Error in cheap DMM manuels from China - Page 1
Forum section: electronics / testgear

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 01, 2018, 03:08:26 pm
content: When using a 15usd DMM from Harbor Freight on continuity mode the manuel states "The read out gives the voltage drop across the diode." It doesn't it give the resistance in ohms. These meters don't give any indication on the display of the what the meter is reading, just a number, no decimel point, no anything.Hope this helps.

author: mtdoc
author_rank: Super Contributor
timestamp: August 01, 2018, 03:15:30 pm
content: Ironec ...

author: Fungus
author_rank: Super Contributor
timestamp: August 01, 2018, 03:59:03 pm
content: I'll be sure to watch out for that...

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 02, 2018, 01:42:05 am
content: If you're talking about one of those 2000-count 7106-based meters, it is typical for the diode test setting on those units to configure the meter exactly as it would for 2000 ohms resistance, with a current source of about 1mA. Which means the display can be interpreted as mV, as well as ohms.The single-resistor current source typically used is sloppy, so the mV reading will be sloppy too. You get what you pay for.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 02, 2018, 12:20:28 pm
content: Quote from: ModemHead on August 02, 2018, 01:42:05 amThat is indeed useful, but not being that aware of what the meter is doing when taking a measurement and responding in millivolts which is the same as ohms does make sense. So if I was to remove the battery from the DT 308 Harbor Frieight meter and measure the resistance in the diode check mode I should see 2k?I will give it a try.

author: crazyguy
author_rank: Regular Contributor
timestamp: August 02, 2018, 12:35:52 pm
content: measure the diode in terms of resistance is not a useful parameter, the diode is nonlinear device, no one care about the resistance of a diode

author: bugi
author_rank: Regular Contributor
timestamp: August 02, 2018, 12:59:52 pm
content: Quote from: crazyguy on August 02, 2018, 12:35:52 pmSlightly nitpicking here, but there are cases and over limited range(s), where the diode's "resistance" (i.e. current vs. voltage) is useful info and is very much cared about. A typical DMM won't do that kind of measurement by itself, though, and the measurement mentioned above is not about such case.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 02, 2018, 01:02:11 pm
content: Quote from: crazyguy on August 02, 2018, 12:35:52 pmI was thinking about this differently. I was not using the continuity check, with the beeb, for diodes.Its useful for determining if some circuit is shorted and how short it is. If its dead short, zero ohms, you see that on the display, if its over about 50 ohms, no beeb, but you see on the display says 100ohms. This works when tracking down shorted components without switching to the ohms range. This approach works if you know approximately what the resistance through a transistor should be and you get a different reading from what you expected. Thats why its useful in troubleshooting.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 02, 2018, 01:54:50 pm
content: Quote from: joseph nicholas on August 02, 2018, 12:20:28 pmNo, I was just trying to point out the meter is measuring ohms with a very approximate 1mA current source. So the reading will be an equally approximate representation of the millivolt drop across the device under test.At least that is the case with the inexpensive (or sometimes free) meters I've looked at. Not familiar with the exact model you mentioned.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: August 02, 2018, 03:30:10 pm
content: Quote from: ModemHead on August 02, 2018, 01:54:50 pmI did measure the resistance of the DT 838x in its continuity mode and it is approximate, but the most interesting thing, for me at least, was how closely the ohms are to the correct value listed on a resistor. As a bonus, it will measure up to about 1k resistance values without changing any ranges. So it turns out that if you use it to see if it lights up a white led, (it does) only thats only a small part of its usefulness.For a cheap crap 4 dollar multimeter from China knowing how versitile the continuity mode with the beeper makes it and knowing it's putting about 1ma through the dut it sort of gives it added functionality when troubleshooting. Everyone needs one in their backpack. IMHO.BTW, as a throw away piece of junk I found a great use for it when buying low voltage batteries. To test coin cell batteries, put it in amps range and dead short the battery and it will give a rough estimate of the power rating.