How bad can a cheap multimeter be? - Page 1
Forum section: electronics / testgear

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 19, 2013, 05:14:18 am
content: I had heard that Harbor Freight sells really cheap multimeters, and you get what you pay for, so I wondered just how bad could one be?To find out, I bought a $4.99 meter, item 92020, a Cen-Tech 7-Function digital multimeter with backlight, as described herehttp://www.harborfreight.com/7-function-digital-multimeter-92020.htmlI expected lousy build quality, lousy leads, and no input protection. This meter delivered on my expectations. But I wondered, is this thing OK for checking low energy voltages?I set my power supply to 10.00 Volts, as measured by my Fluke 179 and Fluke 16. Their calibration has expired, but they agree to the last digit, so I'm confident they're not too far out. Out of the box, the Cen-Tech reads 10.09V, or about 1% off. That's good enough to do a lot of around-the-home diagnostics, even though it's not lab quality.But my cheap meter is brand new with a brand new battery. What happens as the battery ages?I powered the meter with a separate, completely isolated adjustable supply instead of its 9V battery, and start dialing the supply voltage down to see what happens, while the meter is trying to measure the test voltage of 10.00VA perfect meter would consistently read 10.00V until it decided the battery was not sufficient, at which time it would stop reading. How close to perfect is the Cen-Tech?At 9.0V supply, the meter reads 10.09VAt 8.0V supply, the meter reads 10.10Vat 7.1V supply, the meter reads 10.11VAt 7.0V supply, the meter reads 10.11V, but a rectangular icon that looks vaguely like a car battery shows up down in the corner of the display.At 6.0V supply, the meter reads 10.56V, with the battery iconAt 5.0V supply, the meter reads 12.43V, with the battery iconAt 4.0V supply, the meter reads 15.13V, with the battery iconAt 3.3V supply, the meter reads 17.73V, with the battery iconAt 3.2V supply, the meter stops reading and the display goes blank.As a contrast, I tried the same experiment with my Fluke 179. As I dialed the "battery" supply down, the measured voltage kept reading 10.00V until the battery was down to 5.3V. At that point, the display started reading "bATT", and no more numbers were shown. No matter how low the battery got, I couldn't get the Fluke to display a reading that varied even in the last digit.So the Cen-Tech meter delivers only a bit worse than 1% error as long as you pay attention to that battery icon and know to stop using the meter and replace the battery as soon as that icon shows up. But if you don't understand what that rectangular icon means, your readings could be up to 77% over the actual voltage! Seeing a little picture of a battery doesn't necessarily trigger a sense of urgency if you haven't read the manual to know what that picture means. After all, some user might thing the designers could have put the picture of the battery there to indicate "battery ok", or "the meter is set to measure a battery".If you're aware of the limitations, I suppose there are quite a few jobs you can do with this meter. Many diagnostic jobs I do don't need better than 5% accuracy, and I don't always work around circuits that have enough energy to hurt me. But the behavior as the battery voltage drops is a real trap for the inexperienced, and somehow I think they're the target market for this meter.Of course, this isn't a full blown test. I'm sure the meter has other faults I haven't noticed yet.

author: Wytnucls
author_rank: Super Contributor
timestamp: September 19, 2013, 05:42:55 am
content: For safety reasons, the display should blank out before the measured voltage falls out of tolerance. Most meters will do just that, even cheap ones. I doubt that you will be able to power up the meter when the depleted 9V battery voltage drops below 5V, because of internal resistance, so it might not be a serious issue.

author: Lightages
author_rank: Unknown
timestamp: September 19, 2013, 05:55:15 am
content: Yes it works, but for how long? Will it get die without warning? Maybe not. Actually when things are made in such high quantities as these things are they can actually end up being reliable as they work out the problems with them. That is theifmanufacturer cares to correct problems with something as cheap and disposable as these things are.

author: Bored@Work
author_rank: Super Contributor
timestamp: September 19, 2013, 05:59:23 am
content: The behavior is typical for meters that use an IC like the 7106 or a clone of it. 35 years ago that was the way things were. However, today there is a reason some us us call that stuff crap and rubbish.Now, watch this space until the first "but it is cheap" apologist shows up. Yes it is cheap, cheap rubbish.

author: Wytnucls
author_rank: Super Contributor
timestamp: September 19, 2013, 07:30:05 am
content: It looks like that meter or its clone was withdrawn from the European market a while ago:(Of course, they are still all over eBay and probably in most shops under a new guise)

author: amyk
author_rank: Super Contributor
timestamp: September 19, 2013, 11:00:35 am
content: These are popular among the Chinese (and those in other developing countries) who can't afford anything much better.I think it's still better behaviour to overreport than underreport upon low battery.

author: Neilm
author_rank: Super Contributor
timestamp: September 19, 2013, 05:52:23 pm
content: Quote from: Wytnucls on September 19, 2013, 07:30:05 amI saw something very similar for sale from Farnell here in the UK. As the company I work for does import and re-badge this sort of equipment, I bought two. One I tore down to check it wasn't compliant to 61010, the second I got the marketing people into the High Energy test observation room - connected it to the test equipment and applied a single HE pulse at what ever the rating of the meter was (300V CAT III from memory which would be 4kV?)We then spent a long time sweeping up the remains and kept finding bits of it months later.

author: toli
author_rank: Frequent Contributor
timestamp: September 19, 2013, 06:12:18 pm
content: I have a couple of good DMM's, and a couple of bad ones. One of these bad DMM's wasn't powered up for years, I gave it to my little brother and I don't think he ever took it out of the tool box. The other one wasn't cheap at all (~45$ at the time I think, this was ~7years ago or so). Its a JT-158 (link), and while at first I was happy with it (I was a kid, I didn't know any better), I quickly learned of its shortcomings.IMO the biggest issue with these DMM's isn't accuracy, not even safety - when probing low voltages or measuring temperature I doubt these things will hurt anyone. The biggest issue IMO is simply the fact you can't trust it. If I knew its 5% off, I could live with that - in some cases 5% is good enough just to know the circuit is alive and voltages are in the ball-park, but with these you can't tell what the error will be 5 minutes from now.BTW, I also had one time when the battery died and I got a significant error in the reading, took me some time to find the reasonNow days I only use it when 2 DMM's are simply not enough, and even then I usually compare it to one of my other DMM's before using it just to make sure its still good. Even then I only measure DC values with it, as its not True-RMS so for AC signals its worthless.Heck, I even have a UT-61E which I still don't trust 100% so I compare it to my U1253A every 6-12 months just to make sure everything is still more or less within spec - so I might be too paranoid

author: AG6QR
author_rank: Frequent Contributor
timestamp: September 19, 2013, 07:31:43 pm
content: Quote from: toli on September 19, 2013, 06:12:18 pmAgreed. Displaying grossly wrong readings is worse than just shutting down and refusing to display anything.I recall a thread from a year or two ago, on a different board, where someone was saying "I've wired up a 7805 voltage regulator according to this sample schematic, and the output is 8.2 volts instead of 5.0V. What did I do wrong?" People offered helpful suggestions trying to diagnose the problem, checking the ground, getting a different regulator, measuring the input voltage, etc. Finally, the troubled newbie came back and said, "I replaced the battery in my meter and now it shows I've got 5.05V coming out of the regulator".I don't know exactly what brand of meter it was, and I don't know if the meter was showing some sort of low battery indication that the user didn't notice, or didn't understand the importance of. But the story stuck with me. A meter should stop displaying readings as soon as the battery is so low that the readings are useless. I didn't realize until then that some meter designers don't understand this, or don't care enough to do it right.Also, FWIW, I finally went and looked through the owner's manual of that Cen-Tech meter. There is no mention anywhere of a low battery indicator. So the user has to figure out or intuitively know that the little rectangle means it's time to stop using the meter and replace the battery.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 19, 2013, 09:26:45 pm
content: Quote from: AG6QR on September 19, 2013, 05:14:18 amWhile there are internal pictures of the HF meter already posted here, I'm wondering if you could post a few photos of the insides so we can see if anything has changed recently?Thanks.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: September 19, 2013, 11:21:28 pm
content: Quote from: Wytnucls on September 19, 2013, 07:30:05 amOK this long and no jokes about Rape X?

author: staxquad
author_rank: Frequent Contributor
timestamp: September 20, 2013, 02:56:52 am
content: Quote from: AG6QR on September 19, 2013, 05:14:18 amI have a couple of cheap $9 meters, use them to check the voltage on batteries before and after charging. Absolutely no point having better meters for that purpose. Bought at Canadian Tire, probably made by Uni-T since it's probes were included.Cheap meters have their place.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 20, 2013, 06:41:35 am
content: Quote from: staxquad on September 20, 2013, 02:56:52 amMultimeters that are branded Mastercraft are made by Colluckhttp://www.colluck.com.hk/diyhp/9359/enus/l25520-0/Multimeters.html

author: synapsis
author_rank: Regular Contributor
timestamp: September 20, 2013, 07:00:42 am
content: The local electronics store here sells meters that don't even have a brand name. They have the Fluke-ish yellow rubber surround, and I've found them as Mastech MY-61 online. They keep them in an overfilled shopping cart by the front door, which tells you exactly how the quality is.I picked one up to use as a continuity tester, though. I haven't even put it on my voltage standard.

author: staxquad
author_rank: Frequent Contributor
timestamp: September 21, 2013, 12:59:07 am
content: Quote from: retiredcaps on September 20, 2013, 06:41:35 am(picture scrounged from the net)similar to Colluck's

author: elliott
author_rank: Regular Contributor
timestamp: September 22, 2013, 06:08:22 am
content: Just about every flyer from Harbor Freight has a coupon to get one of these free with any purchase. You could buy a candy bar and get a free multimeter. Since I buy other things from there often, I'll usually grab a multimeter too. They definitely have their place if you know their limitations.

author: madsci
author_rank: Regular Contributor
timestamp: September 22, 2013, 07:17:53 am
content: They're toys....deceptively dangerous toys.I was in my early teens when I had one of these POS meters read 0V on a 240V circuit. This convinced the parents to help me out a little and get me a new instrument.The Radio Shack 22-801, being a Cat II device, wasn't ideal for fixing mom's dryer (I would later learn that this is a Cat III area) but it was certainly better than the $5 HFT deathtrap that I had. It's also not very accurate...but it doesn't try to lure me to my doom either.As an aside...my 22-801 still works just fine. It doesn't see much use these days in light of my two bench meters and the Extech 430 but it's more than sufficient to keep in the car with the jumper cables.

author: Bored@Work
author_rank: Super Contributor
timestamp: September 22, 2013, 07:20:08 am
content: Quote from: elliott on September 22, 2013, 06:08:22 amYes, this place

author: Robomeds
author_rank: Frequent Contributor
timestamp: September 22, 2013, 01:10:16 pm
content: Quote from: elliott on September 22, 2013, 06:08:22 amI was in the same boat for a while. There was a HF near where I was commuting and I had a free with any purchase coupon. I think I had 8 of those things but the time the coupon expired. I've given a few away* and others became stashed meters just in case.*I've always cautioned people that these meters are very cheap. I've found them to be sufficiently accurate but I wouldn't trust them for high voltage work. On the other hand when my brother was complaining that his baby bottle warmer wasn't working we were able to determine it was a thermal fuse that had blown. We didn't fix it deciding that a blown thermal fuse might mean a bigger problem. When suggesting he buy that meter I also warned him about the safety and suggested he never use it on things that are plugged in.

author: amyk
author_rank: Super Contributor
timestamp: September 22, 2013, 02:11:59 pm
content: Quote from: madsci on September 22, 2013, 07:17:53 amHas this also convinced you to verify the correct operation of the instrument too? I.e. the 3-step live-dead-live test?

author: madsci
author_rank: Regular Contributor
timestamp: September 30, 2013, 07:34:13 am
content: Quote from: amyk on September 22, 2013, 02:11:59 pmYes; that's how it was discovered, it failed the sanity check at the mains input which was known to be at least half live (split phase 120/240V) because the timer and drive motor motor worked. The damn thing read fine in continuity mode but was out to lunch in voltage mode.It ended up being a bad thermostat, which is a typical problem on ancient Whirlpool dryers.

author: PA4TIM
author_rank: Super Contributor
timestamp: September 30, 2013, 08:48:58 am
content: We have a cheap (20 euro) DMM at work. We use it maybe once a couple a month ( for instance to check is a flashlite lightbulb is dead ). For serious use I bring one of my Flukes .Two years ago they gave me a powersupply and a Coleman peltier camping cooler that they told me had died but they did not know if it was the box or the PSU because the thing was not at stock at that moment they asked me if I could test/fix it. I took that DMM and measured a voltage that was a bit wrong but not enough to make it stop working. So I switched to AC to look at ripple and it showed over 35V. So the PSU must be realy dead. The customer needed the box soon again so I took the PSU and box to my home lab and measured the PSU with my Agilent. The Voltage was perfect. I measured zero Volt AC and only a very small ripple when loaded.Turned out to be a problem in the box and I fixed it.I think the CMRR and EMC rejection of that meter just sucks.

author: amyk
author_rank: Super Contributor
timestamp: September 30, 2013, 10:01:50 am
content: Those cheap DMMs are definitely not true RMS, the AC range is just a rectifier and a scale factor correction that assumes sinusoidal input. I'm guessing that's a 25V PSU?

author: PA4TIM
author_rank: Super Contributor
timestamp: September 30, 2013, 02:48:55 pm
content: No, 12V.