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Please keep in mind this is my very first electronics project so it’s entirely possible I’m doing something wrong. Here’s the deal: I’m building a contact microphone from a kit that came with four 150R resistors and four 3.3M resistors. My digital multimeter shows a correct reading for the 150R resistors but does not react at all when I test the 3.3M resistors. To cover my bases: I have the red lead plugged into VΩ and the black lead plugged into "COM". I have the dial set to "200" when measuring the 150R resistors and set to "2M" when measuring the 3.3M resistors. I have touched the leads together before each reading.

When I touch the leads together at 2M the display changes from "1" to ".000". But when I touch the leads to the resistors, the display simple reads "1".

Any help is much appreciated!

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    \$\begingroup\$ at the 2M range, it can only read up to 2 MΩ. Your resistors are 3.3 MΩ. Put it on the 20M range. If your meter doesn't have a 20M range, you can't measure these resistors with that meter. \$\endgroup\$ – Hearth Dec 19 '19 at 19:43
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    \$\begingroup\$ Thank you! Obvious beginner mistake I guess. \$\endgroup\$ – Michijake Dec 19 '19 at 19:59
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    \$\begingroup\$ It is an obvious beginner mistake -- but all of us have been obvious beginners at some point in our lives. \$\endgroup\$ – TimWescott Dec 19 '19 at 20:14
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    \$\begingroup\$ If you can't get a multimeter w/ adequate range, you still can measure TWO of those resistors in parallel. They should be something like 1.65 MOhm. Combining every two of them, you can make sure they all are of proper resistance value. \$\endgroup\$ – fraxinus Dec 20 '19 at 10:00
  • \$\begingroup\$ What is YOUR resistance? As a sanity check for the multimeter, it should be greater than 10kohm and less than 1Megohm. Do not tell a lie when doing this test; it causes your resistance to drop. \$\endgroup\$ – richard1941 Jan 2 at 5:40
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Assuming you don't have a 20M range, put two of the identically marked 3.3M resistor in parallel on the 2M range, and you should read about 1.65M.

The "1" with the rest of the display blank means that the resistance is too high for that range. For example, if you try to measure a 330\$\Omega\$ resistor on the 200 ohm range you'll see the same thing (with the decimal point in a different position).

Avoid touching the leads (or touch no more than one of the connections to the multimeter leads) lest you affect the reading.

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    \$\begingroup\$ Just to point out: Of course the 1.65MΩ parallel reading only tells you that both resistors have at least 1.65MΩ. \$\endgroup\$ – Michael Dec 20 '19 at 7:56
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    \$\begingroup\$ @Michael as the OP has 4 (3 would be enough) it's fairly simple to measure \$\left(\frac{1}{R_1}+\frac{1}{R_2}\right)^{-1}\$ and \$\left(\frac{1}{R_1}+\frac{1}{R_3}\right)^{-1}\$, which should be equal. Playing with the simultaneous equations shows that if they are,\$R_1=R_2=R_3\$, and if not you can solve for the actual values \$\endgroup\$ – Chris H Dec 20 '19 at 10:42
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    \$\begingroup\$ @ChrisH when paralleling resistors, convert to Siemens. Then it becomes simple addition of conductances instead of all that 1/1/x business. G1 + G2 = 606 nS. G1 + G2 + G3 = 909 nS. Solve for G3, G3 = 303 nS = 3.3 MΩ. \$\endgroup\$ – Harper - Reinstate Monica Dec 20 '19 at 22:31
  • \$\begingroup\$ @Harper thanks, I'd forgotten that approach, which would be good here. I haven't had to do any complicated parallel calculations for a while, and for simple ones it's exactly the same steps, just a matter of whether you write the units down on the intermediates. \$\endgroup\$ – Chris H Dec 21 '19 at 7:22
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The range indications on a multimeter are the approximate limits of the range. Typically the actual reading limit is one count lower than the number indicated on the dial. So on a 3½ digit meter:

  • the "200" range can read up to 199.9Ω
  • the "2K" range can read up to 1.999kΩ
  • the "20K" range can read up to 19.99kΩ
  • the "200K" range can read up to 199.9kΩ
  • the "2M" range can read up to 1.999MΩ

Similarly if your multimeter had them:

  • a "20M" range would read up to 19.99MΩ
  • a "200M" range would read up to 199.9MΩ

A "1" on its own with no other digits active is typically the over-range indication. If you want to make an actual measurement then you need to move to a higher range. Unfortunately at some point you run out of ranges to move to.

Many cheap multimeters have "2M" as their highest range, though it's not too hard to find cheap multimeters with "20M" and even "200M" ranges if you look for them. If you want more than that you are generally getting into the realm of specialized insulation test meters.

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