30 minute multimeter backlight hacking

[cybergibbons]

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.

[Zero999]

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.

[Mechatrommer]

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.

why you call it shitmeter? is it really bursting if got shorted?

[Zero999]

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.

[JohnS_AZ]

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.

The input protection is very bad - 20A is not even fused at all.

It 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.

[cybergibbons]

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.

I'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.

[JohnS_AZ]

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) 

[cybergibbons]

Chunky wire, soldered to good 4mm plugs. Short it across the battery...

[Zero999]

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.

[JohnS_AZ]

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.

[david77]

http://www.all-sun.com/products/DM830G_d.jpg

I've seen several meters like the one above plugged into a typical domestic 230V/16A power point at the
10A setting. Usually if the supplied (really crappy) leads are used they are charred and the insulation
molten. 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 input
jacks will be molten.

In the first case the user will get an almighty shock because of the sparks and the bang. I'd actually rate
that 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 cause
burns 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 with
bits flying everywhere.

[JohnS_AZ]

Here's the actual meter in question...

http://www.harborfreight.com/7-function-digital-multimeter-90899.html

I 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.

[Fraser]

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 receive  

I 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'  

[Mechatrommer]

http://www.all-sun.com/products/DM830G_d.jpg

the OP's Uni-T UT50A is 5-10 times the cost of the DM830G? at least here
i owned one like the dm830G, but it died already... gracefully this one is more deserved to be called a shitmeter.

[EEVblog]

I 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 !

Meters 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.

[JohnS_AZ]

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 )

[sonicj]

Here's the actual meter in question...

http://www.harborfreight.com/7-function-digital-multimeter-90899.html

I 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.

that little meter is quite popular for modifying into a high current watt meter. link

[Fraser]

DaveLJ

You 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.

[.o:0|O|0:o.]

It seems to me, and it's been my experience, that a copper trace on a PCB makes a -VERY- effective fuse.

In 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.

[kd5jha]

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 story just 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

[Kiriakos-GR]

designed to fail safe

logically 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 .



 

 
  

[cybergibbons]

Every day is like a philosophy lecture with Kiriakos around.

[Kiriakos-GR]

Keep the good one tips , and trash the unwanted .

This is, what I do too. 

[xani]

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

[gobblegobble]

<snip>thick wire and no fuses at 10 A range<snip>

Some sparks, main fuses blowed, got lucky and nothing happened to me but suprisingly multimeter survived

You 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.