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Old 01-06-2003, 01:15 PM
Drum God is offline
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Okay, I have a multimeter. Now what?


Maybe this is MPSIMS, but I thought I'd start here.

For Christmas, my father-in-law got me a multimeter from Radio Shack. It appears to be pretty nice, with a digital display. He got it for me because we were having trouble with a kitchen appliance and he wanted to check the continuity. He asked for my multimeter and I didn't have one. Now I do.

Anyway, I read the instruction book and am more confused than ever. It was, quite literally, Greek to me. I've never seen so many Greek letters since my days pledging Phi Mu Alpha (OAS, AAS. LLS, by the way).

What could I, the lay homeowner, do with a multimeter? About the most electrical work I do is changing a light switch, electrical outlet, or lamp socket. I already have a simple device to test if the line is hot (besides the tried-and-true touching the wire and swearing method). What jobs will this multimeter help me do better?

Thanks.
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Old 01-06-2003, 01:31 PM
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One of the best things is checking for shorts or to make sure a circuit can be completed. I had motion sensor floodlights that wouldn't work. I used the multimeter to step thorough the connections until I finally found out that the contacts for the halogen bulb were corroded. I polished them a bit and the light worked. Without the multi-meter I would have probably spend $40 on a new lamp.

You can also test batteries with the multimeter, it is more than just "good" or "bad" but actually tells how much charge is left in the batery.

You can also use it to test the polarity of your electrical outlets in your home.

The electrodes are also useful for body pierc..
uh, perhaps I've said too much.
  #3  
Old 01-06-2003, 01:31 PM
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It can help identify problems with electric lines where the voltage is too low for the touch-and-swear method, (car for instance), though I've found that method interferes with my hairstyle somewhat.

In my (admittedly limited) experience, a high-dollar state-of-the art multimeter is mainly useful to the hobbiest or engineer.
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Old 01-06-2003, 01:37 PM
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The fact that you ask the question suggests you may not get much use out of it. I use multimeters quite a bit on the job (auto repair), but very rarely at home.

Nevertheless, if you're so inclined, you can use it to test the function of switches, outlets, and sockets, perhaps avoiding "diagnosis by parts replacement." A simple and helpful thing would be testing the rest voltage of questionable batteries, to determine whether or not they're good.
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Old 01-06-2003, 01:44 PM
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You gotta change the old tried and true method of testing. Instead of touching it yourself, have someone else touch it for you. :P

It sounds more like your father-in-law bought you something he wanted for himself. Have him play with it too.

A multimeter checks 3 things. Voltage, Amperage and resistance. Voltage is how much "GAAAHHK" is on the line, Amperages is how much OOmmp is pushing that "GAAAHHK" and resistance is what stops the other 2.

Voltage is a "V" and amperage is an "A". Make sure you are not checking resistance on a hot wire because that will fry your little toy, to know that set the selector switch to V and find out.

When checking for voltage, set the selector to a high enuf setting to make sure you dont fry the meter. You dont want to test 120 volts at the 50V setting. Higher than what you expect is better for the life of the meter.

To check for continuity (or resistance) turn the selector to the highest setting of the resistance ot the symbol of a zigzaggy line. Touch the 2 leads to each other and the meter should say 0.000, that will indicate no resistance or continuity. Resistance is measured in ohms which uses the omega greek letter. then Check the thingy you wanted to test when you see youre meter is selected right and the leads work.

Use the meter to check fuses, light bulbs, wall switches, wall outlets (when you have no volunteers for the new method), check out batteries (no more 9V tongue actions), diagnose toys, wall adaptors, use them on science experiments, electronics hobbies, computer projects, or you can just practice your chopsticks techniques.

  #6  
Old 01-06-2003, 03:00 PM
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Another thing to remember with a meter is "voltage across, current through".

Read voltage across terminals or devices, from one side of the bulb/battery/whatever to the other.

Always read current (amperage) "through" a path. Open the path and insert the terminals so that the electrons in the wire flow in one terminal, through the DVM, and back out to where they were going before.

If you try to read current across, you'll fry the fuse.

I checked my household current with a DVM one time to find it almost 20% too high. A call to the electric company dropped it to only about 10% too high but that's as good as they would do for me. It did explain why I bought my light bulbs in bulk, though.

DVM's are rarely useful in a regular home environment but when you need one, they're invaluable.

BTW, I always liked the plumbing analogy for electricity:
Voltage=pressure
Amperage=flow rate
Resistance=blockage like a valve
Wattage=quantity/time (4 gal/hour)

-B
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Old 01-06-2003, 03:06 PM
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Quote:
Originally posted by Belrix

I checked my household current with a DVM one time to find it almost 20% too high. A call to the electric company dropped it to only about 10% too high but that's as good as they would do for me. It did explain why I bought my light bulbs in bulk, though.


BTW, I always liked the plumbing analogy for electricity:
Voltage=pressure
Amperage=flow rate
Resistance=blockage like a valve
Wattage=quantity/time (4 gal/hour)

-B
I believe you meant "household voltage", not current. The power company could not adjust your current.
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Old 01-06-2003, 03:09 PM
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Iím an EE. I (not surprisingly) use a DVM very often at work, yet rarely at home. But when I need it, I need it.
  #9  
Old 01-06-2003, 05:23 PM
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Quote:
Originally posted by whuckfistle
I believe you meant "household voltage", not current. The power company could not adjust your current.
Yup - you're right. I had something like 142 volts at my wall sockets. They told me there was a bad voltage regulator on my pole and they adjusted it down.

Apparently this small town used to run their lines pretty hot in areas to compensate for the voltage drop over distances.
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Old 01-06-2003, 08:10 PM
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I also keep a multimeter at home but I find it useful. As has been pointed out, they can be used to test batteries, check which side of an outlet is the hot one, check non-transparent fuses, etc. Of course, you don't need a multimeter for these things, but it is handy. I can think of two things that a multimeter is pretty much essential for: 1. Fixing electronic devices. But a multimeter isn't of much use if you don't know much about electronics. 2. Testing ac/dc converters. This is one use where a multimeter pays for itself. Say that you've got a printer that uses an ac/dc converter and it stops working and the lights go out. How can you tell if it is the converter or the printer? You can buy a new converter and see if that works but that can cost as much as a multimeter and if it is the printer then you're stuck with two useless converters and one dead printer. With a multimeter you can check the DC output in a minute.
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Old 01-06-2003, 08:48 PM
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Just remember, your house is AC, your car is DC.
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Old 01-06-2003, 09:35 PM
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You can also go through this intro to electronics at Siemans. If you like it, you can take classes at the local Junior College. If you don't like it, then you can try and return the meter to radioshack and get a battery charger and some AA batteries.

Here's the link to the intro...
well shoot, I can't farging find it now. I have it at work and I'll put it up tomorrow. But anyway, as Gary T said, you'd be better taking it back and getting something else.
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Old 01-07-2003, 01:02 PM
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Found it! It's so cool!

Check it out

Click on Basics of Electricity for an intro course to electronics.
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Old 01-09-2003, 02:53 AM
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Have a multimeter,now what---

Fool around with it. Try some of these:

Shuffle your feet across a carpet, touch something grounded, get a little zap.
Set the meter on Voltage (DC), do it again, hold one lead and touch the other to the grounded item, see how charged up you are. (If you see 10,000 volts, remember it's static electricity--if it was 10,000 volts AC or DC, the current would be deadly.)
Set the meter on current, do it again, hold one lead, ground the other, and watch the _momentary_ reading of current. Didn't take any time for your static charge to drain off, did it?

Check a power outlet for voltage. An outlet has a charged lead, a return lead, and many have a neutral. Set the meter for AC and test between each combination of two. Is it 110 V or somewhat different? Which is the hot lead? It's supposed to be at the smaller slot.

Note your meter leads can be handled so they don't touch each other while testing the outlet. If they do you get a short which will blow the fuse, heat the leads, and may do other damage. Avoiding this is not just a matter of controlling your hands, you may need to take environment into account, ie your younger brother may bump you at the wrong moment, so you don't give him the opportunity.

Use the DC to check batteries. Does a freshly dead battery have voltage? Does it supply any current worth mentioning? It may supply enough to give heat. I wouldn't try the current test on a good battery, since basically you'd short it out and there would be too much heat.

Set it to ohms and see how much resistance there is in metal, cantaloupe, etc. See how much resistance there is in one inch of dry skin vs. one inch of sweaty skin. You have just invented the lie detector. Try tap water vs. water with salt in it (the salt dissolves into ions with charges, so current can pass using the ions).

See what you see. And if it's the least bit interesting, get a book or a mentor and learn the fundamentals.
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Old 01-09-2003, 04:47 AM
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Quote:
Shuffle your feet across a carpet, touch something grounded, get a little zap.
Set the meter on Voltage (DC), do it again, hold one lead and touch the other to the grounded item, see how charged up you are. (If you see 10,000 volts, remember it's static electricity--if it was 10,000 volts AC or DC, the current would be deadly.)
Is this not likely damage a multimeter? The manual for mine advises against going more than +- 1000V from ground on either terminal. That said, modern multimeters might be better made than when I bought mine.
  #16  
Old 01-09-2003, 05:03 AM
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Quote:
Originally posted by Belrix
I checked my household [voltage] with a DVM one time to find it almost 20% too high. A call to the electric company dropped it to only about 10% too high but that's as good as they would do for me.
Heh; I tried this and they told me to tough it; apparently they only guarantee supply to 240V +/- 20% (UK)
Great.
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Old 01-09-2003, 07:31 AM
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Let's see ... what do I use a multimeter for ... Mine is auto-ranging so I just set it to AC Volts and it'll measure from 0.01 to 1000, but I'll pretend I have a manual one and mention the range. If yours is auto-ranging, ignore the range settings I mention; if it's not it'll have ranges in increments of 10x (i.e. 2V, 20V, 200V, etc.)

Ok, first, I'll see if I can summarize the volts, amps, and ohms scales on your meter in a couple sentences. Voltage is a difference in electrical potential, measured in volts. Current is a electrical flow from one level of eletrical potential to another, measured in amps. Resistance is the resistance to current, measured in ohms.

To measure voltage, you put one probe on a conductor and the other on a different conductor and measure the electrical potential difference between them. To measure current, the meter acts like a flow meter, so you need to open a closed circuit and use the meter to complete the circuit (i.e. to check the current through a light, turn off the switch then connect the two leads of the meter to the two terminals of the switch; the meter will complete the circuit and measure the current through the wire and the bulb.) To measure resistance, connect the meter to an unpowered circuit (the meter actually supplies its own voltage ... you can check it with another meter if you want) with one lead on one conductor and one lead on another ... this is the resistance to flow between those two points (lower values mean less resistance, so more flow.)

So here's some practical things to do:

1. Checking to see if a circuit is live--Set the meter on AC volts above your normal line voltage ... heck, just set it for the highest setting (which better be more than your household voltage) ... you're not looking for accuracy. The probes on the meter are long enough to reach into a household plug so you can measure the voltage there. If it's less than a couple volts the circuit is off, and if it's well above 90V, it's on. If you're testing a lightbulb socket, be careful not to short the power line together ... remember that touching the plastic handles of the probes is safe, so get a good grip on them.

(As a side note, after you've checked the circuit and are really sure it's dead, short the wires together with an insulated tool like a screwdriver. If huge sparks come off you've made an error, but now you know and you won't get electrocuted. If nothing happens, you can continue with confidence.)

2. Testing lightbulbs--Set the meter on Ohms (or the omega character) or set it to the "continuity" setting which will beep when the probes are shorted together. The range isn't too important, but remember that a 1 watt bulb will be about 100 ohms, so using a range above 1K ohm (1000 ohms) is wise; also, there might be some internal resistance above about 20K ohms so setting the meter below 100K ohm is also a good idea. Take the bulb out of the socket and put the probes on the terminals of the bulb. If it's good it will read less than about 100 ohms, but if not it will read more than 5K ohms, and most likely "infinity" (open circuit.)

3. Finding the dead bulb in strings of Christmas lights--Use the same range as for testing a lightbulb. Now, look at the plug--one wire coming out will go to the first bulb and the other will snake all the way to the other end to the last bulb. There might be a third wire that's used if the string has a plug at the other end. Connect one probe to the side of the plug that connects to the short wire to the first bulb. Pull out the middle bulb in the string and check continuity from there to the plug by shoving the probe into the bulb socket ... touching the side closest to other probe is what you want, but it doesn't matter if the probe touches the other side too. If it's good (low ohms) the broken bulb is in the latter half of the chain, so go halfway to the end of the chain. If it's bad (high ohms) the broken bulb is between the middle bulb and the start of the chain, so go halfway toward the plug. Pull the bulb and check there. Repeat the same thing, dividing the distance between the known-good bulbs and the unknown ones. You'll find the dead bulb in under log[2](number of bulbs) tries ... so for a string of 50 bulbs, that's a maximum of 6 tests.

4. Testing batteries--Use a low range of DC volts. Single-cell batteries like N, AAA, AA, C, and D are 1.5 volts (alkaline) or 1.2 volts (NiMH or NiCD.) 9-volt batteries are 9 volts. Most cars are 12 volts with very rare ones being 6 volts and some big trucks being 24 volts. Set the range for the battery (or batteries if you're testing more than one at once.) The voltage should be very close to the typical voltage (off by not more than 5% to 10% or so.) Even better is to test the batteries under load. If your meter has probes that are small enough, you can check the voltage on a battery in a piece of equipment while it's on ... this is the best bet because sometimes batteries will recover their voltage when not under load, so it's hard to tell when they're low.

Aside from that, put it in a drawer and take the batteries out because often they'll leak their goo after a couple years of sitting there.
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Old 01-09-2003, 09:01 AM
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Thanks everyone for suggestions. This has been helpful and I'll look forward to working through some of these things.

Thanks.
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Old 01-09-2003, 09:20 AM
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Quote:
Originally posted by MaryEFoo
Shuffle your feet across a carpet, touch something grounded, get a little zap.
Set the meter on Voltage (DC), do it again, hold one lead and touch the other to the grounded item, see how charged up you are. (If you see 10,000 volts, remember it's static electricity--if it was 10,000 volts AC or DC, the current would be deadly.)
Set the meter on current, do it again, hold one lead, ground the other, and watch the _momentary_ reading of current. Didn't take any time for your static charge to drain off, did it?
I would not do this.

First of all, a DVM is not capable of reading 10,000 V. Secondly, even if it were capable of it, it would not display 10,000 V. This is because the equivalent source impedance is much, much higher than the meterís input impedance, thus creating a voltage divider. Thirdly, DVMís utilize FET inputs. While there is usually some transient protection built in to the meter, itís never wise to subject FETís to static voltages.
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Old 01-09-2003, 09:38 AM
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Here's what I did when I got a multimeter:

Bought a mid-eighties Volvo 240. These have notoriously bad wiring harnesses, and by the late nineties, the insulation was worn in lots of places.

Then I used the multimeter all the time to diagnose electrical problems with my car. I got 54,000 more miles out of it, before I finally replaced the harness.

It's pretty much like the other respondents have said; there aren't a million uses for one, but when you need a multimeter, only a multimeter will do.

It's a very nice gift, actually!

- bjorn240
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