Switches, Relays and Fuses

In this page are presented:
1. Switches
2. Relays
3. Fuses

SWITCHES

A switch is a manual control component. It transforms our intelligent decision into one electrical On/Off state. A switch in your design is a component as important as any other electronic component, possibly even more. Always study the data sheet of each switch you use.

Despite their simplicity, please remember that switches' contacts are made of:
1. copper--the very bad ones;
2. silver--most switches have silver contacts, and they are fairly good;
3. gold--these are very good switches, and only a bit more expensive;
4. platinum--these are excellent switches;
5. iridium--these are the best switches possible, and only few industries can afford buying them.

The importance and the value of a switch is given by:
1. the function we, the electronic designers, assign to it;
2. the adverse/constraining environmental conditions;
3. the number of On/Off cycles it is capable of;
4. the requirements of the Regulating Authorities;
5. the characteristics of the load.

What you need to know is, switches are specifically designed to operate in:
1. DC circuits only;
2. AC circuits only;
3. in both of the above, within electrical limitations.

In addition, each switch may handle a limited amount of pure inductive or pure capacitive currents. Pure inductive or capacitive circuits are extremely dangerous, and only specially designed switches may be employed to handle them.

Always study carefully the Data Sheet of the switches you intend to work with, in order to understand their range of operation, and their limitations.

ATTENTION
A switch in a DC circuit changes electrical behavior of the DC circuit into an AC one!

ATTENTION
Not all switches are capable to open inductive or capacitive loads. Between the two, the capacitive loads are particularly difficult to open. Pure capacitive current lags the voltage by PI/4. That means, in the moment the voltage becomes zero (opened switch) the capacitive current is at its maximum, and that will create arching between contacts, which will close the circuit again. The arching is intense, and it will burn the contacts, destroying the switch.

Depending on the function and the importance of your switch, always take little time to study it, because a switch is no joke. Never consider electro-mechanical components, like switches and connectors for example, that they are simple and not important. Fact is, they are some of the most important, most expensive, and very difficult to procure electronic components. You will see.

RELAYS

A relay is an electro-mechanical switch capable of being remotely actuated. The schematics involving relays could be incredibly complex, and they employ the well-known "relay-logic". The first computer was built out of only electro-mechanical relays.

We can differentiate relays as being:
1. electro-mechanical devices
2. electronic

Between the two, the first category is the "true" relays one. All logic electronics components behave similar to relays, or to "relay-logic". Take the transistor for example: it is a perfect current controlled relay.

A relay contains two parts: a switch (or a system of switches) controlling the power/primary circuits, and a digital remote control part. Regarding the switch, please read carefully the Design Notes above, because they apply to all relay switches as well.

The second part of the relay, the (digital) control one, could be very complex, and specialized on detecting:
1. ground faults;
2. minimal voltage;
3. maximal currents;
4. increased temperatures;
5. mechanical movements;
6. time actuated;
7. the number of counts;
8. and many more.

There is a strong demand from the electronics industry to replace electro-mechanic relays with electronic equivalents, but the Regulatory Authorities refuse to accept them. The requirements for power utility grid relays (and switches) are very clear: a relay (or switch) must separate electrical circuits mechanically, and that operation needs to be visually noticeable.

Although we can replace (almost) all electrical relays with electronics equivalents, there are still many types of mechanical relays that cannot be replaced; for example electrically actuated pneumatic or hydraulic ones.

We will discuss more about switches and electronic relay equivalents in the following Design Notes. This page highlights just few topics related to switches and relays; however, these few topics are very important. My intention is to encourage you to study them. Please be aware there are thousands of beneficial industrial applications employing relays, which could be greatly improved. All it takes is little investigations.

FUSES

Protecting electrical circuits and devices using fuses is such a complex issue, that a so-called "Fuseology" scientific branch was developed to deal specifically with it. That is no joke, and the theory behind fuses it is very, very important because they work as "protection elements"! My intention is to present here an example of using fuses, which is, unfortunately, little known.

So; the fuse is a protection component in electric and electronic circuits. In electronics, the fuse plays an important role, since it protects various circuits from overcurrents which could destroy an expensive electronic device.

Let's look at the fuses in your home. Their role is to protect the CABLES (the wiring) running in your home. That means fuses do not protect your fridge, not your TV, not your computer, and not your life; only the CABLES! In order to protect your life your need GFCI (Ground Fault Circuit Interrupter) relays or RCCB's (Residual Circuit Current Breaker)

The aspects of protecting people's lives and electrical devices using fuses and relays are very serious issues, and you need to know them well, before starting designing electrical/electronic circuits. A person could be killed by voltages as low 24 V and by 80..100 mA currents, in some particular conditions.

Please study electricity first, before designing electronics. Besides, if you come up with a commercial product, please remember it needs to be certified by a certifying agency. There, all good rules of safety design are analyzed first, and before anything else. Your product could be revolutionary, technologically, but if it is not safe for use (this is considering many adverse situations) it is not going to be certified.

Schematic Symbols
Symbol	Description
	SPST (Single Pole Single Throw Switch)
	SPDT (Single Pole Double Throw Switch)
	NO - PB (Normal Open Push Button Switch)
	NC - PB (Normal Closed Push Button Switch)
	DPDT (Double Pole Double Throw Switch)
	Dual position Relay
or:	Fusible Fuse