0
\$\begingroup\$

I have a 12V, 15A power supply. Using a multimeter I found that there is continuity between the output terminals (2 positives and 2 negatives) of my power supply. Also, I did the same test on my RAMPS (RepRap Arduino Mega Polulu Shield) 1.4 and I found that there is continuity in the first three terminals(negative, positive & negative) with each other but not in the fourth(positive).

Why is there continuity between the negative and the positive terminals of both the RAMPS and the power supply? Furthermore, why is there continuity on RAMPS with only b/w one positive and the two negative terminals and not with the second positive?

Just a side note my multimeter does not beep when I connect the negative and the positive terminal on the power supply (only shows some value) but does sometimes beeps in the ramps 1.4 (along with a value).

enter image description here

\$\endgroup\$
3
  • \$\begingroup\$ Before asking why your multimeter shows continuity, it would help to understand why you are measuring the continuity, and also determine how your multimeter decides if there is continuity or not. Do you have the manual for your multimeter? \$\endgroup\$
    – Justme
    Aug 6 at 13:50
  • \$\begingroup\$ I am measuring continuity in order to see whether there is any short circuit in my DC supply so that the circuitry does not fry up. The manual for the multimeter states buzzer sound will beep at < equal to (70 ohms +_ 20 ohms). \$\endgroup\$ Aug 6 at 14:51
  • \$\begingroup\$ So, you only know if the resistance measured by multimeter is either above or below the threshold. You don't know if it is a short circuit or not. \$\endgroup\$
    – Justme
    Aug 6 at 14:54

1 Answer 1

3
\$\begingroup\$

You are using the diode test option of the multimeter. In this mode the multimeter provides a current limited test voltage at the terminals and the display shows the voltage in [mV] it could reach. If the terminals are connected to a very low resistance, an additional feature invokes the beep sound.

If you connect it to a complex circuit like the power supply, the reading is unpredictable because the small test voltage can activate components and there are several paths for a small current. All you can say is, that there are some more or less conducting elements in there, pretty normal for most circuits.

If there are big capacitors in such a circuit, the multimeter will beep until they are charged up to the test voltage. This is not an indication for a failure.

If you read values below, say, 100 mV for several seconds and hear a continous beep, you may be concerned and further investigation may be necessary. However, this is guess work, just a first impression.

The diode test mode is very useful for testing recifiers, diodes and transistors, where in one applied polarity the display shows between 300 and 850 mV "forward voltage" and in the other polarity some "overload" or "end of range" symbol for "not conducting". But if those components are connected to other parts inside a circuit the reading is not reliable.

This mode is not useful to detect hard shortcuts, the 200 ohm resistor test mode is the best mode, this multimeter provides to detect true short circuits.

Summary: All your readings are typical values, no concern.

\$\endgroup\$
2
  • \$\begingroup\$ Thank you for properly explaining it. \$\endgroup\$ Aug 6 at 23:04
  • 1
    \$\begingroup\$ Some multimeters have a low power resistance mode where applied voltage is limited to about 200 mV, so most semiconductor junctions will not be forward biased, and more realistic resistance values can be measured. \$\endgroup\$
    – PStechPaul
    Aug 7 at 0:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.