30 minute multimeter backlight hacking - Page 1
Forum section: electronics / projects

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 07, 2010, 04:15:56 pm
content: I needed a multimeter one evening, and could only get one from Maplin. I didn't want to spend a lot, so got a UT-50A manual ranging meter. It did the job, and I have found it useful for high current (20A range) and automotive work.The input protection is very bad - 20A is not even fused at all.But, one of the "features" it has is an auto backlight, which is meant to turn on as it gets dark. This would be great if it worked. It needs to be almost totally dark though, and it turns on and off if you are using a headtorch as well. A manual backlight would be much better. I tried putting tape over the sensor window, but enough leaks it for it still not to work.So, for a 30 minute project this afternoon, I opened up the meter and changed it to manual backlight. The meter has a hold button that I have never used, so I simply cut the tracks around this switch, removed the LDR detecting the light, and used some kynar wire to connect the two together.And hey presto, a much better meter.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2010, 07:07:56 pm
content: Good hack but using a shit meter for automotive work is not a good idea because if it shorts across the battery terminals the current will be high enough to cause it to meltdown at best, blow the battery to bits sho2wing you with acid at worst.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 07, 2010, 07:16:02 pm
content: Quote from: Hero999 on November 07, 2010, 07:07:56 pmwhy you call it shitmeter? is it really bursting if got shorted?

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2010, 07:36:04 pm
content: It's shit because it has no protection as the original poster stated.I think adding a fuse would be a good idea, even if it's just a polyswitch, lol.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 07, 2010, 08:32:42 pm
content: It seems like a number of you guys go on about explosive failure of cheap DMMs when used for automotive tasks and other higher current measurements, and maybe I just don't get it.QuoteIt seems to me, and it's been my experience, that a copper trace on a PCB makes a -VERY- effective fuse.Granted, the meter would then be totally smoked, and probably not worth repairing. But I can imagine no situation where a short in a DMM could ever cause destruction that could seriously harm the operator ("seriously" = needs a doctor or hospital), much less cause a car battery to explode.I have an ULTRA-cheap DMM (free from Harbor Freight) that I've never even taking out of the package. Might be fun to try a destructive experiment.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 07, 2010, 08:57:38 pm
content: Quote from: Hero999 on November 07, 2010, 07:07:56 pmI'll take the risk - there's simply not the explosive energy in 12V at 800A compared to 415V at several thousand amps. I've shorted a car battery with wire of a similar diameter, and all it did was rapidly heat up and melt. The chance of it causing the battery to explode? I'm just as likely to drop a spanner across the terminals and cause it.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 07, 2010, 09:01:29 pm
content: Actually, I'll toss it back to you guys ...I have an ultra-cheap DMM, and I have a fully charged 800A car battery.How can I make an explosion? (Or at least the most energetic failure possible)

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 07, 2010, 09:11:48 pm
content: Chunky wire, soldered to good 4mm plugs. Short it across the battery...

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2010, 09:50:59 pm
content: It depends on the condition of the battery, a knackared battery will not do much, a brand new one will.Get the impedance and calculate the short circuit current, if the impedance isn't listed in the spec' calculate it using the cranking capacity and Thevenin's theorem.12V at 800A is still 9.6kW and is enough to cause fire and severe burns.If the PCB trace melts and burns away it's still possible for an arc to complete the circuit, gas discharges have a negative voltage to current coefficient so at high currents even a low voltage can arc a considerable distance, once the circuit is completed.There's also a greater risk of arcing with DC than AC because the current isn't interrupted twice every cycle. A 24VDC breaker with a interrupting capacity of 10kA will be more heavily engineered than a breaker with the same rating but at 240VAC.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 07, 2010, 10:00:39 pm
content: The a battery is virtually new (and currently in use in my wife's car) It's fully charged but I'll throw a hygrometer on it before the test to make sure.The little meter says it can measure up to 10A DC. There is no AC current setting or rating.The sticker on the front of the meter says that it's a Cat II device.Oh, and the instructions say to always wear appropriate eye protection while using the device. (hehehehe)I'm thinking the worse case would be to apply the car battery directly across the 10A DC terminals.

author: david77
author_rank: Frequent Contributor
timestamp: November 07, 2010, 10:52:35 pm
content: http://www.all-sun.com/products/DM830G_d.jpgI've seen several meters like the one above plugged into a typical domestic 230V/16A power point at the10A setting. Usually if the supplied (really crappy) leads are used they are charred and the insulationmolten. The meter will be knackered, too.I've also seen some of those meters used to "check how much amps are left in a battery". The meters don't look good after that, the leads will be molten beyond recognition and the 10A range is usually broken.I haven't analysed the failures any closer yet, but I'd guess the 10A shunt, the PCB traces or the inputjacks will be molten.In the first case the user will get an almighty shock because of the sparks and the bang. I'd actually ratethat quite dangerous, but mostly because someone could be on a ladder checking a light fixture. The MCB will shut the circuit down quick enough not to cause too much damage.The second case will generate quite some disgusting smoke while the insulation melts. It could causeburns through molten PVC or touching a hot lead.In neither case can I see how a really dangerous explosion could occur that might rip the case apart withbits flying everywhere.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 07, 2010, 11:16:39 pm
content: Here's the actual meter in question...http://www.harborfreight.com/7-function-digital-multimeter-90899.htmlI got it for free with a coupon from their mailer.I don't think blasting this across the DC A terminals is going to be all that impressive. There's just a big hunk'o'wire from one jack to the other to form the shunt. There are interesting little crimps in the wire, maybe that's how they "calibrate" the current readings.At any rate, this wire will just go POOF where one of those crimps are.

author: Fraser
author_rank: Super Contributor
timestamp: November 07, 2010, 11:45:04 pm
content: At the high risk of being seriously flamed I would just like to say that if someone can't be trusted to place the right 4mm plug into the right multimeter socket they shouldn't really be using one. I know instruments should be made as idiot proof as possible for safety sake but lets not condemn a manufacturer for expecting a reasonable amount of intelligence and competence. And anyway, if someone were to use the high current range across a mains outlet or a very high current source, they deserve the Darwin Award that they are likely to receiveI have to be honest and say when watching Dave.L.J. review multimeters and commenting on how well (or not) they respond to abuse by the owner selecting the wrong range etc, I am left thinking that he must expect some pretty poorly trained people to use these meters so my comment above applies.... if you don't know how to use a multimeter, don't use a multimeter !My first multimeter was an ICE Microtest 80 unit that didn't have a range switch. You actually plugged the leads into the various sockets to select the function and measurement range.... I certainly learnt to be careful when inserting the leads and never once did I pick the current range when wishing to measure voltage. I still have it and it's big brother in perfect working order some 30 years after buying them. Pictures of ICE meters attached.Excuse me while I 'Duck and Cover'

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 08, 2010, 12:03:52 am
content: Quote from: david77 on November 07, 2010, 10:52:35 pmthe OP'sUni-T UT50Ais 5-10 times the cost of the DM830G? at least herei owned one like the dm830G, but it died already... gracefullythis one is more deserved to be called a shitmeter.

author: EEVblog
author_rank: Unknown
timestamp: November 08, 2010, 01:38:24 am
content: Quote from: Aurora on November 07, 2010, 11:45:04 pmMeters can also fail when used on the CORRECT range.If you don't know how that can happen then I'd have to say you don't know how to use a meter either!Having a meter that is designed to fail safe in various circumstances and designed and tested to certain standards is a GOOD thing that should be encouraged.If anyone wants to use a meter that's not designed to fail safe then that is entirely their choice.But anyone who wants to argue any case against those who chose to use (or encourage people to use like I do) a meter that is properly designed for safety is just a moron who is just making themselves look very foolish indeed.Dave.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 08, 2010, 02:09:54 am
content: I agree with you Dave. To that point the meter I'm talking about was tossed in the back of a junk box unopened while I use my Fluke and Tek DMMs daily.My singular point was that I am skeptical that you can get a life threatening high energy failure by connecting a crap DMM to a car battery. (and if you can, I want to see one)

author: sonicj
author_rank: Frequent Contributor
timestamp: November 08, 2010, 06:51:29 am
content: Quote from: JohnS_AZ on November 07, 2010, 11:16:39 pmthat little meter is quite popular for modifying into a high current watt meter.link

author: Fraser
author_rank: Super Contributor
timestamp: November 08, 2010, 06:01:46 pm
content: DaveLJYou will get no argument from me on appropriate safety being important. Regarding my comments on the range selection. I may have put it poorly but my comment related to users who basically have no idea about correct mode/range selection or socket use and your multimeter test videos 'appeared' to be accepting of such behaviour but I see now that you were just making sure that the meter was intrinsically safe.... fair comment. I use a Fluke 87 Series III as I know it is both safe and accurate.

author: .o:0|O|0:o.
author_rank: Regular Contributor
timestamp: November 10, 2010, 05:12:51 pm
content: Quote from: JohnS_AZ on November 07, 2010, 08:32:42 pmIn my experience, call me lucky, this is quite true. A while back I was curious to see who would win between my cheap Digitech multimeter and the charging capacitor of a microwave oven... The loud bang in my hands was instantaneous and a learning experience that will not be forgotten easily. I opened the unit and beneath the plastic dial were two interrupted tracks, two mini craters, exposing the clear fiberglass. Interestingly, the surge left a snaking pattern on the bottom surface of the dial and managed to heat and warp one of the teeth on the dial in an incredibly short space of time. Had the tracks not been so thin perhaps it might have caught fire. The plastic case was not damaged though..o:0|O|0:o.Ironically, I had already figured out that the fuse had gone and replaced it without needing the meter. I think I must have been really determined to put the meter to use that day.

author: kd5jha
author_rank: Contributor
timestamp: November 15, 2010, 05:16:03 am
content: I also had one of those crap red harbor freight meters, and really found that it was basically useless. One day at work we decided to set it on the bench and hook it up to the 75kv hipot to see where it would flashover... I think ours met it's demise at around 1kv which surprised me (I thought it would be lower)... The funny part was instead of stopping we just kept turning it up more till it arced again! Keep in mind we were standing 10 feet + away at the time doing this from a shielded control panel.I feel pretty safe using even a crap meter like this on a car, heck if your not good with this meter, then just get a good old automotive test light and that will do almost everything you need unless you are troubleshooting fuel injection stuff or something.I will say, I have also had a bad experience with a car battery and a CB radio... Where the battery became shorted through the radio and the fuse installed was not small enough! The PVC insulation melted off the RG-8x coax and filled the cab of the vehicle with toxic fumes to where I could not see! It finally burned the coax in two after about 30 seconds along with melting a trace in my carpet! The main line fuse (dont remember value) never blew! When stuff does go wrong, it can really suck! If I had been driving rather than working on my vehicle in a parking lot I might have run off the road from not being able to see...This is exactly why I get worried every time I hear one of these kids with the big stereo systems telling me that he has a two Farad cap on his wiring to his Amps... I hope that thing is in the trunk in an enclosure so it never gets a penny that fell through the seats and shorts one of the terminals to the frame of the seat or something...that could really suck! By the time most slow blow automotive fuses cut the circuit the carpet might already be on fire!I actually have met two people personally that have had jewelry they were wearing short across a high energy circuit and neither one of them will do that again! One guy had his wedding ring get shorted across a car battery while tightening the terminal clamps with a wrench and it burnt a circle on his ring finger where it cauterized the skin! Now he does not need the ring cause it's branded into his finger permently... The other was a girl I went to school with that leaned over a HV transformer in a lab (which she thought was not energized) and it arced to her gold necklace which was hanging down from her neck! She was in a coma for 3 weeks, had to relearn how to walk and talk again and had a piece of her foot the size of a quarter blown out the side where the arc went to ground.Guess the order of the day is better safe than sorry!Now if you want to blow something up on purpose that's a different storyjust don't make an ass out of yourself on YouTube by getting lye or battery acid all over your face screwing around like an idiot,like the kids making these chemical bombs for fun.Do it as a controlled experiment from a safe distance, and whatever you do, Don't do it to your wife's car! Then you really might get hurt

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: November 16, 2010, 12:28:23 am
content: Quotelogically everything we say , it haves an value , if the measurement conditions demands it.Most people buy an 5 Amps adjustable PSU , and they debate , that they need an Fused 20A DMM ..Is this logical ?The only "fail safe" comment that I can make , are that they are lots of different target groups,that they need to use an multimeter .And so, for its one target group, there is products that cover those special needs.And the one who needs this special product he will find it.I am still standing over my opinion, that the brand name by it self its not the warranty about safety.The model does the difference .

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 16, 2010, 03:26:28 pm
content: Every day is like a philosophy lecture with Kiriakos around.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: November 16, 2010, 08:12:24 pm
content: Keep the good one tips , and trash the unwanted .This is, what I do too.

author: xani
author_rank: Frequent Contributor
timestamp: November 18, 2010, 03:26:56 am
content: Heh I remember that as a kid (something like 10 years old, we had some kind of brownout, voltage dropped to like 100-130V and I couldn't play my PC games) I managed to connect my father's multimeter, el cheapo $5 one, with a piece of thick wire and no fuses at 10 A range, to 220V and yes, with probes connected on that 10 A range.Some sparks, main fuses blowed, got lucky and nothing happened to me but suprisingly multimeter survived

author: gobblegobble
author_rank: Regular Contributor
timestamp: November 18, 2010, 09:47:54 am
content: Quote from: xani on November 18, 2010, 03:26:56 amYou were lucky allright, but the saving grace was that you were indeed "fused" if you think of it: the meter is specced for 10 amps and the fuse you blew probably was a 10A fuse as well. So essentially the current grew higher than expected and the mains fuse blew around the same time your meter fuse would have blown.Had you been further than CATII area you wouldn't probably been as lucky. The fuses and short circuit currents climb quickly to devastating numbers at the distribution network.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: November 18, 2010, 10:27:40 am
content: You can't really rely on a breaker to protect you though.A 32A MCB in a house in the UK will normally still take about 0.1s to trip with the current at 20x i.e. 640A. You can easily get fault currents in the range of thousands of amps for a short time.

author: gobblegobble
author_rank: Regular Contributor
timestamp: November 18, 2010, 11:24:12 am
content: Quote from: cybergibbons on November 18, 2010, 10:27:40 amAs I said, he was indeed lucky. Had he put the probes into much higher current branch he probably wouldn't be here telling the tale...The fuse size was a pure guess as it's the most common size we have around here. Also, the fault current in case of a fused range is often taken into account when designing the meter: think of a current in a Fluke with fraction of a second if fed with 100 amps of current. Seriously doubt the cheapest ones are given thought on that era, hence the "Do you feel lucky, punk?" -meter.Edit: wonder why do these discussions always seem to end as a disgussion about meter safety? People should already realize they're gambling when using the cheapest meter they can find for measuring mains...

author: tyblu
author_rank: Frequent Contributor
timestamp: November 19, 2010, 03:32:56 am
content: I've been using meters before I knew the difference between voltage and current to fix household crud (replacing old wiring and sockets, checking cords, lights, replacing fuses and breakers). I blew the house/etc. breakers dozens of times, cranked the range switch around while hooked into the load, jammed probes into mains lines, and blew the meter fuses several times, all to do what needed to be done. Better than sitting in the dark or giving up several days of pay just to get someone to plug in a light. These tools need to be bombproof; mine wasn't, which is why it often tripped the breakers when measuring mains V[line-to-line] straight from the wires. A multimeter enables people to fix things themselves, so DMM designers need to expect every type of person to be using them, including tired-and-cranky-overwraught-dad-who-just-wants-to-get-the-stove-working-so-his-kids-will-quit-whining-and-he-can-get-back-to-the-game people.

author: tyblu
author_rank: Frequent Contributor
timestamp: November 19, 2010, 04:05:27 am
content: Also, about blowing up your meter: a car battery won't do it, only higher voltage (and low impedance). If the fusible link (10A+ copper jumper) and the fuse melt in such a way that they still conduct, a car battery will make the probes wires melt and warp the box. It happens often with non-HRC 5A+ fuses, especially some automotive fuses. If you're lucky, the probe lines will catch on fire. You've likely all seen the result of high voltage, low impedance sources. In my experience, 240V mains usually just makes the meter jump a bit, then acts like the car battery if it still conducts and the house breakers don't trip -- it only happened to me once, though, while looking for live mains to put in garage door opener on a line that apparently wasn't hooked to the grid through a breaker. Industrial voltages here in Alberta are 240V, 400V and 608V/620V, and are often a bit inductive/lagging, so shorting your meter across them gives you a bit more of a jolt than you'd expect as the flyback voltage spikes. Never did it with a meter myself, but have fried a number of items (mostly wrenches...) in a surprisingly exciting manner on these lines.

author: david77
author_rank: Frequent Contributor
timestamp: November 19, 2010, 04:18:01 am
content: I don't know about that. I always get nervous when someone who has evidently absolutely no idea abouthow electricity works wants to do mains wiring or fixing stuff in the house. I have seen some verydangerous stuff done by people who don't know what they were doing. My advice is: If you don't knowwhat you're doing get a man in who does, it might save lives.There was one gentleman who asked me why his wive gets a shock everytime she touches the stove andthe sink... It turned out he installed the stove himself. In Germany an electric stove is usually connectedto all three phases, N and PE. So there's some potential for death to occur. And still not every home hasan RCD switch!Or people who want to put up a lamp and cram all three wires into one terminal and then wonder why thebreaker trips and the light switch welds shut everytime they turn the light on.Or the one guy who needed a bigger fuse because all the wiring in his workshop started to cook once hegot his new lathe in and the old 16A fuse always blew.Or the guy who wanted three black, one blue and one yellow banana plug and socket... I got suspiciousand asked him what he wanted them for. He wanted to use them instead of a 3 phase CEE plug and socketbecause the CEE plug was so bulky. Jesus!Or the Chinese restaurant I was in last week. They had a row of power points wired up with speaker (!)cable (no PE!) and plugged metal table lamps in them. All the lamps had 3 pin plugs, so the manufacturerwanted them to be connected to PE.I could go on...People should leave their hands off stuff they don't know shit about and let the professionals deal with it.Especially when it's potentially lethal stuff they know shit about.When I see what stupid and dangerous stuff some people produce, I think it would be better the meterblew up in their hands and gave them an almighty shock so they'd stop endangering the innocent publicor their families. 230V at 16A makes quite a loud pop and nice sparks after all.

author: tyblu
author_rank: Frequent Contributor
timestamp: November 19, 2010, 04:45:54 am
content: Folks are most definitely better off not doing what they can't do safely, but do you really think that is going to stop anyone? People aren't idiots, despite the growing number of Darwin defiers and jaded professionals, and will continue to use whatever they have access to, including DMMs and electricity. It's not much of a leap to believe that multimeters are safe. I mean, you can buy them in the auto section at Wal-Mart, right? So, this tool the public believes to be safe had better not have a 'melt and catch fire' setting.

author: quantumfall
author_rank: Regular Contributor
timestamp: November 19, 2010, 10:35:11 am
content: Quote from: EEVblog on November 08, 2010, 01:38:24 amCould you list some failure modes for the correct range I'm not sure I could think of them all ?The voltage range I would think unlikely to go short but I suppose it can in some failures. The current range could have a high impedance and catch fire or get very hot what am I missing out ?

author: House91320
author_rank: Regular Contributor
timestamp: November 26, 2010, 09:12:05 am
content: nice hack

author: gushh
author_rank: Newbie
timestamp: November 26, 2010, 07:13:11 pm
content: Quote from: House91320 on November 26, 2010, 09:12:05 amIndeed, when was the last time you used the Hold function?, I would rather see a REL function on cheaper meters than a HOLD. But that's probably just me.

author: RJSC
author_rank: Regular Contributor
timestamp: June 22, 2011, 08:01:56 pm
content: Do you have any idea which trimpot calibrates what?I'd like to order a precision reference from maxim to calibrate my UT50A but i can't find any schematics of it or service manual, and the user manual only tells how to replace batteries and fuses.About the backlight: mine is the contrary, lights up a little too soon, while i can still read it. Maybe your LDR is bad.