Cheap Multimeters Can be Dangerous, Though Inexpensive is not Necessarily the same as Cheap

The person I bought my home from had issues, not worth detailing in depth here. At some point, in selling the house, he took fasteners, screws, expensive cement anchors, bolts, nuts, grease, lubrication sprays, and much more, stuff he didn’t want to move and threw them along the property lines, in the alley, of two homes behind me. When I realized that, I recovered most of them.

What caught my attention to those items even being there was a carrying case and leads for a meter older guys at work had at one time, yet no meter. One person living on the street below, i.e. behind me, sometime ago, found the meter, but not the leads. So I gave him the leads I found, and out of the blue, he gave me an inexpensive digital meter that works and appears quite new.

Though the meter is rated for 250 VAC and 250 VDC, I wouldn’t expose the meter to that, possibly not even to 120 VAC, though 12 VDC, and voltages to 50, would probably be OK, if you’re careful, because if you’re not, it could be dangerous.

Meter Covers View

It appears to be externally virtually identical to a Cen-Tech Meter, and was discussed on this thread:

Link: Harbor Freight Cen-Tech 90899 Small Multimeter Teardown

The Meter discussed in the thread Measures:

  • To 1000 VDC and 750 VAC, and to 10 Amps DC 

My Meter Measures:

  • To 250 VDC and 250 VAC, and to 5 Amps DC.

Externally Identical Meters are not necessarily internally identical meters, as the following image shows, this image is not of my Meter. Nevertheless, I am no more reassured that the meter shown immediately below is any safer than mine, I cannot tell from the Image if the Depicted Fuse has anything to do with the Higher Current Input.

Different CB -1

On the topic of Current, that’s especially where the problems arise with this meter. If you remember, in a previous post, I discussed Clamp Ammeters, that is one way to measure the current in a circuit, and it pertains to the Magnetic Field generated by the Conductor. The simpler form relied on the expanding and collapsing magnetic fields of AC Current. Eventually, DC Clamp Ammeters were built to determine the steady state DC Current, using Electronics in the Meter, to determine the current flowing through the wire within the clamp.

But not all applications permit measuring with a Clamp Ammeter, many uses exist for accurate current measurements, such as on Circuit Boards, in Industrial Processes (Current Loops, a method to indicate how far open a valve is, for example), etc., but are Inserted into the Circuit.

Meter Connection in CKT

Meter Connection Into a Circuit Shown Above: 

A situation encountered in Industry where a Multimeter is Used to Measure Current in the Circuit to Calibrate the Transmitter is shown below: 

Science_of_4_20_Loops_Fig1

Inside this multimeter is a Shunt. A very low resistance jumper that produces a specific voltage or millivoltage across it in response to a specific current. This permits the meter to handle higher currents for measurement then would otherwise be possible. It’s essentially a short circuit, for all intents and purposes. It allows the current to flow into the meter and immediately back out without interface with the Meter’s Electronic Circuits, save for the tiny voltage produced across the shunt that is equated into a current flow. With Higher Currents, the User Manual will tell you how long you can let the current flow, it may be as little as ten seconds with some meters.

Before retiring, I was an electrician, but we had people that specialized more in electronics. It was common for them to measure current on a Multimeter (and that Multimeter was calibrated by an independent company and given a sticker listing the date and that it is certified as calibrated), so they’d leave the leads in the meter, in the Common and Milliamp Positions. One day, I needed a meter, I borrowed one of theirs. The leads were in it, I set it to Volts, placed the Probes on a Voltage Source, and Voila, nothing. Hmmmm, why isn’t it reading? Oh, the leads are in the wrong jacks on the meter. On no. That means I blew the internal fuse. And I seem to remember them telling me the meter didn’t work after replacing the fuse. My bad.

But there was a Fuse. Look at the pictures of the inside of this meter, that section of the meter HAS NO FUSE, and it’s a Short Circuit from the Bottom Jack (Com) to the Top Jack (5 Amps) via the Shunt. If you put the Leads in the Top (5A) and Bottom (Com) Jacks, then place the leads across 120V, or even a Car Battery’s 12V, something terribly bad is going to occur. If across 120V, if you’re lucky, the Breaker in the Breaker Panel will Trip. If not, the Meter itself could experience a Catastrophic Failure, blowing apart forcefully.

Meter Inside Views

The SparkFun Meter I reviewed has Fuses on Both Inputs. So, though it’s inexpensive, it’s Protected, and Accordingly is rated Cat III (Category III), making it, by rating at least, suitable for Commercial and certain Industrial Exposures.

Listening to the reviews of others, some Meters seem to rely on self sacrifice, and Meters that do so have reduced width tracks on the board in association with this input, so that a short due to putting Test Leads across a voltage source, when plugged into the Amp Socket, obliterates the Track, fuses if you will, destroying at minimum future use of that input, or ruining the meter entirely. This is not recommended.

The Meter reviewed here today is Cat II, and IT IS NOT SUITABLE FOR INDUSTRIAL AND COMMERCIAL EXPOSURES.

In addition, the Meter Reviewed here, look at the Circuit Board. Awkward solder joints common to lower quality workmanship, and a strange point I noted, there is a break away daughter board, if you look where the Jacks connect to the board, that rectangle of circuit board was broken off the right hand side of the Main Board, and moved to the left side, as viewed, but there it’s not held on by the integrity of the board, but only via Solder Joints, and as such, is subject to flexing and failure. The only thing that limits the flexing is a single narrow stalk of plastic from the back of the meter.

Also shown on the Inside Views is the Circuit Board to Display Interface. It’s a piece of pink rubber (in this meter) that has carbon tracks passing through it. It’s a flexible connection. In many instances, displays that are missing segments or small symbols or words, can be fixed by carefully removing the circuit board, as needed, and this flexible rubber unit, gently cleaning it, and very gently cleaning the Display Mating Surface (You’ll usually not see metal tabs on the display mating surface, but you should see conductors of sorts) and gently clean the metal contacts on the Circuit Board. Very easily wipe the mating ends of the flexible rubber unit. Be sure all the lint and dust are wiped away, and Reassemble. I’ve definitely had success with this.

Display Flex Pad

Inexpensive Meters have a use, but we must differentiate between decently designed inexpensive meters and poorly designed meters. Some of these Meters sell for less than $10, delivered. This meter reads well, I just used an expensive meter to measure some batteries I knew were essentially dead, it read the same voltage. It can be used to fix Table Lamps unplugged from the Socket and the Meter Applied to the Plug end of the Cord to look for Continuity. A number of things that doesn’t require it to be subject to higher voltage. But be wary of low voltage with High Current Potential, such as a Car Battery, Laptop Batteries, etc. One Careless Move and the meter shorts the battery, bad things can happen. I may fill the 5ADC Jack with Epoxy so no one else uses it incorrectly.

To be safe, we must be aware of the equipment we own, limitations on it, possible dangerous things that can inadvertently be done, and how to use it to eliminate any chance for harm to ourselves, others nearby, or even to eliminate the chance for harm to the meter.

The following link is an individual I watch often. He does reviews on Dangerous Consumer Electronics, taking apart the items, showing why they’re dangerous, and offering hints to the manufacturers on how to make it right. He likes inexpensive meters, but he has one that was destroyed by the application of 220V even though the Instructions seemed to indicate that was acceptable to input up to 250V. He found the error, how to do it correctly, using an identical meter, but the instructions were misleading. He also talks about Meter Test Lead Failures.

Dangerous Multimeters (+ Explosion, Smoke & Fire)

And another Vlogger I’m not familiar with, he has a Presentation from a visit to Fluke, using a meter seemingly nearly identical to this. They rather graphically show the meter when exposed to 660V after being placed in the Resistance Range. He takes a Fluke Product, and holding the Meter, apples 660 Volts, then switches it to Resistance, nothing bad happens.

Inside Fluke – The Measure of Innovation 2015 – Pt

Author: Dr-Artaud

A Doctor that is not a Doctor, but named after a character in the movie "No Such Thing", as is the Avatar.

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