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WHAT DOES ESR

For ESR is defined as the equivalent series resistance of the components, in practice it is mainly addressed to electrolytic. It is essentially due to parasitic inductance and is all the more critical as the higher is the working frequency of the elements in question.

In modern power supplies, now apart from small niches of equipment that continue to have analog power supplies the rest is fed with switching power supplies for their undoubted advantages for reduction of overall dimensions and costs for their high output capacity due to extremely low in heat losses.

While in the past he straightened with the 50 / 60Hz power supply now modern power supplies work to hundreds of kHz and some even beyond, these frequencies have created a series of new equipment failure. The wear and premature death of electrolytic, because of their parasitic inductance is one of these, they dissipate energy in the form of heat that dries the internal electrolyte, wiping develops gas, once it reaches a certain pressure escapes. All electrolytic now already have of breaking pre-engraved lines already under construction, they facilitate the escape of excess gas without explosions. Once arrived at these conditions, the electrolytic lose their characteristics and must be replaced to return the circuits to them to normal operation downstream, restoring optimal supply conditions.

The basic problem is that not always the escape of gas is so blatant, sometimes an electrolyte may have subtle micro-fractures that do not change the outward appearance, unique aspect is the modification of the electrical characteristics and precisely those to measure to be sure that the component is efficient.

THE METER

This meter ESR provides the measurement of the equivalent series resistance (ESR) capacitors. The circuit diagram is very simple, They are used only two NPN transistors. The device provides measurements of the equivalent series resistance in the range from 0,1 Ω a 23 Oh.

Despite its simplicity has its one study to reduce the essential components and the ability to work with a single AAA battery also download ( tested up to 0,9V power).

The heart of the circuit is a Colpitts oscillator transistor Q1 configured with common collector . The oscillator frequency is determined by the components L1, C1 and C2. The operating frequency is about 15 kHz. The capacitor Cx under test is connected in series with C1. Since the capacity of Cx is much higher than the capacitance C1, the ability to Cx does not affect the oscillator.

The oscillator produces oscillations only if the equivalent resistance of the series capacitor Cx under test is low. With the growth of the Cx series resistance, It decreases an amplitude of oscillation. At a certain point, if the resistance of the series is too high, there will be no oscillations. The series resistance of Cx is inversely proportional to the amplitude of the oscillations.

The D1..D4 diodes are used to discharge the capacitor to be tested was loaded, they protect the circuit from damage but are irrelevant in normal operation, In fact, their threshold voltage is higher than the amplitude of the oscillations.

The transistor Q2 operates as a rectifier and current amplifier, the capacitor C4 suppresses the ripple voltage on the collector of Q2. The milliammeter PA1 provides an indication of the resistance of the capacitor in series in the test. As PA1 is possible to use any suitable milliammeter with a deflection value of full scale of 0,5 mA … 15 mA. Instead of milliamperometer, for the measurements it is possible to use a digital multimeter (DMM) configured as an ammeter.

In my case I used a tool of an old Vmeter, if the indication of the pointer reaches the red zone, the capacitor is good, given the low resistive values ​​you can safely run the test without removing this product from the circuit. A great time-saver when you do repairs.

For the measurement of ESR, it is necessary to short-circuit the probes, adjust the potentiometer RV1 to deflect the needle of the instrument until the end of the scale, finished calibration which essentially depends on the battery charge can perform the test. Now connect a capacitor to the probes and see the ESR reading. More needle is close to the scale, the lower the value of ESR. If the needle is in the first two thirds of the scale, then the tested capacitor is not good.

The current consumption of the circuit is approximately 1,2 mA. View my proverbial “dowry” in leaving the tools turned on only to realize that they are useless when they are needed because of the battery I used a normally open switch button instead, the instrument needs to operate the button was pressed. I will never leave leaving lights!!!

A BIT’ THEORY

This final section can be considered as general culture and it is not critical to the realization, if not interested immediately passed the final part of the greetings

There are many different configurations of oscillators based on a single transistor. In my case I chose the configuration Colpit of oscillators The three versions are: common base (CB, left), common emitter (THIS, medium) and common collector (CC, to the right). All of these circuits contain a resonant LC circuit composed of an inductor L in parallel with C₁ e C₂ serially, with a resonance frequency

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C_S It is the equivalent capacitance of the series combination of C₁ e C₂ .All the others CAPACITORS (without a subscript) are coupling capacitors that have a large enough capacity and can therefore be treated as a short circuit for AC signals. .

Here are the requirements for these oscillator circuits:

1. a resonator of the LC tuning circuit that generates sinusoidal oscillation at its resonant frequency

2. a positive feedback circuit that maintains the oscillation.

How does each of these circuits can be understood qualitatively as follows:

• CB with the base massa per AC: the collector voltage V_c is the output, a fraction of which the center point between the two capacitors, “touch point”, It is fed back to the emitter with a positive feedback circuit.
• EC with the emitter connected to ground for AC : the collector voltage V_c is the output, that is transmitted through the circuit of the LC resonator at the base. Since the gripping point is grounded, the sinusoidal voltage across LC produces voltage of opposite polarity to the remote ends of C₁ e C₂, that is, V_{c 1} = V_B e V_{c 2}= V_C They have opposite phases and therefore form a positive feedback circuit.

• CC with the collector grounded for AC: it is a voltage tracking circuit in which the emitter voltage V_C that is the output follows the input voltage V_B . The feedback from the emitter through LC at the base form a positive feedback circuit.

More specifically, we consider the common collector circuit used in this circuit. To find out why the circuit oscillates and which is the resonant frequency, unlink the base path of the circuit and consider the open loop gain of the circuit H= V_O / V_I feedback. further we model the transistor with a Thevenin voltage source V_I with a R, as shown in Figure:

As borne by the Thevenin source, the resonant circuit receives an input V_t to the common point of the two capacitors and produces an output V_O through the parallel combination of L e C₁ in series with C₂ . Applying KCL at the junction and we get:

that means,

Solving for V_t you get

that is maximized if the frequency is such that the imaginary part of the denominator is zero:

therefore

It is the resonant frequency, in which the voltage V_t becomes equal to the source voltage V_t –V_i , since the impedance of the resonant circuit as a load of the Thevenin source is infinite:

The denominator becomes zero and , that is, there is no current drawn from the source from the resonant circuit. Consequently, the voltage drop is nothing. Now the output voltage can be found from the voltage divider:

The open loop gain is:

We see that the open loop gain is real but greater than 1. However, the non-linearity of the transistor in the feedback path will force him to become 1.

Amilcare Greetings