How does the MrCarlsons lab capacitor leakage tester work without high voltage?

[bitseeker]

It seems to me that low voltage testers like MrCarslons or mine are usefull only for specific problems and are not giving ultimate answer.

Correct. It's not a complete replacement for high-voltage leakage testers. It's safer, due to the low voltages involved, but there are trade offs as a result. As with any tool, it's important to understand what it is and isn't good at.

[Kalvin]

It's only four transistors, no crystal or any  oscillator I could see. It seems like it amplifies the leakage and has this diode that had to be very stable and not exposed to heat while soldering.

Measuring a leakage with a diode or two is be quite simple as shown in this EDN article:
https://www.edn.com/design/test-and-measurement/4443319/Multi-decade-current-monitor-the-epitome-of-simplicity

Probably it is a simple R+D circuit charging the capacitor and measuring the charging current, and transistors are there just for amplifying the voltage across the diode.

[Beamin]

You're looking at this the wrong way.

What someone reading this thread would do, is watch public videos showing the operation of the device, then sit down with pencil and paper and work out how to design something that behaves the same way.

You posted in another thread, "How would you go about designing something?" Well, here's your opportunity.

Start in this thread. Begin by describing exactly how the device behaves (that's your objective).

Then start proposing functional blocks that could reproduce that behavior. Get feedback from the forum if you get stuck.

Next start thinking about how to implement those functional blocks.

Last, build a prototype and test it. If (when) the prototype doesn't work, debug it, adjust it, and iterate until it does.

Do that and you will be way ahead.

I guess I don't know where to start because I don't understand what i's looking for. In the HV design it's simple. You add the voltage then measure how much is leaking across by just looking at how much current the circuit is pulling. The eye opens up at first because current is being used to charge the amp its current goes down as the charge levels out. If that current never goes to zero or starts leaking after its charged the eye starts to close. Simple.

In this circuit we have 36 volts. We also seem to have a circuit that looks at the small voltage difference from leakage and then amplifies using a transistor it and feeds it into another circuit that creates the led bar graph based on how much voltage it is getting. There is no section that steps up the current or creates AC as far as I can see it's just four transistors three look like stages in an amp. Can you assume leakage at HV by detecting small amounts at low voltage and that number scales directly with voltage? As far as my understanding this wouldn't work because the dielectric material will leak nothing until it hits some magic break down voltage hence why it has multiple voltage switches on the old equipment.

Maybe this is too advanced a project but I even figured out the super probes and went from no understanding of tube to a reasonable level just by watching his videos; I can explain to someone the differences in a tube and a transistor how a tube inverts the signal and how feeding current into the tube lowers output instead of gain like a jfet.


 

[IanB]

You may be assuming too much from this circuit. It may not be equivalent to a high voltage tester, and the results from using this tester may not tell you about high voltage failure modes.

Now I haven't looked at the actual circuit, and I haven't seen Mr Carlson's video on how it is designed, so I am basing everything on observation of him using the tester.

But what it looks like from the outside is that the circuit charges up the test capacitor and then tests to see if it maintains voltage under charge. If the capacitor holds the voltage it is good. If the capacitor does not hold its voltage when charged then it leaks. This can be made to drive a display.

Let's say we wanted to design a circuit around this idea. One way we could do that is to charge up two capacitors side by side. One is a known good capacitor and one is the capacitor under test. After they are charged, compare their voltages using a differential amplifier. If the voltages remain the same, the test capacitor is good. But if the test capacitor leaks its voltage will decline and the differential amplifier will see a voltage difference. This difference can be used to drive the bar graph on the display.

I've no idea if Mr Carlson's tester actually works this way, but it is one way I can think of to replicate the function of his device.

[joeqsmith]

I made a few attempts to see if there was some sort of general trends in the leakage.  You may find that thread here: 

https://www.eevblog.com/forum/testgear/using-low-voltage-levels-to-test-hv-capacitors-leakage-current/

I watched his transformer video.  As he was showing the meter in resistance, I was surprised he never switched it to conductance as the meter had that feature.  It still may not have real anything but would have been one more bit of data. 

[oPossum]

It is a sensitive current meter, nothing more. There is no reference cap. Two transistors are used to amplify the current. There is no feedback, so the characteristics of the transistors used will determine the gain. The transistors used in the final version are not what is on the publicly available parts list. They have much higher Hfe.

The first private video explains and demonstrates the difference between a meter that detects dielectric breakdown (Heathkit IT-11) and one that detects very small amounts of leakage at a relatively low voltage (his design). He tests several different caps with the IT-11 and his tester. Leakage at low voltage was detected by his device every time. The last demo shows a cap that the IT-11 shows as good at 600V and once again his tester shows the cap to be leaky.

[joeqsmith]

It is a sensitive current meter, nothing more. There is no reference cap. Two transistors are used to amplify the current. There is no feedback, so the characteristics of the transistors used will determine the gain. The transistors used in the final version are not what is on the publicly available parts list. They have much higher Hfe.

The first private video explains and demonstrates the difference between a meter that detects dielectric breakdown (Heathkit IT-11) and one that detects very small amounts of leakage at a relatively low voltage (his design). He tests several different caps with the IT-11 and his tester. Leakage at low voltage was detected by his device every time. The last demo shows a cap that the IT-11 shows as good at 600V and once again his tester shows the cap to be leaky.

I suspect with the caps I damaged that I have some that would read false on the low voltage and others that would read false on the high voltage tester.   He mentioned the threshold for the alum elec but not for the others but he did go on to talk about how you could easily adjust the threshold.  I took this to mean that he is suggesting that it's not a one size fits all solution.    If he offered to sell a completed working unit, I would pick one up.  If I build one, there will always be a question if I used the right parts or if there was a problem with my construction practices.