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In this answer it is mentioned that the EEVBlog-branded BM235 meter has a threshold of between 30 and 480 ohms for its continuity beeper (as seen on page 25 of the datasheet).

My first thought was "why isn't that an exact value?", but I realized soon afterward that of course it's not exact, as any meter with a good continuity beeper uses analog circuitry to get a fast beep response, and that analog circuitry inherently has tolerances. So I understand why they give a range.

What I don't understand is why the range is so large. I would understand something like 30~60 Ω, but this is more than an order of magnitude! What is it that causes this extreme variation in the beeper threshold?

Is this sort of variance normal? What sort of circuit is typically used for this, and what makes it so sensitive to component tolerances? Is it even that sensitive to component tolerances?

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  • \$\begingroup\$ I see someone has voted to close this as opinion-based; should you see this, could you explain how it's opinion-based so that I can edit to ask something more objective? \$\endgroup\$ – Hearth Jun 17 at 15:39
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    \$\begingroup\$ Maybe ask for typical circuit schema for this function. I'm wondering if that's mostly hysteresis and it's around 30 for turn on and 480 for turn off. \$\endgroup\$ – Transistor Jun 17 at 15:46
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    \$\begingroup\$ I did not VTC, but I think that the question can be seen as opinion based because to answer it speculation is needed. Nobody really cares if your tester beeps below 1 ohm, or 10, or 100... So any reasonable threshold is fine. Why they picked that range, which is reasonable? That is impossible to answer without guessing. \$\endgroup\$ – Vladimir Cravero Jun 17 at 15:49
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    \$\begingroup\$ The DCR of an audio transformer can be well above single digit ohms, so its all relative I suppose \$\endgroup\$ – sstobbe Jun 17 at 15:54
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    \$\begingroup\$ Some possible factors: battery voltage variation, battery internal resistance variation, test lead variation, contact resistance variation, current required to cause a beep. Some meters may do that function cheaply. How many meters have you surveyed. Which is more important, detecting that the circuit is definitely open or definitely closed. \$\endgroup\$ – Charles Cowie Jun 17 at 16:42
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The reason the range is so large is that few people care what the range is.

The 'continuity beep' is a 'nice to have' feature, not an essential feature. It doesn't do anything that can't be done on the ohms range, albeit avoiding the need to read the display.

When a large enough segment of the market starts demanding continuity beep thresholds with a tolerance of x%, then the manufacturers will start specifying and providing them.

Test is a significant part of the cost of making equipment. If a feature doesn't need testing, because it's not accurately specified, then that feature can be added very cheaply.

The reason the ABC company might add a continuity beep function is that XYZ company have one on theirs, and we don't want to lose out on a sale when customers hold the data sheets up side by side and do a comparison. "1% on DC voltage, check, at least 1000v, check, continuity beep, check!" If there's no standard, it just doesn't get standardised.

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  • \$\begingroup\$ Sure, but I would assume there's a physical reason behind this. They're not going to just stick random resistors in to get their division; they're going to use the same parts, probably 10% tolerance, for every meter they make. I wouldn't expect that much variation unless they used parts with different nominal values, not just different actual values. \$\endgroup\$ – Hearth Jun 17 at 17:23
  • \$\begingroup\$ Just curious: Is this answer based on your own thoughts or experience because e.g. you/your company sells multimeters or based on research? \$\endgroup\$ – Huisman Jun 17 at 19:28
  • \$\begingroup\$ I worked for a test gear company, not quite DMMs, our boxes sold with 5k to 50k price tags, but the principles were the same. If the customers want it, build it, charge for it. If your competitors have thrown something 'nice to have' in theirs, then we have to as well to protect our market, but maybe make it slightly different if there's no standard. Test is a very large part of production cost, if you don't need to test it, features can come really cheap. \$\endgroup\$ – Neil_UK Jun 17 at 19:34

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