Voltage reference and resistor measurement - Page 1
Forum section: electronics / general

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: September 09, 2021, 04:19:12 pm
content: Hello,I have a stupid question.I would just like to know if there is not some kind of standard on the maximum voltage to be applied during a resistance measurement ? ( for safety or obvious reason ? )On my low cost DMM, according to the selected range, the voltage reference is 0.5 or 1V.But on a higher end DMM, the voltage reference can be between 0.2 to 2.5V.This can be annoying in in situ measurement if silicon components are on the way for example.Thank you

author: bob91343
author_rank: Super Contributor
timestamp: September 09, 2021, 05:53:24 pm
content: I don't think there is a standard. Each device has its characteristics, including applied voltage, and so each situation needs its own evaluation. Too low a voltage impairs resolution. Too high a voltage risks excessive dissipation or, as you note, exceeding diode thresholds.A classic case is the antique capacitance bridge which applies 50 V or more to the unknown and has been known to damage DUTs.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 09, 2021, 06:55:54 pm
content: Some higher resolution meters may apply even more than 5 V to the DUT. They usually give the maximum votlage. Special meters for high resistance may use some 200 V.Usually the reference resistors will tell - sometimes with the damage level and the much lower level for good accuracy. More voltage may not be a good thing. Some meters use quite some current in the ohms mode and this can be a slight problem with self heating. Usually not enough to damage a resistors, but possibly to effect the accuracy. Less current may be better. Using 1 mA with a 2 K resistor would be 4 mW and this is a bit on the high side to get an accurate reading from a small resistor that does not have super low TC.

author: guenthert
author_rank: Frequent Contributor
timestamp: September 09, 2021, 10:54:43 pm
content: Quote from: Kleinstein on September 09, 2021, 06:55:54 pmor rather the max. current and the (considerably lower) current for which the resistor exhibits the nominal resistance. Sometimes it is specified whether those values apply to an oil or air bath or a specific ambient temperature (might be different values).Some multimeters offer a choice ('low current mode') of how much current to use when measuring resistors in order to minimize self-heating.

author: David Hess
author_rank: Super Contributor
timestamp: September 10, 2021, 12:17:50 am
content: For multimeters, the compliance voltage depends on when it was designed. Early ones from the 70s often apply a voltage limited only by the internal supply so 9 to 12 volts. Later this was reduced to 2 to 3 volts to prevent damaging bipolar transistors through base-emitter breakdown. More recently, the compliance has been increased to 4 to 5 volts to support testing blue LEDs.

author: bob91343
author_rank: Super Contributor
timestamp: September 29, 2021, 03:59:28 am
content: As I remember it, the standard voltage to test a resistor was that requred to dissipate 25% of its power rating.

author: David Hess
author_rank: Super Contributor
timestamp: September 29, 2021, 04:15:40 am
content: That would be a different sort of test.