Capacitance leakage tester build input

[Jordanaudio]

Hi all,
First, thank you for this wonderful resource and your willingness to help. 

I am clearly a beginner and had minimal electrical engineering training many years ago in college (meaning I'm not an electrical engineer). I am attempting to build a capacitor leakage tester, the Amateur Wireless Association's design https://www.antiquewireless.org/homepage/pcb-item-4/. I have purchased their PCB. I saw YouTuber xraytonyb's build of their project and really liked his modification. The original design just has the metering through a DMM or Analog meter that you attach to the circuit. Xraytonyb's modification added a large analog volt meter, used to determine the leakage current, and a digital volt meter to determine the voltage across the DUT.

I understand the logic (or math) behind the need for a buffer circuit so the analog meter's low resistance doesn't load the capacitance tester circuit and have modified the design of xraytonyb's buffer. His design used a +/-9V VCC for his op amp because it was something he already had on hand from a previous build. Since I didn't have any -9V regulators, but had 5, 6 and 12 V +/- regulators in my parts bin, I modified the design to take a +/- 12V VCC. I have drawn up a schematic in KiCad for both the power supply and buffer circuits. I think, based upon the rotary switch I've ordered, that I can mount the entire small board to the front panel of the case, so I kept the power supply board separate, which I also would imagine is a good practice anyway.

I have attached screen shots of the schematics I drew up, as well as a picture of the AWA's PCB page and xraytony's finished leakage tester.

I intend to purchase a Jameco Valuepro 102111-R Power Transformer, 24 VCT/1A, 115/230 Vac transformer. For the analog meter, I am planning on the Adafruit 5V large panel meter. And the digital meter is an inexpensive Amazon buy - YB4835VA Digital Voltmeter Ammeter DC 600V 100A.

Would this transformer be adequate to power the buffer circuit and also provide a +5VDC output to the digital meter? The UA741 op amp output supply current is listed at 3.3 mA full range and the digital meter says 20mA for regular operation or 200mA isolated operation. The transformer specs it is 1A, so that should plenty of extra supply to account for any losses?

The capacitance leakage tester circuit is powered separately by a Hammond 229A230 as spec'd out in their build sheet. Because I'm dealing with mains AC I don't want to do anything dangerous or hazardous. They provide no schematics, but a BOM which does not include the AC parts. I know I'll need an IEC connector, a power switch, a fuse and a varistor for the mains input. Is this the sort of MOV i should use? https://www.mouser.com/ProductDetail/YAGEO/361KD20?qs=PzGy0jfpSMuY6yzw3dD%2FOw%3D%3D Soldered across the L and N post fuse?

If I used a fused switched inlet like this? https://www.mouser.com/ProductDetail/Bulgin/BZH01-Z0000-10?qs=klolToFEUMVTQKfIDu%2FEyQ%3D%3D

Do you see any problems with my choices (other than a novice dealing with mains voltages)? Thank you for your input.

[Kleinstein]

The supply circuit could drop one of the rectifiers. The transformer center tap can be directly connected to the DC side ground. If one looks careful in the circuit with 2 rectifiers there are diodes anti-plarallel that could be repleced by the direct link.

The other point is that one should avoid tantalum capacitors at a supply that is capable of providing high current (e.g. more than 1 A). 330 nF is anyway an odd / small values. Some  1 µF ceramic or maybe just 100 nF (maybe 2 x in parallel if a bit paranoid) as at the output side would be more suitable.

The amplifier part is drawn a bit odd. This starts with an odd symbol for a normal 741 OP-amp.
The actual isolation tester part is somehow still missing. One may have the gain switching aready as part of the circuit there and may not need the extra amplifier part. Depending on the meter used, one could get away with just series resistors to reduce the sensitivity and no actual gain.

With such a simple ciruit, one could probably have all on one PCB. If one wants to reuse just the power supply part, one could still leave the small rest not populated.


The digital volt + amps meters can differ in how the connections are made. One should check the data-sheet there if it is suitable and how to connect with the supply an measurement current.

[watchmaker]

Here is mine.  https://www.eevblog.com/forum/repair/my-take-on-a-capacitor-leakage-tester/msg5689261/#msg5689261