Fluke and Brymen seem to have different ideas about LoZ implementation - Page 1
Forum section: electronics / general

author: cedd
author_rank: Newbie
timestamp: October 10, 2024, 10:35:48 pm
content: I'm a novice in this field but I've been researching DMMs the past few days to eventually buy a nice one for general use. My use case is mostly general home maintenance and being able to also use it for circuit boards would be a big plus.I would have already bought the BM786 if it had LoZ implementation parity with Fluke. The Brymen offerings just seem better in every conceivable way and are more fairly priced. In my research though, I happened upon the following article & video describing when LoZ is commonly used:https://techcircuit.org/how-lowz-works-and-why-its-useful/#google_vignetteThey're both great demonstrations if you aren't familiar with the LoZ function.Not to bury the lead here, Fluke implements LoZ with what appears to be a simple ~3Kohm resistor whereas Brymen implements it (in the meters that have it) as a 1Kohm + 1.2PTC thermistor which effectively creates a 2.2Kohm initial resistance that ramps up to a few hundred Kohm as current passes through. The added complexity of the Brymen implementation seems to add extra limitations on the input voltages and frequencies it can meaningfully read (>12V & between 50-400Hz). Additionally, I have concerns that Brymen's more complex implementation would be an annoyance/hindrance when dealing with LoZ use-cases beyond ghost voltage, such as current leakage and compromised voltage sources. For the latter would I only have 1-2 seconds of pre-ramp-up time to troubleshoot those events? And would I have to wait for the thermistor to cool down before getting the ~2.2Kohm impedance again for another 1-2 seconds? Many of Fluke's implementations also include a LoZ capacitance mode which is cool but a bit niche and not really important for my use-case.I wonder if this is an instance where the simplicity of Fluke's implementation made for a more versatile function. Sure the Brymen LoZ provides some extra convenience for ghost voltage readings, but did it come at the cost of greater versatility?To me, the LoZ function seems really handy for residential electrical troubleshooting. I want my chosen meter to have this function for that reason. Are my concerns with Brymen's implementation warranted? Or maybe I'm giving Fluke's implementation too much credit and the perils of a simple resistor outweigh the greater versatility I'm attributing to it? (possible risk of upstream circuit damage if not careful which the Brymen implementation gives some protection for)And overall which do you think is the better and more versatile LoZ implementation: Fluke's or Brymen's? I suspect personal preference and use-case will weigh heavily here.Thanks for reading!

author: TheDefpom
author_rank: Frequent Contributor
timestamp: October 11, 2024, 02:38:27 am
content: I think Brymen's approach is done to protect the resistor.If the Fluke is a constant 3K input (which sounds awfully low, but I cant say i've ever actually looked), then at 240V that is approx 80mA of current passing through...thats a fair bit of power going through a resistor, safe if not actively powered and to deal with ghost voltages only, but if used on actual voltage by mistake... hmm.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: October 11, 2024, 03:44:34 am
content: the Fluke 12, and presumably all Fluke models do use a PTC for their LoZ ranges. see the below graph from the Fluke 10,11,12 Service Manual, PN 900824, March 1992 Rev.1, 7/93.cheers,rob :-)

author: floobydust
author_rank: Super Contributor
timestamp: October 11, 2024, 06:12:27 am
content: A limitation is there are very few PTC's that will be comfy with 600V-1,000V applied to them continuously. Even fewer that have safety approvals. Even fewer high ohmic value over say 2,000 ohms cold.Because it takes time for them to heat up to ~80°C, inrush current can be very high (if they see HV) so a surge resistor is added in series. That also divides down the overall voltage seen by the PTC.The Fluke graph does not show time.

author: cedd
author_rank: Newbie
timestamp: October 11, 2024, 03:43:15 pm
content: I actually wasn't watching the video carefully because if you look at the demonstrations for leakage current and compromised voltage, the voltage is indeed continually increasing which indicates that there is PTC in series.

author: electr_peter
author_rank: Unknown
timestamp: October 11, 2024, 05:32:17 pm
content: There is nothing "exact" about 2.2k\$\Omega\$ or 3k\$\Omega\$ input resistance in LoZ mode - it is just a good enough value (compromise) to eliminate ghost voltages from mains circuits. Initial value will change wildly with input voltage due to PTC action - if there were no PTCs inside, DMM would just melt after some time.Fluke and Brymen implementations of LoZ are very similar and both fit for purpose, I do not see any significant difference to the user.

author: coppercone2
author_rank: Super Contributor
timestamp: October 11, 2024, 05:45:04 pm
content: I see good reason to build a external low z circuit with a resistor that can stand up to the heat indefinitelyseems to be a problem with the usual make it as small as possible design methodthis should workL50J3K0Efor higher voltages it ends up being bigger then the meter, but a 225W one isL225J3K0E-NDThat should work for many volts, if you really need it, that should work to around 800V continuousfor 1200V, you would need 485W dissipation capability ;P (400mA)

author: ArdWar
author_rank: Frequent Contributor
timestamp: October 12, 2024, 05:44:02 am
content: LoZ function is meant to discharge weakly inductive or capacitive coupled voltage. Essentially a high impedance source with short discharge impulse at worst. It is not meant to be used to measure leakage current or compromised voltage (whatever that is). It isn't meant to be precise load either.As a practical case: Even the usual limit for leakage current at 30mA will dissipate 2.7W across 3k resistor. While not unreasonable to design for, that's too much of a design creep. With several safety regulatory implication too I'm sure. You need the resistor to be fused and rated at 300W impulse to get 1000V CAT rating.