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| Gratuitous Continuity Speed Test and Comparisons |
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| Antonio90:
--- Quote from: KungFuJosh on August 12, 2024, 01:30:03 am ---I want to get deeper into the methods people are using with an AWG. Some of the claims I've seen are questionable. --- End quote --- I can have a look later for a screenshot. You really just need to setup a pulse with a positive baseline, so that the meter detects voltage (drop) on the terminals, and assumes high resistance. Then the pulse has to drop to around zero volts, often slightly negative, which the meter reads as very low voltage drop, and assumes very low resistance. The pulse can most likely be set up with any decent AWG, but it is faster to set a regular, positive pulse with negative offset and reverse the leads. That's What I did with all DMMs except for the UT181A, which worked without reversing the leads. In any case, this doesn't take into account the source characteristics of the DMM, which might make a difference. The proper way to test it is with a MOSFET, as was already stated. For the delay measurement one would probably need a microphone and a buzzer to check for the inherent delay of the measurement, and then trigger the scope on the AWG pulse while recording the output of the microphone, calculating the interval and substracting the inherent delay. |
| Njk:
The fundamental problem is that a standardized method assumes a standard setup. It's a project that will require significant investment of time and effort, to say the least. Will never materialize, I'm afraid. Continuity test is usually about a resistance measurement. It was suggested to use a generator. The generator generates a voltage pulse, not a resistance pulse. But it's a minor problem, it's easy to convert a voltage pulse to a resistance that the DMM can sense, with FET or something. Usually, input of such a device is quite delicate so a reasonable input protection circuit will be required (a bit more additional components). In DMMs, the result of the continuity test is indicated by sound so the only non-intrusive way to convert it to a machine-readable form is to use a microphone. Therefore, a microphone amplifier will be required. I think it'll be better to stay away from any sort of digital audio (PC sound cards, etc.) because digital processing is associated with some latency and annoying calibration procedure must be followed to take the latency time into account. A simple analog amplifier seems a better solution (add a few more components). Anyway the sound is generated by a beeper, internal location and characteristics of which is up to the DMM's manufacturer. That means that for each particular DMM, operator will have to identify optimal spatial location of the microphone relative to the DUT. The chances are in the optimal position, the microphone will be sensitive to a not related environmental sounds which can compromise the measurement accuracy and make the results debatable. That leads to the idea of a standardized fixture, e.g. in the form of wooden cabinet acoustically isolated from the environment. The microphone (or an array of them) are placed inside at a fixed position. The cabinet is of enough internal volume to accommodate a handheld or a bench-type DMM (if we would like to test the latter type as well). The problem with a stationary DMMs is that many of them are fan-cooled. A high pass filter in the mic amp will be required to filter out a hum from the rotating impeller (even more additional components). From the electrical perspective, there is nothing complex. A mic(s) followed by an amp followed by a rectifier followed by a comparator that produces a sharp edge at the moment of sound attack, so the time position can be easily measured. BTW the comparator output can be routed back to the input of the voltage-resistance converter device (perhaps through some delay circuit) to create a positive feedback oscillation that eliminates the need for external generator. It can be more complex to define the test criteria. The DMMs can be very peculiar. The beep first starts for several hundreds of ms, then interrupts for tens of ms, and only then starts sounding continuously. No idea how typical it is but I've one evidence. Another DMMs, like my Fluke 189 generates not a tone, but a sound of quite weird cadence. I think it's a feature. In a really noisy environments, like a server room, that irregular cadence is easy to notice while a musically perfect tone would be a disadvantage. It will be nice if the test setup can differentiate between a well-tempered DMM and a poor one. |
| Demon Xanth:
The method I would use: Set up a DPDT or DPST relay with one pair of contacts going to a set of banana jacks, the other going to an LED. Monitor with a high enough frame rate camera as you energize the relay. Review the video to look at the difference in frames between the meter's response compared to the LED lighting up. The relay's speed would be largely irrelevant since both sets of contacts would have (for all practical purposes) the same delay. |
| Fungus:
--- Quote from: nctnico on August 12, 2024, 08:37:42 am ---I wouldn't use a relay as this has a delay in 10's of milliseconds. --- End quote --- Not from the sound of the click. The problem is knowing when the meter responds, hence using something audible and looking at the audio recording. |
| BeBuLamar:
--- Quote from: Fungus on August 12, 2024, 06:19:23 pm --- --- Quote from: nctnico on August 12, 2024, 08:37:42 am ---I wouldn't use a relay as this has a delay in 10's of milliseconds. --- End quote --- Not from the sound of the click. The problem is knowing when the meter responds, hence using something audible and looking at the audio recording. --- End quote --- You can hook a scope to the 2 test leads and see the voltage accross it drops when the relay closes. On the second channel you feed the signal from the microphone for the beep. |
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