WTF Philips bench voltmeter ?? reverse volt in ohm/diode - Page 1
Forum section: electronics / testgear

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 31, 2022, 07:49:15 am
content: in this video : a huge What T F Philips ??user needs to swap red black wires when performing Ohm / Diode, since this meter reverse voltage ??How the .. H.. is that a good idea ??All my other Philips Multimeters works fine and are designed correctly,I have never seen anything like this before.https://youtu.be/hbOnKYrWjz4

author: wasedadoc
author_rank: Super Contributor
timestamp: August 31, 2022, 08:36:23 am
content: ISTR that every analogue multimeter I used did that. No problem once I knew about it.

author: SeanB
author_rank: Super Contributor
timestamp: August 31, 2022, 12:12:10 pm
content: Yes, looks like they are emulating all analogue meters in resistance range there, to make the change from analogue to digital easy, without rewriting all the manuals set up for analogue meter use.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: August 31, 2022, 03:28:47 pm
content: I dont understand Sean ?if i take an analog meter, what ever type and brand,diode / ohm go from red to black terminal, right ?this one however, diode / ohm, go from 0 to + input !!that is what i am "complaining" about :-)that is just wierd

author: wasedadoc
author_rank: Super Contributor
timestamp: August 31, 2022, 05:49:27 pm
content: Quote from: oz2cpu on August 31, 2022, 03:28:47 pmNope. Suggest you try it yourself.

author: Gyro
author_rank: Super Contributor
timestamp: August 31, 2022, 07:22:15 pm
content: Quote from: wasedadoc on August 31, 2022, 05:49:27 pmLikewise on AVOs.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 01, 2022, 09:53:14 am
content: Once you understand how a passive analogue multimeter works it is obvious that the "polarity reversal" on the resistance ranges is inherent.In a passive analogue multimeter the needle deflection is proportional to the current flowing through the meter. The current that goes in to the red/+ jack and comes out from the black/- one. The meter movement is actually measuring current whether set to voltage, current or resistance.Current ranges use various parallel shunts so that the current through the meter movement is a known fraction of the current in and out of the jacks.Voltage ranges use known value resistors in series with the meter movement. Then simple application of Ohm's law gives the voltage from the measured current and the known resistance.For the resistance ranges an internal cell/battery of known voltage causes current to flow though the resistor being measured. Ohm's law again is used. R = known voltage divided by measured current. For that current to flow into the red/+ jack the internal battery and wiring must be such that the black/- jack is at a higher voltage than the red/+ one.Not a problem once you know about it. Merely, as Dave Jones frequently says, "A trap for young players".

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2022, 06:21:57 pm
content: With analog meters it is common to have the polarity reversal and thus the positive voltage on the black lead. There may be a fre exceptions, but it seems to be the more common way.For my russian brand meter it is reversed polarity for the higher ranges and "normal" prolarity for the low ohms range, but this is only some 150 mV open circuit and thus would hardly see a diode anyway.I am not sure if they choose the polarity because they wanted to behave similar to the analog meters. It would as well be that a negative polarity current source could have a few advantages (e.g. use of a N FET instead of P channel). The current source seems to be from a special chip and this may just have been available for negative current.

author: dl6lr
author_rank: Frequent Contributor
timestamp: September 01, 2022, 09:34:45 pm
content: The old Fluke/Philips PM2525 uses reverse polarity like the plain analog meters. The old McVoice M890G (maybe a Metex device) uses "normal" polarity, the Fluke 23 too.