The voltage developed across these resistors is fed back to the inverting input of U3, thus completing the feedback loop. Within the diode bridge, the AC is rectified and passed through the front panel meter, which responds only to the average i. By enclosing the bridge within the op-amp feedback loop, most of the non-linearities inherent when a bridge is used to drive a moving coil meter are removed. Switch SW1 puts R20 in parallel with R21, reducing the value of the current-sense resistor combination, thus increasing the sensitivity of the meter.
With it open, an ESR of five ohms is required to drive the meter to full scale. The gain of this stage is set by R17, R18, and R The latter is a 10K ohm trimmer potentiometer used to set the calibration of the ESR meter after the circuit is built. If the ESR instrument is powered up with no CUT connected, R24 limits the average current through the panel meter to a maximum value of about 2 mA, thereby making life a bit easier for the meter.
This simplifies the circuit design and makes it easier to follow, in my opinion. A single-supply approach would require the additional complication of providing a virtual ground reference throughout the ESR meter. The -5V bus is easily supplied by U4 — a dandy component from Texas Instruments TI that conveniently puts out a DC voltage equal in magnitude to its input, but with a reversed polarity.
I used the services of ExpressPCB www. Their standard low cost MiniBoard fits very nicely into a 3 x 4 x 5 inch aluminum enclosure, with plenty of room for a mA meter and two binding posts to be mounted on the front panel. Refer to Figures 4 , 5 , and 6. ESR meter after calibration.
The meter is displaying the value of the one ohm test resistor. Internal wiring, showing the mounting of the circuit board and the cabling to the front and rear panels. One end is soldered into a hole in the PCB, and the free end is formed into a loop for easy grabbing by clip leads or test probes. Figure 6 is an inside view of the enclosure, showing the internal wiring. Here you can see that connections to the front panel meter and binding posts are brought out from the PCB by four-pin male connector J2, and power from the rear panel via two-pin male connector J3.
The current requirement is fairly modest. The whole circuit runs on less than 40 mA. A good quality wall wart type of power source works very well, as does a 9V alkaline battery.
The front panel label sheet and a new face for the panel meter were drawn using Microsoft Visio, printed on heavy paper stock, and glued in place. There are two adjustment trimmer potentiometers on the circuit board. One R8 is used to adjust the output of the phase-shift oscillator to about 1. Full details of this procedure can be found in the downloads at the article link. Figure 4 shows the result of this setup with a one ohm resistor connected across the CUT binding posts.
Most projects hit a snag or two along the way, and so did this one. If you look carefully, you may spot a small inconsistency between the photo of the printed circuit board in Figure 3 and the ExpressPCB layout file included in the online files. This is the result of an initial design goof on my part, which required me to cut a couple of PCB traces and re-locate components R7 and C4. I revised the PCB layout after the fact, and the ExpressPCB layout file in the downloads has these corrections and agrees with the schematic.
The impedance of the surrounding circuitry is normally much higher than the ESR being measured, and the voltage developed across the CUT is quite small: less than millivolts — much too low to switch on any semiconductor junctions in the vicinity.
Power to the equipment should be off, of course, and the ESR meter should probably be running off an isolated power source like a 9V battery. I have not tried this type of measurement myself, but I see no reason why it would not be successful.
You can use an ESR for other experiments like checking low ohms resistors such as 0. Various factors contribute to power loss in capacitors. You can sum these factors up as ESR. A good example of an oscillator circuit runs at approximately kHz.
Also, kHz is the industry standard for making ESR measurements. One section of this circuit acts as a phase-shift oscillator. The other section works as an amplifier and a buffer. Due to the phase-shift oscillator having a high output impedance, this section prevents the oscillator circuit from overloading. Also, you can adjust the level of the kHz signal with the gain-control potentiometer.
The ESR detector is where most of the action happens. The first section is a voltage-to-current converter that converts the kHz signal from the oscillator to a peak-to-peak current of about 7Ma. So, you can connect the CUT capacitor under test inside the feedback loop of this stage.
Also, you can do this with two front panel binding posts so the CUT receives the same current. In this section, a single-supply approach requires you to provide a virtual ground reference throughout the ESR meter.
On the other hand, the -5v bus gets its supply from a dandy component that dishes out a DC voltage equal to the magnitude but with a reversed polarity to the input. First, the TR1 on the above diagram and the fastened NPN transistor create simple feedback that triggers a blocking oscillator.
The blocking oscillator starts oscillating at a high frequency. Additionally, the oscillations create a consistent magnitude of the voltage over the 5 turns of the secondary transformer. Also, the oscillator applies the induced high frequency across the CTU. Also, you can see an op-amp joined with the low voltage high-frequency feed. Plus, it works there as a voltage amplifier.
If the ESR is absent or you have a working capacitor, then the meter will suggest a complete deflection. Thus, it shows a tiny ESR over the capacitor, which in turn goes down to zero for various capacitors—with different levels of ESR. Now, a lower ESR might cause a moderately higher current to build up over the op-amp inverting sensing input. Also, it correspondingly displays on the meter along with a lofty degree of deflection and vice versa.
I was expecting the repair to be short lived. It depends on the design excitation voltage and the dut surrounding circuitry. If the excitation voltage is low, it may not turn on a transistor or diode, sand there isn't other resistors or inductors around, it can be fairly accurate. Sign up to join this community. The best answers are voted up and rise to the top.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. ESR meter: Are in-circuit readings accurate? Ask Question. Asked 7 years, 8 months ago. Active 4 years, 9 months ago.
Viewed 9k times. Also, three close votes for the question being about the use of electronics? For questions about test equipment that would only be used in an electronics design context, I think the close votes are uncalled for. There is a nice cirquit as well. In any case do not confuse real ESR meter with a square wave voltage drop detector ESR is quite complex issue. The meter I'm using claims to use a sine wave rather than a square wave , however upon analysis it's more like a rounded triangle-wave.
Add a comment. Active Oldest Votes. Spehro Pefhany Spehro Pefhany k 12 12 gold badges silver badges bronze badges. Nice experiments. Andy aka Andy aka k 21 21 gold badges silver badges bronze badges.
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