Question about my multimeter ?

Thread Starter

Amr_hamed

Joined Aug 11, 2018
4
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 senario 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
 

MrChips

Joined Oct 2, 2009
30,706
1) What is the make and model of the meter?

2) Never attempt to connect your meter to a voltage source while the meter is set to measure continuity, diode, resistance, or current. You can damage your meter. Your meter must be set to measure voltage when connecting to a battery.
 

LesJones

Joined Jan 8, 2017
4,174
The mulimeter measures resistance by passing a fixed contsant current through the resistor being tested and mesuring the voltage across the resistor. This current is chosen so the voltage developed under test is less than the forward voltage of semiconductor junctions to avoid false reading. I SUSPECT that your meter has a silicon diode across the input circuit for protection and the 614 corresponds with the forward voltage of this diode which would have a forward voltage of about 0.6 to 0.7 volts. You are VERY lucky that you did not destroy your meter.

Les.
 

Thread Starter

Amr_hamed

Joined Aug 11, 2018
4
Its milwaukee multimeter and yes i understand what you saying but i just want to understand whats going on ? Why the continuity go on like that ?

Thanks for your reply
 

Thread Starter

Amr_hamed

Joined Aug 11, 2018
4
The mulimeter measures resistance by passing a fixed contsant current through the resistor being tested and mesuring the voltage across the resistor. This current is chosen so the voltage developed under test is less than the forward voltage of semiconductor junctions to avoid false reading. I SUSPECT that your meter has a silicon diode across the input circuit for protection and the 614 corresponds with the forward voltage of this diode which would have a forward voltage of about 0.6 to 0.7 volts. You are VERY lucky that you did not destroy your meter.

Les.
can you explain in details please , forgive me if i am dumb i just wanna understand , what do you mean by forward voltage ? and from where the multimeter bring the current to pass it through the resistor ? and what's exactly 614 ?is it ohm or volt ? and why ? if you have any video or link to explain more i will be glad .. thanks alot
 

Aswinth

Joined Nov 1, 2016
30
Its milwaukee multimeter and yes i understand what you saying but i just want to understand whats going on ? Why the continuity go on like that ?

Thanks for your reply
LesJones has given the answer. When you put the multi-meter in continuity mode do not connect it to a Voltage source like battery.

The reason why your multimeter read 614 ohms must be due to the diode inside your multimeter as explained by Lesjones. Although what I dont understand here is that, if it was because of the diode you have also reversed polarity of your to check again. In that case there should have been a short. I am really curious to know how your battery escaped the damage
 

LesJones

Joined Jan 8, 2017
4,174
Forward voltage of a semiconductor junction is the voltage across it when the current is in the direction which makes it conduct. It does not vary much between a current of a few mA and the rated current. (In the case of a diode.) There are a few different ways to generate a constant current. You would need the schematic of your meter to know which method it uses. There would probably be two protection diodes cross connected to limit the voltage to about + / - 0.7 volts. These would not be directly across the input. I suspect the reason you got a reading of zero in one direction is that in normal use measuring resistance the input would always be positive so there would be no need to display negative voltages. The guess at the 614 reading was the forward voltage of the protection diode was guessing that if a current of 100 uA, 1 mA, 10 mA etc. was used then the reading would be a direct relationship to voltage. For example for a current of 1 mA through a 614 ohm resistor the voltage would be 614 mV. Again to know if this guess is correct then you would need the schematic of the meter and possibly the code listing for the microcontroller used in the meter.

Les.
 
Last edited:
Top