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Basic Electronics Question

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Basic answer:

Generally, it's a value that represents the manufacturer's suggested maximum voltage that a component can tolerate.

Exceed that level and it is assumed that the component will, at the least, perform erratically and, at the worst, fail altogether.


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Different components, particularly active components like transistors and integrated circuits, are designed for certain applications or a range of applications. Music players and flash drives use a low operating voltage to keep battery costs down, things in cars run on 12 V but need to tolerate twice that, power supplies run on 120V or 240 V AC, etc. The components used need to be able to operate with these voltages safely and reliably for a long time. Most of the time you can use a higher-rated part in a lower-voltage application, but they usually cost more than a lower-rated part (all other things being equal).



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For the low voltage stuff, this has been partly due to evolution of design. As design speed increased, the component size and voltage decreased.


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Hi AS,

Here is my bit on why there are so many different 'standard' voltages used in the electrical/electronics industries. In general terms, there are six factors but some overlap. By the way, the three most common voltages are, 12V DC (automobile battery voltage) 120V AC, 60Hz (mains supply US and other countries), and 240V, 50 Hz (mains supply UK and other countries).

Transportation Efficiency
Electrical power is voltage multiplied by current. So if you had a 1000 Watt electric heater in your house and you lived in the US, where the mains voltage is 120V AC, the current taken by the electrical heater would be 1000W/120V = 8.3 amps. The total resistance between the electric company input to your house and the actual heating element of the heater would be, say, one Ohm. This resistance is made up of the wiring, switches, plugs, sockets, thermostats...

Power is also, current * current * resistance. So in the case of the heater, the power wasted as heat in the wiring etc, would be 8.3A * 8.3A * 1 Ohm = 68.9 Watts.

If, on the other hand, the mains voltage to your house were 70V AC, the current needed for a 1000 Watt electric heater would be 1000W/70 = 14.29 Amps. This would result in 14.29 A * 14.29 A * 1 Ohm = 200 Watts being wasted.

So you can see that the higher the voltage the less power loss between on point and another.

This is why the national grid transports electrical power at many thousands of volts. In the UK for example, 400,000 Volts

Certain voltages are historic- someone decided on a particular voltage way back and that voltage has stuck. This applies to the mains supply voltage. For example the mains supply in the US could equally well have been 240V, 50Hz if that voltage and frequency had been chosen by the original designer.

Power Sources
Voltages are frequently dictated by power sources, mainly batteries. For example, 12V which is the voltage of an automobile lead acid battery. Before that 6V was very common because early automobiles had 6V lead acid batteries

Other common voltages are 24V (two 12V lead acid batteries in series) and 48V (four lead acid batteries in series).

1.5 V is the voltage of a Leclanche cell. later developed into the dry cell torch battery.

Then four dry cells were connected together to make a six volt battery for door bells for example. The early transistor radios suited 6V batteries as well.

Then transistor radios moved over to 9V batteries which comprised six 1.5V dry cells connected in series.

Device Dependency
In the 1970s, 5V became a very widely used voltage, because TTL logic chips required 5V.

At the same time, integrated circuit operational amplifiers became available at a reasonable price, so 15V and -15 supply rails became common because chip op amps are designed to operate from these two voltages.

A less well known standard voltage is -5.2V, which suits the lightening fast and power hungry, emitter coupled logic (ECL) chips. ECL is only used in very high performance applications, often military, but you may find ECL used in normal equipment for a particular circuit function that needs extreme speed.

You may think that getting more and more transistors on to a minute piece of silicon is a challenge, but the ultimate challenge is power dissipation. If you have a chip with a billion transistors on a square millimeter of silicon, getting the the heat away from each transistor becomes a major problem.

But, one way to reduce the power generated is to reduce the operating voltage.

So, although the initial logic chips used a 5V supply, considered very low at the time, the trend now is to move down to 3.3V. And continuing this trend, some logic chips work at 2.5V, 2.2V, 1.8V, 0.9V, and even 0.8V.

One benefit of a lower voltage supply line is that the signal can be faster because it has less amplitude. For technical reasons, it is much easier to generate, say, a 100MHz square wave at 5V peak to peak than it would be to generate a 100MHz square wave at 50V peak to peak. The former would be simple. The later would present quite a challenge.

Non Standard Voltages

Certain equipments run from non-standard voltages- audio power amplifiers are a common example. In these cases, the supply line must be optimized for the design. So you may get audio power amplifiers running from 3V, 11V, 23V, +- 40V, +-100V, or any other voltage.

Component Voltages
Components, capacitors, diodes, transistors..., have a maximum voltage which must not be exceded or the component will be damaged. For example, the ubiquitous 1N4148 high-speed small signal diode has a maximum reverse voltage of 75V. This only means that the 1N4148 reverse voltage must not be exceeded. It does not mean that the 1N4148 will not operate correctly at a lower voltage. Similarly, the common BC546 small signal transistor has a maximum voltage between its collector and emitter of 65V. This does not mean that the BC546 will not operate at lower voltages. In fact, the BC546 could be operated with 1V between its collector and emitter. Likewise, a capacitor rated with a maximum voltage of 650V, would perform normally with only 1V across its two terminals.

So there you have it: There are myriad 'standard' voltages, but if you were going to get into electronics three supplies lines would cover a very wide range of applications: -15V, 5V, and 15V.

Last edited:
How are designs usually carried out? Do you design to certain specifications or do you design what comes to mind and figure out the specifications once you have all the components?

EDIT: Ah spec pretty much answered my question, didn't appear in my browser.

Ian Rogers

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How are designs usually carried out? Do you design to certain specifications or do you design what comes to mind and figure out the specifications once you have all the components?
It's not really that hard... I will take each part in turn.. Power supply... Micro essentials.. ADC inputs.. and use known working examples.... You usually work to a preset path... Normally you are asked to fill a space, you design to fit that space... After several systems, you find designing a new one, really isn't !! All your new designs are a variation of a theme..

At least. That's how I do it!!
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