Passive components
This page covers passive components: switches, push-buttons and connectors, how to identify them and understand their common failure modes, and how to test them.
Summary
You might think of passive components such as switches, push-buttons and connectors as the "country cousins" of electronics, but they are important, none the less. We discover how to identify them, what they do, how they fail and how to test them.
You can read this page on its own if you like, but if you're not already familiar with basic electrical and electronic theory you my find you get more out of it if you first read Electric circuits, volts amps watts and ohms.
Switches
Switches come in many different forms, and being in part mechanical, they are subject to wear and failure.In its simplest form, a switch consists of two pieces of metal that can be made to touch or not, so switching the current through it on or off. Alternatively, a third piece of metal can be moved to bridge the other two.
Often, a single enclosure will contain two switches operated by the same lever or knob. This is known as a 2-pole switch, and it can be used, for example, to disconnect both the live and neutral in a mains outlet, making the outlet completely safe.
A 2-way switch connects one contact to one or other or two other contacts, diverting the current into one of 2 paths. The waveband switch in an older AM/FM radio might well be one of these. You can also have multi-way switches which can switch up to a dozen ways.
Multi-way switches can also have multiple poles, so for example a 2-pole 3-way switch would be able to switch each of two circuits three ways.
A microswitch is often used in electromechanical devices where a switch needs to be operated for example by a safety interlock or some other mechanism rather than directly by a user. They are designed to operate with very little force and reliably at a particular point. They come in a few standard sizes and hence are easily replaced but come with several different contact configurations. There is also a choice of operation by a lever, or a roller (as in the photo) or directly by the button which is beneath the lever in other types.
Many small switches consist of several springy metal fingers which move across one or more static metal contacts. Sometimes the pressure needs to be increased by bending the springy metal finger. If the contacts are dirty or corroded, they can be cleaned with switch cleaning fluid. In some cases the static contacts are simply tracks on a circuit board, and these can wear through. A solution may be to bend the springy metal pieces to make contact at a slightly different point.
If you try to disassemble a switch be aware that it may contain a spring which can easily take flight and small parts which tend to fall out. This can make it challenging to work out the precise arrangement of the parts and difficult to hold them all in position as you try to put it back together again.
When they go wrong ...
(NB This section is work in progress.)
Since switches and push-buttons have moving parts it's almost inevitable that they will sometimes fail. Sometimes this will be a purely mechanical failure, perhaps resulting from excessive force being applied to some plastic part. More often, it will be a problem of wear, dirt or corrosion of the switch contacts.
There are essentially two types of switch or push-button:
- Those that directly control power, whether mains or a lower voltage, and
- Those that are simply used to indicate a logic state to a microcontrolle, or other electroncs.
Before proceding, it's worth briefly delving into switch ratings, which have different significance in the two cases:
- Maximum voltage and current. This is relevant to power switches, and any replacement must have the same or greater ratings.
- Minimum voltage and current. In order to ensure reliable operation, a minimum current and voltage must be applied in order to break through microscopic oxide or other contaminants.
- Maximum number of operations. For something like a gaming mouse button, this needs to be verry high.
- Bounce time. On switching on or off, over a period of a few milliseconds a switch may actually go through many transitory on/off transitions before settling. A microcontroller or logic circuit must allow for this, but this is a compromise between correct operation and the maximum switching rate.
In the case of mains and low voltage switches which directly control the power to a device or a part of it, sparking ay occur at the contacts when it is switched off, or when or is switched on if it doesn't have a good snap action. You can get special switch cleaning fluid, which with a stiff brush, can be used to remove dirt, corrosion and oxide.
Push-buttons
Push-buttons also come in many different forms but are usually single pole single way (on/off) switches. Most rely on the click effect familiar from the safety button on screw-on metal lids to glass jars of jam, baby food and many other food products. At a critical pressure the button flips from a convex to a concave form. In a push button switch it may be a plastic film or a sliver of thin metal. In the case of metal (as illustrated from a digital alarm clock), it may be this that makes contact with a trace on a circuit board beneath, otherwise a piece of conductive rubber may bridge the gap between two circuit board traces when the button is pressed. In either case, cleaning with isopropyl alcohol or switch cleaning fluid can help.
Small push-button tactile switches may not be repairable but can often be easily replaced. Common types are 6x6 or 12x12mm and various heights. The height is measured from the bottom of the body (in contact with the circuit board) to the top of the button. The 4 legs are connected together in pairs. On pressing the button, a connection is made between one pair and the other. Many eBay sellers offer these at very reasonable prices.
Connectors
Connectors are used not only to connect external devices such as a power supply or headphones, but they are also used extensively inside many devices, to interconnect the various sub-assemblies.
Connectors can often fail if they see a lot of use or are subject to strain or contamination with dirt or moisture.
Dirt and corrosion can be removed with switch cleaning fluid, available in an aerosol can, assisted by gently rubbing with a tissue. Corroded terminals in a battery compartment is a common problem, caused by chemicals leaking from long-dead batteries.
Electrical contact is normally maintained by spring pressure in some form. The pressure may be lost over time and can sometimes be restored by gently bending a springy piece of metal.
Modern electronic equipment often uses ribbon cables to interconnect the sub-assemblies. Sometimes the ribbon can be simply pulled out of its connector and pushed back in on reassembly, but often there is a clamp which must be pulled or lifted in order to release the ribbon. Care is required - if the clamp is broken through being pulled in the wrong direction or with too much force, it may be that little can be done.
A separate page lists many common types of connector, and how to disconnect and reconnect them.
And now ...
... you might like to continue by reading about Protection components.