Author Topic: Help with new DMM  (Read 18926 times)

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Offline IanB

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Re: Help with new DMM
« Reply #50 on: July 10, 2013, 02:39:14 am »
Not if you're a scientist. :P Every american academic journal/paper/etc that I've ever read uses SI.

Yes the scientific community has to use SI because of the international readership, but the engineering world plows its own furrow. Interestingly enough, precision engineering is done in metric inches. You can get an engineer's rule marked in tenths of an inch, and accurate measurements are done in thousandths of an inch ("thou" in Commonwealth countries and "mil" in America).
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Offline mtdoc

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Re: Help with new DMM
« Reply #51 on: July 10, 2013, 03:51:24 am »
Why do they put the temperature in Fahrenheit? Seems a bit random for a scientific instrument.

In America all temperatures are in Fahrenheit. (All measurements are in feet and inches and all weights are in pounds and ounces. You get used to it.)

Not if you're a scientist. :P Every american academic journal/paper/etc that I've ever read uses SI.

And in medicine as well.  Vital signs - temperature in degrees C, weight in Kg (though often translated into F and lbs for patients), blood pressure in mm Hg and lab test results reported in SI units.
 

Offline zorthgo

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Re: Help with new DMM
« Reply #52 on: July 10, 2013, 03:59:50 am »
No backlight, not biggie. I was just wondering if it did have it but I just couldn't find that feature. :)
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Offline zorthgo

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Re: Help with new DMM
« Reply #53 on: July 10, 2013, 04:02:30 am »
@IanB, By the way, thanks for the instructions on how to adjust the meter. My pocket meter was just a tad off from the reading of the new meter. So, I adjusted it to match the new meter. Now I'm just waiting for the other meter to come in the mail, so I can match it with these two, and see if everything lines up! ;)
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Offline zorthgo

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Re: Help with new DMM
« Reply #54 on: July 10, 2013, 04:31:14 am »
Can someone tell me how to check if the meter's fuse got blown? I was trying to measure the amps with the new meter but instead of switching to mA (the actual reading was 65mA), I was reading it set to uA. At first it kept giving me some wierd numbers, but now it just stays at 00.00. Is there a way to pull the fuse and test it? Thanks!
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Offline ahnuts72

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Re: Help with new DMM
« Reply #55 on: July 10, 2013, 04:56:46 am »
Remove the back cover have to remove the fuses check for continuity.
Watch out when u remove the back the extra fuses are at the top of the meter and will fall out.

They are hrc in mine if I remember so can't tell if blown visually.

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Offline IanB

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Re: Help with new DMM
« Reply #56 on: July 10, 2013, 05:01:11 am »
Gosh, you're clumsy  :(

But yes, you can open up the meter and see the two fuses inside it. You can pull them and test with the meter on the continuity setting. Luckily for you there are spare fuses included inside the case. Don't blow the other one  ;)

A good safety rule is always to measure current on the 10 A range (using the separate 10 A socket). Only switch to mA if you are sure you will be within the maximum current limit of that range.
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Offline c4757p

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Re: Help with new DMM
« Reply #57 on: July 10, 2013, 05:13:25 am »
Luckily for you there are spare fuses included inside the case. Don't blow the other one  ;)

I've blown DMM fuses exactly twice: once when I was ten years old and dicking around, I thought I'd measure the mains current. I distinctly remember intending to do that - not sure what ten-year-old me thought would happen, but what did happen was that I came very, very close to pissing myself. That was when I learned to double-check around mains.

(A sub-lesson about cheapass DMMs: the breaker tripped when I did that. This means that the circuit breaker, inherently a time-delay device, and a 15A one at that, cut out before a multimeter supposedly protected by a 10A fast-blow fuse. This should not happen.)

The second time was more recently, when I had just bought my first "good" multimeter, with good fuses. (Not on mains this time.) My lesson was learned when I had to buy the replacement fuse. Not only are those damn sand-filled fuses pretty expensive as it is, but they used an uncommon type in that meter. And I was broke as hell, being a student and having just spent a large chunk of my money on that meter.

Every time I switch a multimeter onto the current range I think of experience #2. :-DD

Moral of the story: don't blow the second...
« Last Edit: July 10, 2013, 05:16:39 am by c4757p »
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Offline zorthgo

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Re: Help with new DMM
« Reply #58 on: July 10, 2013, 05:18:31 am »
Thanks! I replaced the fuse and it is working fine... And to my defense, I am mainly a programmer (not counting the theoretical shit that I learned in school for CE). Working with these things on your own is way different than following the labs or solving the problems the teacher gives you in class. For example, I bought this RGB LED at radio shack, and I've spent the last 3 hours trying to get it to work, and the only thing I've accomplished was to blow my fuse! Pun not intended! ;)
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Offline mtdoc

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Re: Help with new DMM
« Reply #59 on: July 10, 2013, 05:25:17 am »
I've blown DMM fuses exactly twice: once when I was ten years old and dicking around, I thought I'd measure the mains current. I distinctly remember intending to do that - not sure what ten-year-old me thought would happen, but what did happen was that I came very, very close to pissing myself.

When my oldest son was about 4 and dicking around, he thought he'd measure the mains current with a bent fork.  I'm not sure what he thought would happen but he did in fact piss himself.  :palm:          Thank goodness for the breaker. :phew:

He's 7 now and whenever he is curious and starts poking around my lab equipment all I have to say is " remember what happened last time you .....?    He immediately drops whatever he's picked up and takes a step back. :clap:
 

Offline c4757p

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Re: Help with new DMM
« Reply #60 on: July 10, 2013, 05:26:50 am »
Don't feel too bad, there are people in my EE class at school right now who probably couldn't figure out how to light an RGB LED. You'll get it eventually.

Does it look like this?



Connect the longest lead to the negative terminal of a 9V battery (or a variable power supply set to at least 5V), then connect the positive terminal to a resistor between 200 and 2000 ohms. Then when you connect the loose end of the resistor to one of the loose leads of the LED, it will light up.

If the leads are all the same length, compare your LED to that picture (especially look at the size of the metal elements inside the dome) and figure out which one matches.

When you connect multiple colors at once, they'll each need their own resistor.
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Offline zorthgo

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Re: Help with new DMM
« Reply #61 on: July 10, 2013, 05:27:23 am »
@mtdoc, that is so funny! I did the same thing when I was a little kid. The only difference is that I gave the fork to my little sister and told her to stick it in the socket. Good thing no one got hurt! lol!
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Offline zorthgo

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Re: Help with new DMM
« Reply #62 on: July 10, 2013, 05:35:17 am »
@c4757p, Are you sure? I get it to light up (but only red), by connecting the positive voltage to the longest leg (the anode), and the other 3 legs to ground. I would think that as a diode, if you plug it the wrong way it doesn't work.

The problem that I am having right now, is that I can't get the other colors to light up. The voltage at the LED only increase to a certain voltage (roughly 2.8V), after that only the amps increase. I set one of my resistors on fire by increasing the power supply voltage to high, when attempting to get the voltage past the resistor going into the LED past 2.8V. Driving me crazy!  |O
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Offline c4757p

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Re: Help with new DMM
« Reply #63 on: July 10, 2013, 05:35:29 am »
He's 7 now and whenever he is curious and starts poking around my lab equipment all I have to say is " remember what happened last time you .....?    He immediately drops whatever he's picked up and takes a step back. :clap:

I've always been completely unteachable with things like that... To this day, I still routinely burn myself on the soldering iron, cut myself on knives, and occasionally zap myself on HV-ish circuits. (Interestingly, all three feel exactly the same when they catch you off guard.) I am, however, an insufferable cheap ass, so I do learn from experiences that make me spend money. :P

Considering stupid things we do when we're kids, I've also burned my lip on a light bulb and my knuckle on a toaster oven (still have the scar on my knuckle), shocked myself on a light socket, shocked myself on a half exposed plug, shocked myself on a damaged VCR that my dad warned me not to touch.... on and on...

And now that I've grown up and I actually understand what was going on, I can remember even more situations that were extremely close, in some cases to more than a little zap. I was one reckless child...

And after all that, my parents still let me work on electronics. Possibly because I was too embarrassed by my idiocy to ever tell them what happened... :-[ :P
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Offline c4757p

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Re: Help with new DMM
« Reply #64 on: July 10, 2013, 05:38:47 am »
@c4757p, Are you sure? I get it to light up (but only red), by connecting the positive voltage to the longest leg (the anode), and the other 3 legs to ground. I would think that as a diode, if you plug it the wrong way it doesn't work.

It could be backwards from that.

Quote
The problem that I am having right now, is that I can't get the other colors to light up. The voltage at the LED only increase to a certain voltage (roughly 2.8V), after that only the amps increase. I set one of my resistors on fire by increasing the power supply voltage to high, when attempting to get the voltage past the resistor going into the LED past 2.8V. Driving me crazy!  |O

They smell good, don't they? :P I just set one on fire too, so you're among friends. Are you connecting all three leads together? Like I said, they'll need their own resistors. If you connect them together directly, only the LED with the lowest voltage threshold (red) will light, because it'll keep the voltage from going above that to the next LED's threshold.

Also - LEDs are not happy with overcurrent. There's a possibility that you blew it.
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Offline zorthgo

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Re: Help with new DMM
« Reply #65 on: July 10, 2013, 05:43:20 am »
Are you telling me I might have killed my LED??? So that makes An LED, a fuse, and a resistor. All in under 5 hours? I must be setting a record. lol!  :-DD
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Offline c4757p

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Re: Help with new DMM
« Reply #66 on: July 10, 2013, 05:49:47 am »
When you're still trying to figure out the LED, stick to a 1k resistor and nine volts. That will definitely be enough (both voltage and current) to light it, and never be enough to kill it (or the resistor). There's no reason to drive it harder to get it to turn on, only do that once you have it working and it's not bright enough.

When you finally want to choose resistor values, measure the voltage across the LED when it's lit (separately for each color). The current it takes will be (Vsupply - VLED)/R - don't let it go above 10mA or 15mA or so (even better, the datasheet maximum, if you have a datasheet). Then just choose by trial and error. You'll find you need different values per color for even brightness.

Some multimeters can measure that voltage VLED directly on diode mode, but some don't go high enough to measure LEDs (especially blue and white ones).
« Last Edit: July 10, 2013, 05:55:07 am by c4757p »
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Offline zorthgo

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Re: Help with new DMM
« Reply #67 on: July 10, 2013, 06:08:53 am »
Thanks I got it working. YAY! I replaced the single resistor between the power supply and the cathode, with 3 separate resistors between the 3 anode and ground. By varying the resistor values I can vary the light color. I guess, that if I can replace the 3 resistors with 3 potentiometers I can vary the color on the fly! Is there any other way besides with a potentiometer? I would guess I could use transistors to turn the different legs on and off. Oh well, at least I can go to sleep. That LED was going to keep me awake all night if you hadn't helped me get it to work. THANKS! ;)    :-+
« Last Edit: July 10, 2013, 12:10:48 pm by zorthgo »
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Offline AndrejaKo

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Re: Help with new DMM
« Reply #68 on: July 10, 2013, 09:26:26 am »
Don't feel bad about burning (relatively cheap) components! You at least managed to set a resistor on fire.

Last time I tried to do that, the damn thing was still within 5% of resistance value after being cooked at 25 V for 30 minutes and it was a 0.25 W 100 ohm 1% resistor! At least I managed to get it out of spec.
 

Offline zorthgo

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Re: Help with new DMM
« Reply #69 on: July 10, 2013, 12:44:52 pm »
@AndrejaKo: Thanks! I am just chucking it up to growing pains. lol!

@Everybody: By what I've heard so far from several people, apparently I shouldn't stick my multimeter in the wall socket to measure the "main" current. But why? If my circuit breaker is 20A and the multimeter is rated at 20A, in my mind the worst that could happen would be to trip the circuit breaker. Am I wrong?

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Offline AndrejaKo

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Re: Help with new DMM
« Reply #70 on: July 10, 2013, 02:34:15 pm »
This is one of the things that come up a lot for no good reason.

First, think what would you gain by measuring the mains current in the described manner? Assuming that you actually get a measurement (and that's a big assumption), all you'd get would be the short-circuit current of that single plug, which isn't going to be very helpful.


Now time to scare you straight!

First, let's think a bit about realistic system: Your multimeter is rated for 20 A, but under what conditions? Did you completely read the manual? Did you fully understand the specifications there? are you 100% sure that you did, if the answer to previous question is yes? Would you bet your life and lives of people around you on that?

Let's take a look at what happens when you're measuring current: First, your multimeter is definitely using some sort of resistive element and when measuring current is actually measuring voltage across that element. That may be a precise resistor or it could be even just a metal bar of (more or less) known resistance.

When I stick one my meter's probes into amperes socket of my other meter (also rated for 20 A), I get around 0.5 ohms as resistance. Do that experiment yourself! Now let's assume for a second that the reading is correct and that resistance of my probes and my meter's socket is zero. If I were in the US and I connected that meter to mains socket to "measure the current", in world of ideal voltage sources, I'd have say 220 A flowing through that meter. That's 11 times the current it's rated for.  If the probes had resistance of say 0.01 ohm, then they'd be dissipating around 485 W of heat just from that connection. The meter would be dissipating around 24 kW! You can of course see that in such conditions it would momentarily melt.

On the other hand, you have circuit-breakers and hopefully fuses in the meter. That's all fine and nice, but!! the circuit-breakers have minimal time it takes them to respond and that time might be too long for your hands or whatever you're using to connect the probes. Next, we have the fuse in the meter. If that fuse is safe (and that's a big, great, huge and extremely dangerous assumption), it should burn out very quickly and open the circuit, hopefully before the rest of the meter vaporizes. Those fuses have maximum breaking current and maximum breaking voltage. You need to be sure that those numbers are realistic and that they will be sufficient for your case. There's a reason why you can buy a whole multimeter (or maybe even more than one) just for a price of a Fluke fuse.

Next, you have all the imperfections along the way. I won't go too much into them, there are other members of this forum who are much more capable of providing relevant information. I'll just mention that in this case, you can expect lots o sparking when making connection, you can expect the multimeter fuse to explode, maybe even eat bits of the PCB with it. Good quality multimeters will often have some sort of blast walls inside, especially around fuses that are meant to prevent the explosion from escaping the meter's enclosure. They will also have specially designed seams, so as to reduce the possibility of explosion getting outside of the meter.


And keep in mind: The time it takes for the circuit breaker to react is short, but the time it takes for the meter to turn you into a cripple is even shorter.  And yes, I've heard of a guy who was scratching his head trying to understand why appliances work from a plug whose current reads as zero. You really don't want to try to find out if you're as lucky as he was.
 

Offline zorthgo

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Re: Help with new DMM
« Reply #71 on: July 10, 2013, 04:35:46 pm »
Thanks for the explanation AndrejaKo! You painted a very understandable picture. It answered my question. I guess I won't go around sticking my probes into any sockets! ;)
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Offline olsenn

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Re: Help with new DMM
« Reply #72 on: July 10, 2013, 06:00:46 pm »
I would recommend the Rigol DM3058E. The only difference between the 'E' model and the regular DM3058 (apart from the price) is that the DM3058E lacks some of the connectivity options like LAN and GBIB (but USB is still present).

Don't listen to people suggesting to buy a handheld meter. If you are an engineer and not a technician or a repair man, then you don't need portability. Bench meters are MUCH more precise and accurate, offer more features, and you can leave them on as much as you'd like without  worrying about batteries dying. They support 4-wire resistance measurements and in many cases offer data-logging and remote connectivity.

With the Agilent meters you are paying for the brand name; the Rigols are no worse spec wise (in fact they're often better), and they will last just as long (decades).
 

Offline c4757p

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Re: Help with new DMM
« Reply #73 on: July 10, 2013, 06:04:34 pm »
He will know when he needs a $450 multimeter.
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Offline olsenn

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Re: Help with new DMM
« Reply #74 on: July 10, 2013, 06:19:38 pm »
Quote
He will know when he needs a $450 multimeter.

And when he does know he needs one, he won't have one! A good engineer makes due with what he/she has to work with, but that's not an escuse to not acquire quality equipment to work with. If he is paying $10,000/year tuition for an EE program, then I think he is committed enough that a $450 lab purchase is reasonable. That being said, if you are just a kid who wants to muck around with blinking LEDs and that's it, stick with you cheap RadioShack POS
 


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