Hmmm, it seems that page
www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/ is down now. I found the google cached page but it does not show the graphics which are the most important part. Maybe it will come online again soon.
At any rate, here is my version as it stands now. Still a work in progress though. Notice some values are not final yet and some labels are not consecutive etc. But I will answer any questions.
The ESR meter has two test probes to connect it to the capacitor under test without removing it from the circuit. This injects a signal of about 100 Khz and under 100 mv(pp). This low level signal will not be enough to trigger semiconductors in parallel with the capacitor and most components one can expect would have higher resistance and would not interfere with the measurement.
The circuit is a bridge of four resistors which is normally balanced and the capacitor is placed in parallel with one of the legs which introduces an imbalance which is amplified by an OpAmp. The lower the ESR, the greater the imbalance and the higher the reading.
OpAmp A, first from the left, divides the 9V power supply into two halves so we have +4.5 and -4.5 with respect to the center point which becomes our virtual ground. This symmetrical power supply is required by the opamps which follow.
Second opamp, B, is the basis for the 100 Khz oscillator. In practice I have noticed that the frequency tends to be not close to 100Khz, probably due to component tolerance values, so it is best to check it and trim values to bring the frequency close to 100 Khz. While this is not essential, it permits better comparisons between units built. At the output of this opamp we should have a square wave between -4 and +4 V pp.
The transistor which follows serves several purposes. It shifts the level of the signal so it is always positive, between 0 and +4 and it isolates the load that follows from the output of the opamp which does not have a low enough output impedance to drive the bridge directly.
At the collector of the transistor we find the bridge which is the central part of the unit. The two upper resistors are much larger in value than the two lower ones so that the voltage which is put to the capacitor under test is a small fraction of the output of the transistor. Any imbalance in this bridge is amplified by the next opamp, C. Let us analyze the different cases carefully.
1- When the leads are open, not connected to anything, the bridge is balanced and the output of the opamp will be a constant zero volts.
2- When we connect a good capacitor we are shorting one branch of the bridge to ground in AC only, not in DC. Therefore the output will be an AC waveform with no DC component, i.e. it will be centered at 0 V and swing up and down from 0V.
3- When we further short the capacitor, then the branch of the bridge is shorted to ground in DC also and the output of the opamp will be shifted upwards so that it has a positive DC component. Now the entire waveform will be above 0 V.
At the output of the opamp we separate the AC and the DC components. First we see a low pass filter, composed by a resistor and capacitor, which allows the DC component to drive the base of the transistor which controls an LED. This LED will light if the capacitor under test is shorted or has high leakage current.
The AC is allowed through a high pass filter, formed by a capacitor (which blocks the DC component) and a resistor, into the input of the last opamp which is a rectifier. The lesser the ESR of the capacitor under test, the greater the rectified voltage will be. I adjust the circuit so that the maximum voltage out of the rectifier is about 1 volt. Then I have inserted a diode in series with the instrument. The purpose of this is to expand the range of the instrument in the low ESR values and compress it in the higher ones. As the voltage rises, at first the needle hardly moves, but as the voltage passes 0.6 volts, the needle moves faster. This means the lower 0.6 volts hardly move the needle and the upper 0.4 volts are expanded to almost the entire range of the instrument.
By adjusting the output to less than 1 V we could make the instrument even more sensitive.
I have built a couple of prototypes using cheap galvanometers I had in my junk box. I have considered modifying the design to use a bar of LEDs but I decided not to do this for two main reasons. One is that a galvanometer can usually be found cheaper and gives more resolution than a bar of, say, 10 LEDs. Another is that a galvanometer will use much less battery power which is an important consideration when using 9 volt batteries.
I am still considering adding a circuit which will make the LED flash briefly at intervals as long as the instrument is turned on. This will remind us to switch it off and therefore save batteries.
Cheaper ESR meter
keep going!
to get some components!
Linear Mode
