before-Make”. Another type of vernacular that can be used to describe this is “Shorting” or “Non-Shorting”. The term “Shorting” refers to “Make-Before-Break”. This means that when the switch moves from one position to another, there is never an open circuit. The switch will temporarily have two active circuits on it until it moves completely to its desired position. The 212T0111S332RA from CTS Electrocomponents is an example of a “Make-Before-Break” rotary switch. The opposite of this is “Non-Shorting” or “Break-Before-Make”. This means that the first circuit will be broken entirely before the switch is in position for its new circuit. The KC52A30.001NPS from E-Switch is an example of a “Break-Before-Make” rotary switch.
Keylock switches are a very common form of switch. A car ignition is an example of an OFF-ON-MOMENTARY switch. When the key is inserted into a car ignition, the switch is in the ON position, but it will do nothing in the on position until the vehicle is started. The MOMENTARY position is what starts the vehicle, at which point the key is returned to the ON position. To turn the vehicle off, it is necessary to turn the key to the OFF position. An example of a keylock switch is the 84829-07 from Honeywell Sensing and Productivity Solutions depicted in Figure 21.
Figure 20: 84829-07.
There are many different kinds of switches in the marketplace today. They all work to achieve the same end, connecting circuits. There are maintained and momentary switch functions. A switch with a maintained function will hold its position until it is manually moved. Conversely, a switch with a momentary function will automatically return to its default state. The application will narrow down the type of switch that is required. The switch circuit, along with physical parameters such as maximum allowable current and voltage, will be some of the greatest determining factors in choosing a switch. The style of switch can be interchangeable; an electronic circuit will work just as well with a toggle switch as it will with a rocker switch. The style of switch will be largely determined by the engineer’s personal preference, and the environment the switch is going to be placed in.