ELECTRONIC DESIGN NOTES #2 - SWITCHES, RELAYS, FUSES
notions highlighted in this page are related to a few electronic
design topics presented in the first part, Hardware Design, of
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electronic design work is conventionally
discriminated as being analog or digital. However, practical designs are almost never only analog or totally digital; in most instances,
electronic designs are mixed solutions. In other words we need to learn both the analog and the digital side of electronic
In this page are presented:
A switch is a manually control component; it transforms our intelligent decision into one electrical
state: ON or
OFF. The variety of switches existing today in the world may discourage even the most enthusiast technical
writer. We have no intention to differentiate, classify, or to discuss much about switches, because that is
totally futile: a switch is just a switch! [This is a joke]. No, please never think that way. A switch in your
design is a component as important as any other electronic component, possibly even more. Always study the
Sheet of each switch you use attentively.
Despite their apparent simplicity, please remember that the contacts of the switches are made of:
1. copper--the bad ones;
silver--most switches have silver contacts, and they are fairly good;
gold--these are very good switches, and only a bit more expensive;
platinum--these are excellent switches;
5. iridium--these are the best switches possible, and only a few industries can afford buying them.
So, a simple switch it is, sometimes, build (partially) out of gold! Why do you think people waste precious, scarce materials to build those boring switches? Please note this: in certain instances the amount of precious
metal on those contacts could weight a lot!
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 Regulatory Authorities;
5. the characteristics of the load.
Particularly important is the last topic: 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.
Please be aware that each switch may handle a limited amount of pure inductive or pure capacitive
currents. Pure inductive or capacitive circuits are extremely dangerous, and only the most expensive,
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.
A switch inserted in a DC circuit changes that DC circuit into
an AC one!
Not all switches are capable to open inductive or capacitive loads. Between the two, the
capacitive loads are
particularly difficult to open. It is mentioned in Design Notes 1 that 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, therefore it will create arching between contacts, which will close the circuit again. The arching is
so intense that it will burn the contacts, therefore destroying the switch.
Depending on the function and the importance of your switch, always take little time to study it because selecting
a good switch is no joke. Never consider that electro-mechanical components, as are switches and connectors for
example, are simple and not important. Fact is, the electro-mechanical components are some of the most
important, the most expensive, and the most difficult to procure electronic components. You will see that for yourself
sooner or later.
A relay is an electro-mechanical switch capable of being remotely actuated/controlled. The schematics involving relays
could be very simple, or incredibly complex since they may employ the
well-known "relay-logic". The first computer was
built out of only
Now, we can differentiate relays as being:
1. electro-mechanical devices
Between the two, the first category is the "true relays" one. Any logic electronic component behaves
similar to the way the relays do. 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/analog circuits, and
a digital (remote) control part. Regarding the switch, please read carefully the design notes above, because they
do apply to all relay switches just 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 motion;
6. time actuated;
7. the number of counts;
8. and many, many more.
There is a strong pressure from the electronics industry to replace the electro-mechanical relays with electronic
equivalents, but the Regulatory Authorities refuse to accept them. The requirements for
the power utility grid
relays (and switches) are very clear: a relay (or switch) must separate electrical circuits mechanically, and
that operation/state needs to be visually noticeable.
Although we can replace (almost) all electrical relays with the electronic equivalents, there are still many types of
mechanical relays that cannot be replaced; for example, the electrically actuated pneumatic or hydraulic relays.
We will discuss more about switches and electronic relay equivalents in the following Design Notes. This page
highlights just a few topics related to switches and relays; however, these few topics are very important. The
intention is to encourage you to study these aspects thoroughly. Please be aware there are thousands of beneficial
industrial applications employing relays, and switches, which could be greatly improved. All it takes is
Protecting electrical circuits and electrical/electronic devices using fuses is such a complex
aspect that a so
called "Fuseology" scientific branch was developed to deal specifically with it. That is no joke;
the theory behind fuses it is very important because they work as "protection elements"! The intention
is to present here an example of using fuses, which is, unfortunately, too 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
Let's look at the fuses in your home. Their role is to protect the CABLES (the wiring) running through your home.
That means, fuses do not protect your fridge, not your TV, not your computer, and not your life;
those fuses protect only the CABLES!
In order to protect your life, your need GFCI (Ground Fault Circuit Interrupter)--these are in fact relays.
The same thing is valid in the industry. If you have a ten thousand dollars electrical engine, the utility fuses on
the cables supplying power to that engine protect only the cables, not the engine, although the cables cost, say,
one hundred dollars only. In order to protect that expensive engine there should be a control panel having all
sort of protective relays, or the protection means could be built inside the engine itself.
The aspects of protecting people's lives using fuses and relays are very serious issues,
and you need to know them very well before starting electrical/electronic
hardware design. A person could be
killed by voltages as low 24 V and by 80..100 mA currents, in some particular circumstances.
Please study electricity first, before designing electronics. Besides, if you come up with a commercial product,
please remember that it needs to be certified by a certifying agency. There, all
the 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.
Switches, Relays, and Fuses Schematic Symbols
SPST (Single Pole Single Throw)
SPDT (Single Pole Double Throw)
NO - PB (Normal Open Push Button)
NC - PB (Normal Closed Push
DPDT (Double Pole Double Throw)
Dual position Relay
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