30 minute multimeter backlight hacking

[cybergibbons]

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.

[gobblegobble]

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.

As 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...

[tyblu]

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.

[tyblu]

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.

[david77]

I don't know about that. I always get nervous when someone who has evidently absolutely no idea about
how electricity works wants to do mains wiring or fixing stuff in the house. I have seen some very
dangerous stuff done by people who don't know what they were doing. My advice is: If you don't know
what 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 and
the sink... It turned out he installed the stove himself. In Germany an electric stove is usually connected
to all three phases, N and PE. So there's some potential for death to occur. And still not every home has
an RCD switch!
Or people who want to put up a lamp and cram all three wires into one terminal and then wonder why the
breaker 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 he
got 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 suspicious
and asked him what he wanted them for. He wanted to use them instead of a 3 phase CEE plug and socket
because 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 manufacturer
wanted 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 meter
blew up in their hands and gave them an almighty shock so they'd stop endangering the innocent public
or their families. 230V at 16A makes quite a loud pop and nice sparks after all.

[tyblu]

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.

[quantumfall]

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.

Could 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 ?

[House91320]

nice hack

[gushh]

nice hack

Indeed, 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.

[RJSC]

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.