Question about multimeter continuity

[Amr_hamed]

Hi guys , i am working in the automotive industry today i encountered a weird electrical issue and i am sure you guys have an explanation , i was using my multimeter to check continuity between different stuff , by mistake i put the leads on the car battery terminals i found the multimeter beeping and reading 614 ohm ! I switched the leads and re-tested back nothing beeping ! So i tried again with the first setup multimeter (+) lead on battery (-) and multimeter (-) lead on battery (+) same scenario the mutlimeter beeping on continuity reading 614 ohm ! Switched leads (+) on (+) and (-) on (-) nothing beeping , so i am pretty confused here , Am i have a problem with the multimeter ? Because i tried the test on AA battery same result ! Can anybody explain , thanks so much

 

[mfb]

To measure resistance your multimeter will apply a given voltage and measure the current or vice versa. That works well with a resistor, but an external voltage source will make that approach go wrong. Applying a given voltage would lead to a very high current, so I guess the multimeter tries a constant current. The voltage is given by a car battery, a 614 ohm resistor would have 12 V at 12V/(614 Ohm) = 19.5 mA. It drives this current through the car battery, measures 12 V and thinks it has a 614 Ohm resistor.
If you turn the leads around the multimeter measures -12 V which makes no sense for a resistor (the multimeter gets energy from the outside, that cannot happen with a resistor).

 

[Svein]

I suspect that there is a diode in series with the meter somewhere. You said multimeter - if that is an old-fashioned analog multimeter a series diode would protect it if someone tried to do what you just did.

 

[Amr_hamed]

it looks like i am missing a lot here ,mfp can you please explain in details what happened consider that i don't know how the multimeter works and what do you means by resistor ? and what do you mean by (the multimeter gets energy from the outside, that cannot happen with a resistor) , forgive me if i am dumb .. thanks

 

[Amr_hamed]

I suspect that there is a diode in series with the meter somewhere. You said multimeter - if that is an old-fashioned analog multimeter a series diode would protect it if someone tried to do what you just did.

can you explain more the purpose of the diode inside the multimeter .. thanks

 

[Asymptotic]

Can you provide the multimeter manufacturer and model number?
Were you on a resistance ("Ohms") range, or on a separate "continuity" range?

When either 12V or 1.5V was applied the meter read "614" which (I'm guessing) may be the largest value that can be displayed. If you have a variable DC supply it would be interesting to see at what voltage under 1.5V the multimeter begins to display a value lower than "614", then plot an X-Y chart of displayed value versus applied voltage.

 

[CWatters]

The continuity setting on most meters is designed to measure low resistances. Meters don't actually measure resistance directly, what they do is send out a constant current and measure the voltage produced. They then effectively "calculate" the resistance using ohms law.

If you connect the meter to a voltage source like a battery the meter is fooled... One way around made it think it's connected to a 614 ohm resistor. Connecting it the other way produces a voltage of the wrong polarity for the voltage measuring circuit. It can't produce a sensible result.

Suppose the voltage was -12V and the current 1A. Ohms law says the resistance should be R = V/I = -12/1= -12R however negative resistors don't really exist.

 

[CWatters]

PS Your meter is working fine. You shouldn't use it to measure the continuity of a circuit with a battery in it. At least not on the continuity setting. Why not use it on the Voltage setting. Eg if the battery is 12 V and you measure 12V at the far end of the wire away from the battery you know the wire is ok.

 

[davenn]

Hi guys , i am working in the automotive industry today i encountered a weird electrical issue and i am sure you guys have an explanation , i was using my multimeter to check continuity between different stuff , by mistake i put the leads on the car battery terminals i found the multimeter beeping and reading 614 ohm ! I switched the leads and re-tested back nothing beeping ! So i tried again with the first setup multimeter (+) lead on battery (-) and multimeter (-) lead on battery (+) same scenario the mutlimeter beeping on continuity reading 614 ohm ! Switched leads (+) on (+) and (-) on (-) nothing beeping , so i am pretty confused here , Am i have a problem with the multimeter ? Because i tried the test on AA battery same result ! Can anybody explain , thanks so much

OK I'm really shaking my head here … you said you measured across the battery when in continuity test mode by mistake

So, why on Earth would you continue to do the same tests ? don't you realize that this is a very good way to permanently damage your meter ?

The reasons for the specific reasons is most likely that you have now done damage and it may never work properly in continuity test mode again …
and maybe other modes may no longer work properly eitherDave

 

[Amr_hamed]

The continuity setting on most meters is designed to measure low resistances. Meters don't actually measure resistance directly, what they do is send out a constant current and measure the voltage produced. They then effectively "calculate" the resistance using ohms law.

If you connect the meter to a voltage source like a battery the meter is fooled... One way around made it think it's connected to a 614 ohm resistor. Connecting it the other way produces a voltage of the wrong polarity for the voltage measuring circuit. It can't produce a sensible result.

Suppose the voltage was -12V and the current 1A. Ohms law says the resistance should be R = V/I = -12/1= -12R however negative resistors don't really exist.

ok first of all thanks a lot for your response but i have several questions , one of the members mentioned that 614 ohms is the diode resistance which used to protect the multimeter ! i tried to look online for an explanation for this but i couldn't understand , 2nd you saying meters measure resistance by sending current and measure volt produced , my question from where the multimeter send the current ? from the internal battery you mean ? you also said the multimeter is fooled when connected with other battery why ? also i am so confused why specially 614 ohm ! is that a non sense no. ? or it has something to do with a diode ? oooh man i have been using the multimeter for ages only today i felt like i know nothing about it.. thanks for help

 

[Amr_hamed]

OK I'm really shaking my head here … you said you measured across the battery when in continuity test mode by mistake

So, why on Earth would you continue to do the same tests ? don't you realize that this is a very good way to permanently damage your meter ?

The reasons for the specific reasons is most likely that you have now done damage and it may never work properly in continuity test mode again …
and maybe other modes may no longer work properly eitherDave

the meter still working fine i kept doing the test out of curiosity to understand from where 614 ohm coming from and why , i don't care about the meter damage i care about understanding what's going on .. thanks

 

[mfb]

one of the members mentioned that 614 ohms is the diode resistance which used to protect the multimeter

No one said that.
A diode could explain why the other direction didn't lead to a measurement, but simple logic in the multimeter would explain that as well.

2nd you saying meters measure resistance by sending current and measure volt produced , my question from where the multimeter send the current ? from the internal battery you mean ?

Yes, they use their internal power to send current through a resistor.

If you set the multimeter to "measure resistance", it expects to be connected to a resistance. To measure that it has to send some current through it and then compare current and voltage. R=V/I. But that only applies to resistors. Batteries have a voltage on their own. Calculating V/I for that case doesn't lead to a resistance.

also i am so confused why specially 614 ohm !

See post 2, could be the result of the specific current the multimeter uses to measure resistances.

 

[Amr_hamed]

No one said that.
A diode could explain why the other direction didn't lead to a measurement, but simple logic in the multimeter would explain that as well.Yes, they use their internal power to send current through a resistor.

If you set the multimeter to "measure resistance", it expects to be connected to a resistance. To measure that it has to send some current through it and then compare current and voltage. R=V/I. But that only applies to resistors. Batteries have a voltage on their own. Calculating V/I for that case doesn't lead to a resistance.See post 2, could be the result of the specific current the multimeter uses to measure resistances.

thanks so much i really appreciate your time to explain to me , my best

 

[Amr_hamed]

last question the multimeter should beeping at the continuity when the resistance is less than 30 ohm , however it was beeping when it read 614 ohm ! any explanation ?

 

[rbelli1]

12V/(614 Ohm) = 19.5 mA.

I would suspect that 19.5mA is far larger than the meter actually uses for any resistance measurement. 12V is many volts above the limit of the meter in continuity range so the actual voltage it is measuring is the voltage of the internal protection device and that gives the 614 Ohm value.

BoB

 

[CWatters]

The short answer to your question is that you are essentially abusing your meter by connecting it to a battery when in continuity mode. What the meter should display in this situation is anyone's guess. Different meters will do different things. I suppose a really clever meter would say "Error" or "Voltage present... help... Save me".

 

[CWatters]

2nd you saying meters measure resistance by sending current and measure volt produced , my question from where the multimeter send the current ? from the internal battery you mean ?

Yes.

you also said the multimeter is fooled when connected with other battery why ?

Iets suppose the meter sends out 20mA when on one of the resistance ranges. If it measures 1V then the resistance is 1/0.02 = 50 Ohms. If there is a 12V battery in the circuit it may be fooled into thinking the resistance is 600 Ohms. 12/0.02 = 600.

also i am so confused why specially 614 ohm ! is that a non sense no. ? or it has something to do with a diode ? oooh man i have been using the multimeter for ages only today i felt like i know nothing about it.. thanks for help

Who knows. Could be nonsense or perhaps the meter does send out about 20ma as much said.

 

[davenn]

i don't care about the meter damage i care about understanding what's going on .. thanks

I am so pleased you are not one of my test technicians, you would be fired for treating test gear like that

What is going on is that you are forcing the poor meter to do something it shouldn't be …. the 614 could be it's final cry for help

 

[CWatters]

I hope someone has told you not to connect it to a battery when set to measure current.

 

[berkeman]

I hope someone has told you not to connect it to a battery when set to measure current.

And with that good piece of advice, this thread is now closed. Thanks everybody for trying to help the tech out.