For years! What am I doing to my fluke!? - Page 1
Forum section: electronics / projects

author: mkiijam
author_rank: Regular Contributor
timestamp: November 07, 2023, 02:31:12 am
content: For years I have been draining caps with my Flukes by using the resistance mode? What am I doing and why does it work? Am I hurting them?

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 07, 2023, 04:10:23 am
content: Any reason you don't just...... use a resistor?

author: uer166
author_rank: Super Contributor
timestamp: November 07, 2023, 04:12:14 am
content: I do that too.. A resistor is not nearly as safe, and doesn't provide almost automatic indication that it's discharged. What I believe happens is that you're triggering the internal PTC protection by applying a high current into it, where it becomes higher impedance and discharges the cap. It is likely a wear item, so eventually with enough energy pulses put into it it'll fail.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 07, 2023, 09:36:28 am
content: I usually use a resistor with my multimeter measuring the voltage across it.Mr Carlson of Mr Carlson's Lab on Youtube has a design for a capacitor discharger that uses some LED 's to indicate when the capacitor is safe to handle.

author: Gyro
author_rank: Super Contributor
timestamp: November 07, 2023, 01:21:41 pm
content: "Why does it work?" Because you're using the protective clamping diodes, PTC, and (fusible) surge resistor to dissipate the energy."Am I hurting them?" Well you're not doing them any good.

author: niconiconi
author_rank: Frequent Contributor
timestamp: November 07, 2023, 10:03:52 pm
content: Quote from: mkiijam on November 07, 2023, 02:31:12 amA safer way is to use the low-impedance input mode as found in some voltmeters. It's designed to distinguish between a real mains AC voltage from parasitic voltage. A ghost voltage (usually from capacitive coupling from nearby powerlines) disappears, while the real AC voltage remains. You can abuse it to discharge capacitors safely.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 08, 2023, 02:29:06 pm
content: Quote from: mkiijam on November 07, 2023, 02:31:12 amJust pointing out that the OP never mentions what meter they are using or what the capacitors are charged to and what sized they are. You can't make blanket statements without added constraints.My old Fluke 8000A has basically no protection for the front end. Even if the OPs Fluke did have a PTC, surge rated resistors, MOVs and high speed clamps, claiming it will survive any capacitance value charged to any level is false.Recently I had to repair our microwave oven. It's fairly common that people are killed working on them (I've posted some links to stats on this before). I think ours has a few uF at a few kV. Now, if I charged that cap on a bench and discharged it into any of my Flukes, would they survive, I don't know.Some of the cheap meters I have looked at will not have a surge rated resistor (even then there are specifics what surge means and it's not a 2uF 2kV cap with very low source impedance). Some use one or even two PTCs in series. Some of these PTCs are only rated to 500V. I think Dave's 1212GW for example uses two PTC in series for the low impedance AC mode. I think when I looked at it, the rotary switch broke down.I pulled an old Fluke 77 handheld from the trash and ran a few tests on it. That meter used spark gaps. It wasn't even in the same class of more modern Fluke products I have looked at, in regards to how electrically robust they are.

author: mkiijam
author_rank: Regular Contributor
timestamp: November 08, 2023, 04:58:18 pm
content: Fluke 77, 23 87I recently did a 220uF @ 450V that was stubbornly sitting at 200V. I jab the probes into the solder pads for a few seconds, then switch back to V to see how it's dropping. Keep doing that till I think it's low enough.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 08, 2023, 07:46:44 pm
content: I can't begin to imagine the thought process that would lead one to doing this. Well, maybe if there were a TikTok video!

author: ejeffrey
author_rank: Super Contributor
timestamp: November 08, 2023, 08:11:00 pm
content: One of the tests on the EEVblog multimeter shootouts was to connect to 240 VAC mains in ohms mode. The idea being that this is a predictable mistake someone might make, so you want to know how the meter would handle it. That is a 300-350 V peak voltage, and of course it doesn't drop off the way a cap discharges. IIRC, most meters handled that OK but some were degraded? So discharging a 300V capacitor is probably no big deal for higher quality or more rugged meters, but could damage cheaper ones, but the higher you go above that, the more likely you are to cause problems? Also, dave only did the test once, when you are stressing protection devices there is a chance they can be degraded slightly each time and eventually not be able to protect the meter.

author: mkiijam
author_rank: Regular Contributor
timestamp: November 08, 2023, 08:17:36 pm
content: Quote from: Conrad Hoffman on November 08, 2023, 07:46:44 pmme neither, that's why I asked!I started my "career" as an oderley-jr. tech-lackey-go-for-gohper.I can't fathom or remember having any coherent thoughts about technical stuff, as I was hired to answer the phones... but I think I maybe "accidently" measured a cap in resistance mode instead of voltage mode and noticed it dropping after switching back???

author: rsjsouza
author_rank: Super Contributor
timestamp: November 08, 2023, 08:30:58 pm
content: What is wrong to use a resistor in parallel that prevents stressing the DMM's internals like this?(values can change depending on your voltages and availability of junk bin parts)

author: mkiijam
author_rank: Regular Contributor
timestamp: November 09, 2023, 01:09:20 am
content: LazynessI left the spelling error cause I'm too lazy to right click and choose lazyness See? I did it again...

author: joeqsmith
author_rank: Super Contributor
timestamp: November 09, 2023, 02:36:30 am
content: Quote from: rsjsouza on November 08, 2023, 08:30:58 pmIf C = 1MF (not m) and V = 1MV (not m), will any meter survive? Will the resistor? The human?Quote from: mkiijam on November 08, 2023, 04:58:18 pmI doubt many of even the weakest meters I have looked at would fail with 200VDC applied to the meter in any mode, even for days on end. Someone was telling me once about wearing out the MOVs. I ran one of the Fluke meters for several days at over a kV to prove the point. Normally even the worse meters limit the current to the MOVs.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 09, 2023, 03:28:58 am
content: Quote from: joeqsmith on November 09, 2023, 02:36:30 amHehehe... In this case the human can be a very effective capacitor discharger. :grin:Quote from: joeqsmith on November 09, 2023, 02:36:30 amYou need to go out more and see what kind of junk is being peddled as a DMM these days... Over the years I tested a few über-cheapo meters (Surpeer AV4, Aneng M118A) that fainted when "high voltage"1was applied to a mismatched setting of the meter...160~80V

author: joeqsmith
author_rank: Super Contributor
timestamp: November 09, 2023, 01:13:10 pm
content: Quote from: rsjsouza on November 09, 2023, 03:28:58 amThat's pretty poor. Does "fainted" mean damage to meter or did it recover?We could run another pile of sub $50 meters. It's been eight years. Maybe there is something else out there that would compete with the Amprobe and Fluke.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 09, 2023, 09:13:49 pm
content: Quote from: joeqsmith on November 09, 2023, 01:13:10 pmThe "faint" were fully recoverable, but meaning the meter was either in an unstable/locked state or its display went blank completely. A power off/on restored it.That indicated to me there was something bleeding to the low voltage part of the meter, causing the microcontroller to give up.One remark, though: the fully automatic Aneng M118A failed when it was measuring resistance on the output of a power supply and, when the PS output was turned on, the DMM "fainted". Quite unremarkable for an automatic DMM.I caught that on video and it has been one of my most disfavourable videos of my channel. People tend to think I am being too harsh simply because the meter is "cheap". Oh well...

author: bdunham7
author_rank: Super Contributor
timestamp: November 10, 2023, 12:51:23 am
content: Quote from: joeqsmith on November 08, 2023, 02:29:06 pm1µF @ 2kV is pretty common, so 2J total energy. A typical modern design Fluke rated for 1000VAC might have a PTC/resistor combination with about 3k total resistance and 2 910V MOVs in series, so while the capacitor might just tickle them a bit, I don't think the MOVs will be overtaxed. You'd have at most about 600mA of current until the PTC warms up and reduces it in a few tens of milliseconds. A BAV99 can take 1A for 1s, so if that is what they use to clamp I'd guess it would be OK.An even better bet is to use a meter with the Lo-Z feature. The 116 and 117 even have a Lo-Z capacitance mode that is specifically for measuring capacitors that may be charged or even have one side still connected to mains power. For all the howling people do about Fluke selling 'old design' meters, the one thing I don't hear anyone asking for is Lo-Z on so-called top end meters like the 87V. If they'd added it to the 87VMAX when it came out, I might have bought one. Maybe there's a reason.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 10, 2023, 01:03:11 am
content: Quote from: bdunham7 on November 10, 2023, 12:51:23 amI think you missed my statement about my 8000A.

author: bdunham7
author_rank: Super Contributor
timestamp: November 10, 2023, 01:10:15 am
content: Quote from: joeqsmith on November 10, 2023, 01:03:11 amNot at all! I was responding to "any of my Flukes" by discussing what might happen with more modern design versions, which I presume you have some of. Obviously the chances of the vintage ones surviving is not as good depending on the specific model. Your 8000A would likely die of fright before you even connected that capacitor, but what might be interesting are the pre-CAT handhelds like the original 27 and 87 types.

author: joeqsmith
author_rank: Super Contributor
timestamp: November 10, 2023, 01:18:17 am
content: Quote from: bdunham7 on November 10, 2023, 01:10:15 amAny of my Fluke meaning I would need to test them. It's not a given.