Low pass filter and voltage stabilization

This article is intended to explain voltage stabilization since it is important in order to provide electronic device stability. Without voltage stability, proper functionality of the electronic device is question mark, since voltage noise can appear on the voltage signal.

Ideal power supply voltage signal is nothing else than constant signal in time, without changes. In reality, 100% smooth signal all the time does not exist. In some period voltage noise appears and voltage oscillate around ideal voltage point. For example, ideal voltage is set to 18V. At the beginning of the time frame voltage is ideally smooth, but at time t1 noise appears and voltage level is dropping to the 4VDC (VDC = Volts DC). This kind of oscillations are not welcome since variations under power supply voltage signal is automatically reflected to the rest of the circuit which leads to the improper electronic device stability.

Figure #1. Low pass filter and voltage signal stabilization (click on the image to zoom in).

The voltage divider presented in figure #1 is a perfect example about how voltage noise is automatically reflected in the rest of the electrical circuit. Pay attention on the time frame signals presented in the figure #1.a, 1.b and 1.c. Figure #1.a is power supply voltage signal and it is perfectly smooth till voltage noise at t1 has been occurred.  Since voltage divider is resistor network consisted of the two resistors and nothing else (voltage divider with resistors does not provide any kind of capacitance), voltage noise is automatically reflected to the VREF_CMP point (as it is presented on figure #1.b) connected to the comparator like LM393 integrated circuit. This kind of voltage noise is not welcome and comparator will not work as expected since VREF_CMP point is not exactly 9 volts DC if noise is presented on the power signal.

To avoid this kind of problem low pass filter is used in order to eliminate noise signals from power supply line. Noise signals are very fast, couple of nanoseconds as it is presented on the diagrams, and low pass filter is good way to eliminate any kind of noise, since low pass filter pass only signal with low frequencies, millisecond or seconds. Low pass filters are implemented with capacitors (C4 on the electrical schematic, Figure 1.d), since it is not possible to charge or discharge capacitor instantly (it is discharged instantly if both capacitor pins are connected with wire - short circuit).

If we are not able to discharge capacitor instantly throughout the resistor network of the electrical circuit, nanosecond noise signals will be filtered. Since capacitor does not react immediately in terms of discharging, voltage at point VREF_CMP is stable like it is presented on the figure #1.c.

Low pass filters are used in many different cases not just for voltage divider VREF_CMP voltage stabilization signal. Low pass filters or capacitors are also used for voltage stability against input and output voltage signal of the linear voltage regulator. Both SERPENT I and II DC motor drivers has electrolytic capacitors regarding voltage stabilization including ceramic multilayer capacitor as additional stabilization at the output stage. Capacitors are also used for integrated circuit stability, in most cases you will see ceramic disk capacitor or multilayer ceramic capacitor as baypass capacitors (or series of different types of capacitors in terms of package and value), placed very closely to the integrated circuit VCC and GND pins (this is very important if device is intended to work under higher frequencies, where switching is performed under MHz scale, in that case bypass capacitor is used to provide additional current to the integrated circuit when it is needed). If you pay attention, multilayer ceramic capacitors are used for LM393 comparators, used as gate drivers, for both SERPENT DC motor drivers/controllers. Capacitors are used for low pass filters but they are also used for bypassing to provide additional electric current when it is needed, but it is important to understand that difference does exist between low pass filtering and bypassing even if the same capacitors are used for both, filtering and bypassing.

SERPENT II - Pit VIPER Rattle (assembled blue-white edition) - Low pass filters / bypass.

Electrolytic capacitors and design rules

The figure #2 presents already used electrolytic capacitors. As we can see electrolytic capacitor is polarized device which means that it is important to pay attention during capacitor assembling to the PCB (printed circuit board). Depending on the capacitor manufactures, in some cases negative lead is marked with a negative sign, and in some cases positive lead (capacitor pin) is marked with a plus sign. Beside polarization mark, there are also labels about capacitance (for example 470 microfarads) and maximum allowed voltage (for example 10V). The maximum allowed voltage is important parameter regarding electronic device design and it should be chosen carefully. For example, figure #1 presents power linear voltage regulator or power supply. The power supply provides 15 VDC at the output. To provide voltage stability we will place an electrolytic capacitor between 15VDC and ground, in following fashion: positive lead of the electrolytic capacitor is connected to 15VDC while negative lead (capacitor pin) is connected to the ground. Let say that electrolytic capacitor capacitance is 100 microfarads, but what about working voltage? That is a good question since we have to choose it and there is a simple rule how to do that. If a linear voltage regulator provides 15VDC at the output, that means that electrolytic capacitor must be rated for 15 VDC minimum, but practically, always higher value is selected in order to avoid electrolytic capacitor explosion. So, instead to choose 15VDC exactly, we will choose 20VDC or more. At this point you have to check, by looking at electrolytic capacitor datasheets, does electrolytic capacitor exist for combination 100uF/20VDC. If does exist, that is great, otherwise select first higher voltage value regarding capacitance of 100uF. If we choose electrolytic capacitor declared on 10VDC maximum (instead of 15VDC minimum), and place it between the output leads of the voltage regulator declared on the 15VDC output, electrolytic capacitor will be permanently damaged by explosion.  

This was only an example regarding voltage regulator, but let say that we are dealing with five different voltage potentials regarding electronic device: 5VDC, 10VDC, 18VDC, 35VDC and GROUND (do not forget that ground is also a voltage potential with zero value). Now, because we do know that the device has five different potentials where the highest value is 35VDC, what does that mean regarding electrolytic capacitor? That means that the minimum voltage rating regarding electrolytic capacitor must be 35 volts DC. But, since voltage oscillations does exists in reality we will pick higher value for electrolytic capacitor, let say 40 volts DC. As for the first example, we will check by looking into the electrolytic capacitors datasheets does combination capacitance/voltage exist for the related electrolytic capacitor. If chosen voltage does not exist we will pick first higher voltage value. Why we do this? Lat say that electrolytic capacitor is connected to 18VDC, if 35VDC by some kind of failure appears on the electrolytic capacitor, it will be damaged permanently by explosion since 35 VDC is overvoltage regarding capacitor declared on 18VDC. That is the reason why we pick capacitor voltage rating by highest voltage potential related to the electronic device (or some part of the electronic device), and by doing so, we are sure that capacitor explosion will be avoided.

Figure #2. Already used electrolytic capacitors (radial package) without etched protection against explosion (click to zoom).
The explosion of the electrolytic capacitor could be very dangerous and it is worse if the capacitor is designed without etched protection against explosion on the top side, as it is presented in the figure #2. The electrolytic capacitor is filled with electrolyte, if overvoltage is placed between capacitor leads, the electrolyte becomes gas and pressure is so high that electrolytic capacitor construction at some point is not able to handle generated (by heat) gas pressure, and that is the reason why electrolytic capacitor explodes. Overvoltage is just one way to blow an electrolytic capacitor, the same is happening when electrolytic capacitor leads are switched, when positive lead is connected to smaller voltage potential relative to the negative capacitor lead. So, it is very important to pay attention on polarity during electrolytic capacitor assembling. In general, always pay attention about maximum voltage rating and capacitor lead polarity.

Capacitor is electronic device like any other device and it is not ideal component. It has internal resistance ESR (equivalent series resistance), capacitance and inductance (capacitor leads - pins are inductors - parasitic) and in general it is a RLC circuit. Because it has internal resistance that means that power dissipation does exist inside capacitor while it is in working state. If capacitor works under normal working conditions dissipated heat will be much smaller in comparison with dissipated heat regarding overvoltage, but anyway, generated heat decrease the electrolyte properties and in time capacitor life cycle will be ended. As for the capacitance and maximum voltage ratings, under electrolytic capacitor datasheets you will find information about capacitor life cycle, and how long it will work under normal working conditions.

Figure #3. Electrolytic capacitors (radial package) with etched protection against explosion (click to zoom).
The figure #3 presents brand new electrolytic capacitors not already used (excluding testing after fabrication). As we can see, the polarity, beside the polarization mark, can be also determined by capacitor lead length. Lead with greater length is positive capacitor lead (or anode) while smaller lead is negative capacitor polarity. Also, information about capacitance as well as the maximum voltage rating are also printed. The big difference between electrolytic capacitors presented in figure #2 and figure #3 is capacitor package construction. All capacitors are packed into the radial package (there is also axial package), but with one big difference. Capacitors at figure #2 are uncontrolled regarding explosion with very high explosion intensity, but capacitors at figure #3 are controlled, which means that the explosion is directed to the top of the capacitor, since the etched surface of capacitors can handle much less pressure compared to the rest of the capacitor package construction. By placing etches on the top of capacitor we can be sure (but not 100%) that explosion will be controlled with much less explosion intensity in comparison with capacitors presented on figure #2. This is also important, and pay attention to buy electrolytic capacitors with etched top surface. The difference between etched surface and the smooth surface is presented on figure #4.

Figure #4. Difference between electrolytic capacitors with etched top surface (left) and without etched surface (right), Etched surface is used as protection against explosion. Left is better choice, so pay attention when you buy capacitors, always choose capacitors with etched surface (click to zoom).

Axial electrolytic capacitor package. Plus sign is related to the positive lead of the capacitor. This capacitor is intended for horizontal assembling but it can be also assembled in vertical position as radial capacitor (click to zoom).

SERPENT I DC motor controller/driver is assembled with radial electrolytic capacitors with etched top surface as protection against capacitor explosion. All zilsel-invent DC motor drivers are assembled with etched electrolytic capacitors (click to zoom).

Electrolytic capacitor PCB symbol and assembling

Depending on the silkscreen design, electrolytic capacitor PCB (printed circuit board) symbol could be different, but polarity label is always printed if soldering place is intended for electrolytic capacitors. Before we continue with electrolytic capacitor symbols, a couple of words regarding PCB. PCB has two major colors: soldering mask color and silkscreen color. In figure #5 white color is dedicated to soldering mask while black color is dedicated to silkscreen, that is the reason why PCB is labeled as White-Black edition (rule: soldering mask-silkscreen edition). Silkscreen is printed on the board and it is consisted of electronic device symbols and values. Silkscreen is a guideline for assembling process. Without silkscreen assembling will be difficult because it is not clear which soldering point belong to which electronic device. That is one of the reasons why silkscreen does exit and why it should be designed carefully. Beside electronic component symbols, silkscreen contains labels about the product and vendor: >SERPENT II - Pit VIPER Rattle< is product codename, >designed by zilsel-invent< is vendor label.

Figure #5. SERPENT II - Pit VIPER Rattle - White Black Edition. Electrolytic capacitor PCB symbol (click to zoom).

Now let’s go back to the electrolytic capacitor. Symbol for electrolytic capacitor is rounded with yellow rectangle (figure #5) and it is consisted of the polarity mark, in this case + sign is printed (positive lead of the electrolytic capacitor) and capacitance value, in this case 100uF (microfarads). During the assembling process (soldering of the electrolytic capacitor on the PCB) it is important to place the positive lead of the electrolytic capacitor pin “through the hole” (through-hole technology) labeled with positive mark sign as it is presented in the figure #6. You should always be careful here, if you change capacitor leads, the capacitor will be damaged permanently by the explosion. And for the last, pay attention on the label “C3” placed on the PCB at figure #5, on the left side of the “C3” label you can see the symbol for multilayered capacitor of 100nF in capacitance, used as an additional low pass filter. 

Figure #6. SERPENT II - Pit VIPER Rattle - White Black Edition. Electrolytic capacitor PCB assembling (click to zoom).
Related articles:  
SERPENT I - DC motor controller
SERPENT I - PCB DIY (do it yourself) assembling - video clips examples
zilsel-invent assumes no responsibility or liability for any errors or inaccuracies that may appear in the present document.
Specification and information contained in the present schematics are subject to change at any time without notice.


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