Multimeter unexplained behaviour - Page 1
Forum section: electronics / beginners

author: higginsdj
author_rank: Contributor
timestamp: April 27, 2025, 07:25:40 am
content: Complete novice and just doing measurements on very simple circuits. I have an OWON XDM1041 and a EEVBlog BM786. I am measuring the voltage drop across an 8.2v Zenner diode - forward and reverse directions. Now for all other measurements I have done, ie basic DC voltage measurements on Power supplies, across resistors transistor bias et al, the 2 multimeters measure to within 0.002v of each other. However, when measuring the same ZD in the same circuit, the BM786 measures 8.066v and -8.066v while the OWON measures 7.965v and -8.057v.Is there something wrong with my OWON? Is there a reason the forward and reverse measurements might be different that would explain the 90mV difference?

author: 807
author_rank: Frequent Contributor
timestamp: April 27, 2025, 11:55:49 am
content: Out of interest, have you tried measuring both ways with other voltages? What would the Owon read directly across a battery for example?

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 27, 2025, 12:14:06 pm
content: Try to connect both meters at the same time. Just to rule out that the meter makes the voltage changed.

author: Zenith
author_rank: Frequent Contributor
timestamp: April 27, 2025, 12:23:43 pm
content: That's not much of a difference, considering Zeners are not perfect constant voltage devices. Something might have happened in the circuit when you were taking a measurement with the Owon, which caused the current through the Zener to change slightly.I agree with the other posters. Try both meters with something like a 9V battery and try them at the same time on the Zener.

author: BennoG
author_rank: Frequent Contributor
timestamp: April 27, 2025, 01:00:53 pm
content: the voltage measurement over the zener highly depend on the internal resistance of your meter most meters are between 1M and 10M or even 1G ohm.For a zener this is a verry unstable current to measure the voltage drop.Benno

author: higginsdj
author_rank: Contributor
timestamp: April 29, 2025, 12:24:40 am
content: No, switching back and forward between both meters a number of times produced identical results - the circuit was perfectly stable.

author: higginsdj
author_rank: Contributor
timestamp: April 29, 2025, 12:26:17 am
content: All other measurements including across the power source showed the Owon and Brymen with the same reading.

author: inse
author_rank: Super Contributor
timestamp: April 29, 2025, 04:33:15 am
content: In both polarities?

author: bdunham7
author_rank: Super Contributor
timestamp: April 29, 2025, 04:51:24 am
content: Try connecting a 10k resistor to one terminal of a 9V battery and then measuring the voltage between the other end of the resistor and the other terminal of the battery. Do this with both meters both ways. What you report is very strange.

author: Zenith
author_rank: Frequent Contributor
timestamp: April 29, 2025, 08:24:02 am
content: I'd swap the leads between the two meters and see if the problem follows the leads or the meters. It may be some sort of galvanic effect.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 29, 2025, 09:45:09 am
content: Quote from: higginsdj on April 29, 2025, 12:24:40 amYou didn't try to connect both meters at the same time?

author: u666sa
author_rank: Frequent Contributor
timestamp: April 30, 2025, 12:19:57 am
content: How much voltage drop is in your probes? Because you are not connecting diode to multimeter, keep in mind that you have two resistors between diode and multimeter, and each resistor set is different on each multimeter. This is how much good probes should read. How much does your probes read on both multimeters?

author: Zenith
author_rank: Frequent Contributor
timestamp: April 30, 2025, 10:23:00 am
content: What current is going through the Zener diode? Don't forget that the Zener voltage isn't a step function, where once it's reached, it's exactly the same regardless of the reverse current. The maker will give a recommended current, which isn't critical. As a WAG and without knowing anything about the Zener or the circuit it's in, I'd say 5mA is a sensible current. If you have it working on microamps, you might see some strange behaviour.What I find odd is that the Owon doesn't show the same numeric value, +ve and -ve when measuring the voltage across the Zener.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 30, 2025, 11:31:05 am
content: The OP still doesn't take my suggestion to connect both meter at the same time. I suspect that either meter shows the correct voltage but the meter causes the zener voltage to change. With both meter connected whatever the voltage is they are the same on both meters.