I'm confused - BM786 - Page 1
Forum section: electronics / beginners

author: Mpegger
author_rank: Contributor
timestamp: March 04, 2024, 10:02:07 pm
content: I'm testing a large capacitor in a old wall wart power brick (simple transformer with 4 individual diodes and a large capacitor) and am getting confused on how to read the measurement with my BM786.The capacitor in question is marked as 16V 3300 uF. When I test the capacitance with the meter, at first the meter displays "disc", but if I remove a test lead and test it again right away, I get 3.51 mF. Great, I do get a consistant value even after shorting out the capacitor to retest it multiple times.For the life of me though, in all my searches, I cannot find what "mF" represents, except that it's a old term that is now replaced with "uF". But the BM786 has a "uF" symbol (and pF), so why would it display 3.51 mF, instead of 3.51 uF? Why would it have 2 different symbols that represent the same value?It's obvious on the BM786 that mF does not equal uF, but I cannot figure out what it's supposed to represent or convert to.Can someone clue me in on what each symbol, mF nF and pF, actually represent on the BM786?Just noticed on my old BK Precision 2709B it's also showing the same symbols. And a cheap-o LCR meter read 188uF on the 200uF scale. I'm still confused.

author: shapirus
author_rank: Super Contributor
timestamp: March 04, 2024, 10:05:44 pm
content: Quote from: Mpegger on March 04, 2024, 10:02:07 pmhttps://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes

author: IanB
author_rank: Super Contributor
timestamp: March 04, 2024, 10:07:30 pm
content: These are standard SI prefixes:m (milli-) = one thousandth (10-3)µ (micro-) = one millionth (10-6)n (nano-) = 10-9p (pico-) = 10-12Hence, 1 mF = 1000 µF (1 millifarad = 1000 microfarads)

author: Veteran68
author_rank: Frequent Contributor
timestamp: March 04, 2024, 10:29:54 pm
content: And in your specific case, 3300uF = 3.3mF, so your reading of 3.51mF is pretty much right on the money and likely well within tolerance of the cap. Different meters may choose to use different scales, and uF has become the more common convention. In the old days it was not unusual to find microfarads actually marked as mF or MFD, in violation of the SI standard, so you don't see mF used that often anymore.

author: Mpegger
author_rank: Contributor
timestamp: March 04, 2024, 10:33:16 pm
content: SI prefixes I'm familiar with, but the wikipedia page on Capacitors also has:https://en.wikipedia.org/wiki/Capacitance#CapacitorsQuoteBut if the SI prefix is to be used, then this near 2 decade old capacitor (that's probably been in use for a decade of that time 24/7) is testing higher then it's rated value according to 2 meters? I'm surprised it isn't dried out or already leaked after so long (it does seem to be loose though so I will need to desolder it to be sure), or test for MUCH less a value. I have plenty of old can capacitors of similar age and use, that test at 1/2 or less thier rated value.

author: IanB
author_rank: Super Contributor
timestamp: March 04, 2024, 10:43:30 pm
content: Quote from: Mpegger on March 04, 2024, 10:02:07 pmYou should not remove the test lead until the meter has finished measuring. When it says "disc" I think that means "discovering" and you need to wait for it to finish (might take a few seconds). If the capacitor is too large, the meter will display "OL" instead of a value.Quote from: Mpegger on March 04, 2024, 10:33:16 pmMaybe you are getting an inaccurate reading due to removing the test lead before it is finished. What happens if you keep the test leads in place until the meter has finished?

author: IanB
author_rank: Super Contributor
timestamp: March 04, 2024, 10:47:37 pm
content: QuoteYou will typically only see these markings on the cans of very (very) old capacitors from over 50 years ago. They are long discontinued in modern instruments or literature.

author: shapirus
author_rank: Super Contributor
timestamp: March 04, 2024, 10:55:00 pm
content: Quote from: Mpegger on March 04, 2024, 10:33:16 pmIt may be leaky, in the electrical sense, meaning that it could have developed a parasitic shunt conductivity, which the meters may see as increased capacitance.

author: Mpegger
author_rank: Contributor
timestamp: March 04, 2024, 10:58:50 pm
content: Quote from: IanB on March 04, 2024, 10:43:30 pmI assumed that "disc" meant "disconnected", as if the leads lost contact in the middle of testing. Leaving the leads on after "disc" is displayed, does eventually result in a capacitance reading. My other meters didn't have such a message, they would just give a read out.

author: Mpegger
author_rank: Contributor
timestamp: March 04, 2024, 11:04:19 pm
content: Quote from: shapirus on March 04, 2024, 10:55:00 pmAs the capacitor is still in the circuit with the diodes, I could understand there being false readings. I took this wall wart apart to see if I could recondition it as it was reading much higher at around 7V to 9V, even though its supposed to be a 5V supply. I'm guessing even if the capacitor isn't any good, it's more then likely the enamel on the windings from the transformer breaking down, and replaceing the capacitor wouldn't do much at all. I'd be better off just buying a new power supply for this unit.

author: BeBuLamar
author_rank: Super Contributor
timestamp: March 04, 2024, 11:08:09 pm
content: I believe "DISC" means discharge. If the cap has some charge in it, it will discharge first before charging it up to make the measurement.

author: radiolistener
author_rank: Super Contributor
timestamp: March 05, 2024, 12:38:33 am
content: Quote from: Mpegger on March 04, 2024, 10:02:07 pmthe unit is the same, just different multiplier.mF = milliFarad = 0.001 FaraduF = microFarad = 0.000001 FaradSince your capactior is more than 1000 uF, it shows it in mF to avoid unwanted zero digits (because it don't have enough digits on the display and cannot measure very precise).

author: 807
author_rank: Frequent Contributor
timestamp: March 05, 2024, 11:19:26 am
content: Quote from: Mpegger on March 04, 2024, 11:04:19 pmDoes the wall wart have a 5v regulator in it, or is it just transformer/diodes/capacitor?

author: 807
author_rank: Frequent Contributor
timestamp: March 05, 2024, 11:29:29 am
content: Quote from: Mpegger on March 04, 2024, 10:58:50 pmDifferent meters take different amounts of time to take a reading. At least the Brymen is showing you that it's doing "something" while taking a capacitance reading. Some meters just sit there for ages without changing the display & you don't know whether it's working or not.

author: Jwillis
author_rank: Super Contributor
timestamp: March 05, 2024, 11:29:10 pm
content: DMMs are OKish for measuring capacitors. But because they only measure the time to charge a capacitor as a DC quantity to give a value they lack the ability to measure other important qualities of the capacitor. In capacitors that are getting worn out can give false positives because the ESR has increased. This changes the charge up time measured by the DMM. So even a Capacitor that reads good on a DMM may still have a large ESR. High ESR on filter caps leads to higher capacitor temperature and poor performance.

author: Haenk
author_rank: Super Contributor
timestamp: March 07, 2024, 06:10:04 am
content: With no load, the output voltage will be higher. The stated voltage will be reached when the stated load is attached.

author: radiolistener
author_rank: Super Contributor
timestamp: March 07, 2024, 11:23:03 am
content: Quote from: IanB on March 04, 2024, 10:43:30 pmNo, "disc" on the display means "Discharge". It appears when DMM decided to discharge capactior before measurement. You're just needs to wait until discharge procedure will be completed then "disc" message disappears and DMM will measure capacitance.Quote from: Mpegger on March 04, 2024, 10:58:50 pm"disc" means "Discharge". Not all DMM can do discharge procedure. Usually you're needs to discharge capacitor manually before measurement. But Brymen DMM can do it automatically and showing "disc" message during this procedure.

author: Mpegger
author_rank: Contributor
timestamp: March 09, 2024, 08:33:13 pm
content: Quote from: 807 on March 05, 2024, 11:19:26 amJust as I said in a previous post. Transformer, 4 individual diodes, a capacitor, and a copper clad double sided board to mount it all to, along with some poor factory soldering. It's a old school wall wart, if the wire wasn't so thin you could swing it around and put someone in the hospital with. I had to break out a dremel with a saw blade just to cut through the plastic case which was almost 1/8" thick at the joining seam.

author: Mpegger
author_rank: Contributor
timestamp: March 09, 2024, 08:36:34 pm
content: Quote from: radiolistener on March 07, 2024, 11:23:03 amI can understand discharge, but I discharge the cap before I even measure it. The Brymen DMM when testing show's no value, but does go through the capacitance symbols (uF > nF > mF), THEN, shows "disc". Could it be the Brymen DMM is both charging up AND discharging to try and get as accurate a value as possible? It does take longer to get a reading with the Brymen as compared to my other 2 meters.

author: Mpegger
author_rank: Contributor
timestamp: March 09, 2024, 08:40:52 pm
content: Quote from: Haenk on March 07, 2024, 06:10:04 amIts the same voltage, with or without a load. Correct me if I am wrong, but this would be because it's a transformer being used to get the lower voltage from mains voltage, and not a modern switching power supply.And now I realize this old power supply being transformer based, would have it's output voltage affected by the input voltage. In my old home, the voltage was regularly around 107-110VAC, while in this new home the voltage is around 118-124VAC. I'm guessing that's probably why it's putting out 7-9VDC instead of being closer to 5-6VDC.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 09, 2024, 08:51:28 pm
content: Read the wikipedia page more carefully. "mf" is obsolete for use as "uF", but "mF" is still normally valid as just plain regular SI as milli Farad.

author: 807
author_rank: Frequent Contributor
timestamp: March 10, 2024, 01:14:29 pm
content: Quote from: Mpegger on March 09, 2024, 08:40:52 pmWhat kind of load did you put on the supply? What is the current rating printed on the supply?

author: 807
author_rank: Frequent Contributor
timestamp: March 10, 2024, 01:28:29 pm
content: Quote from: Mpegger on March 09, 2024, 08:40:52 pmHave you ever measured the output of this PSU as being 5-6v?The higher voltage at your new property will have a slight impact on the output voltage but, as mentioned, an unregulated supply will have a significantly higher output voltage until it draws close to it's rated output.

author: Mpegger
author_rank: Contributor
timestamp: March 11, 2024, 01:27:28 am
content: Quote from: 807 on March 10, 2024, 01:14:29 pmIts a 2A power supply for a 4 port USB hub. I don't have 4 USB devices that can pull the full 500mA per port, but with whatever devices I do have, the voltage barely changes. We're talking about 0.2-0.4VDC drop when in use on all 4 ports.Quote from: 807 on March 10, 2024, 01:28:29 pmI never measured the voltage till recently when some USB controllers were having issues on that hub. I was also having issues with a RPi board on it's USB power supply (or so I thought I was having issues cause USB devices were having issues on the RPi), so I was going around testing different 5V power supplies I had around.

author: radiolistener
author_rank: Super Contributor
timestamp: March 13, 2024, 03:28:01 pm
content: Quote from: Mpegger on March 09, 2024, 08:36:34 pmNo, usually it happens one time. Your case looks like the capacitor is broken or has some semiconductor circuit in parallel which prevents proper measurement. If it happens when no other circuit is connected to the capacitor, then it means that capacitor is dead.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 13, 2024, 03:46:02 pm
content: Quote from: Mpegger on March 11, 2024, 01:27:28 amA USB hub needs a regulated 5 Volt power source. Either the external PSU has a regulator and outputs 5 Volts irrespective of load (up to its maximum current) or the external PSU provides an unregulatedhighervoltage and the hub contains a 5 Volt regulator.If the hub does not contain the regulator do NOT use an unregulated PSU as that can damage the hub and/or the USB devices plugged in to it.The PSU under discussion in this topic is an unregulated one.

author: BTO
author_rank: Frequent Contributor
timestamp: March 15, 2024, 11:33:40 pm
content: Quote from: Mpegger on March 04, 2024, 10:02:07 pmNO NO....Now you need to be old enough to know this. (I'm 50) as the forum has stated, TRADITIONALLY in todays day the S.I. Units for Capacitors areFarads (F)milli (mF)micro (uF)pico Farad (pF)YOU NEED TO UNDERSTAND IN OLDER CAPACITORS THEY USED mF to denote MICRO FARADSNow DISC on the Brymen DMM means "DISCHARGE" , it means the capacitor still have voltage.DO THIS SAFELY, Get a high value resistor (don't hold it with bare hands) and with the resistor, Short the leads of the capacitorUse a 1M Ohm Resistor or there abouts, if you like a spark, try a 100Kohm But be carefulthen test the voltage across the capacitorbut yeah, old caps have mF to denote microfaradsthey also used MFD to denote microfaradsGoogle "Vintage Capacitors" and search via imagesis it possible to see a photo of the capacitor ?

author: BTO
author_rank: Frequent Contributor
timestamp: March 15, 2024, 11:35:11 pm
content: DISC on Brymen means DISCHARGE The Capacitor

author: shapirus
author_rank: Super Contributor
timestamp: March 15, 2024, 11:50:32 pm
content: Discharge or not, but it often shows it on bigger caps that were last used a long time ago (or even never) and can't have any charge. Must be just the way it works.

author: Mpegger
author_rank: Contributor
timestamp: March 21, 2024, 01:25:02 am
content: Quote from: BTO on March 15, 2024, 11:33:40 pmI prefer the tongue discharge method, but when that's feeling numb I just use a screwdriver across the leads.QuoteI highly doubt this is a vintage capacitor. As I said, the device is around 20 years old as the USB hub has "07 13" silkscreened on the board which is the closest indication to a date I can find. And for wasedadoc, neither the power brick or hub have any voltage regulation between the USB ports and power input. The only voltage regulator I can find is a 3.3V regulator which feeds the main chip on the hub. The USB ports have a direct (0 Ohm resistance) connection to the power supply input, both on the positive and negative pins.And here's some pics of the naked wall wart.