The “Not So Simple Guide” to choosing resistors - Part 4
Understanding voltage parameters on resistor datasheets.
Hi fellow Electronics Engineers.
To date in this resistor selection blog series we have discussed EIA decade values, physical properties and power ratings of resistors. Now let us look at how manufacturers represent voltage on their datasheets and how these parameters should be interpreted for your design.
Rated Voltage
Another quick reminder, a Volt is the electrical unit for potential difference between two points in a circuit. In SI units it is how much energy (J) per unit of charge (C, the coulomb).
Note In case this is the first time you have read SI units, here is a description of what SI means, courtesy of the National Institute of Standards and Technology ‘The International System of Units (SI), commonly known as the metric system, is the international standard for measurement. The International Treaty of the Meter was signed in Paris on May 20, 1875, by seventeen countries, including the United States and is now celebrated around the globe as World Metrology Day’.
Please take note, the physical size of the component is a key driving parameter to dimensioning what most resistor manufacturers describe as ‘rated voltage’ (this is not the only parameter, but we will discuss later). This is basically what voltage the resistor can electrically accommodate across its two terminals, with either a static Direct Current (DC) operating voltage, or a Root Mean Square (RMS) Alternating Current (AC) voltage.
Remember, normally the manufacturers datasheet ‘Rated voltage’ does not include any de-rating (none of the parameters do as they are normally stated as absolute maximum values). i.e., it would be wise to make sure you have some design margin to ensure the component does not go anywhere near its maximum rating.
De-rating is common practice in many fields of engineering and helps ensure the design is reliable. I will detail good de-rating practices in a future blog. However, for now, I would suggest you choose components that have at least 30 % margin. For example, a 100 V component should never have any constant voltage greater than 70 V applied across it.
Permissible voltage
The last aspect to consider with respect to datasheet interpretation, is ‘Permissible voltage’. This is slightly different to the rated voltage, as now the datasheet introduces a time element. For instance, you may see a component that is rated for 75 V (non-derated) operating voltage, but also has a permissible voltage rating of 100 V for 1 minute. This dimension relates to something we often call the ‘transient operating condition’.
In engineering you will commonly see / hear the terminology static conditions, interlaced with transient conditions. With respect to voltages, a static condition generally refers to operating conditions that are stable, think about the voltage from a bench power supply. Now think about the observed voltage of a lighting strike in the time-domain (if captured on an oscilloscope). This is an example of a transient condition.
Note For a rule of thumb in electronics engineer we refer to events lasting longer than 1 second as static conditions. If the event lasts less than 1 second, then conversely, we refer to this as a transient condition..
Often designers must consider both operating scenarios. Therefore, the component manufacturers normally provide information about how the component can be ‘pushed’ for short durations. In later blogs we will explain the physics of what happens and how we can model / analyse these transient conditions.
What is the difference between DC Voltage, AC Voltage and Transient Voltages
As a slight deviation from the task of exclusively imparting knowledge on how to select a resistor, let us make sure you are comfortable with some basic understanding regarding how we describe different voltages.
Okay, let us start with a DC voltage source, this should be the easier one for you to comprehend. There are many common voltage sources including power supplies; linear voltage regulators; IC (Integrated Circuit) outputs and batteries etc. All these devices operate as a constant-voltage sources (also known transimpedance), i.e., considering a battery, if you place a load across the battery terminals, the voltage should remain constant (ignoring the battery discharging, or internal resistance causing the voltage to drop etc.).
However, for an AC voltage, e.g., your mains outlet in your house, this voltage is a sinusoidal waveform (as it is represented by the mathematical function sin, that you may remember from high-school trigonometry). This is because the voltage was generated from a rotating machine (we will cover this in detail in a separate blog). As the machine rotates, the voltage that is generated fluctuates.
Normally when an AC voltage is written we write it as its RMS value. There are many sources available online to show you how to derive RMS (of which I am sure you are capable of reading about), but for brevity, if you want to convert to or from a peak (pure) sinewave to its RMS voltage then here is a quick example
Note A word of caution, the derivation in the video shows that to calculate the RMS voltage from a peak voltage you simply multiply by 0.707 (or from an RMS to a peak, multiply by 1.414). Remember this is only true for a sinewave, any distortion or other waveforms will not yield the same results.
Engineers don’t normally use the mathematical average function when representing sinewaves as they have the same amount of area under the curve on the positive half (integration) of the cycle as it does on the negative half-cycle. Therefore, if you were to average these two halves out the answer would be zero. Try telling that to every person who has experienced an electric shock from a home appliance!
Note for the eagle-eyed you may have noticed in engineering textbooks that you see voltages represented in both upper and lower case, which may look contradictory to the note in power ratings. This is intentional as when we switch to lower-case in our mathematic notation (i.e. v instead of V), we are informing the reader that there it is a function of time (transient) as opposed to a static condition. For instance you may charge a capacitor from a DC source, the voltage of that source would be #.## V, but as the capacitor charges with an exponential time function (to be discussed in a future blog), it would be normal to say that the voltage at any given point in time is #.# v. To summarise, DC voltages and RMS voltages use upper case, transient voltages use lower case, what could be simpler?
Throughout Europe, houses commonly have 240 Vac electricity. To be precise, that is 240V RMS. The actual peak voltage is 340 volts.
Note Peak voltage is often abbreviated to pk. Also, peak-to-peak voltage is often written as pk-pk.
Back to our blog thread with respect to voltage. When considering what resistor is suitable for your circuit application, make sure you consider the peak-voltage, otherwise you may inadvertently choose a component that is not cable of handling the circuits voltage.
More to follow on this subject, but for now I would also consider placing multiple series resistors when connecting to line, so that if one component fails short-circuit, the rest of the circuitry (or the user / operator) doesn't inadvertently become exposed to the 'high' voltage.
For now, I bid you farewell and hope you are looking forward to the next instalment.
Until the next time... ut vis vobiscum