Some questions on my new multimeter

[Anonymous]

Actually, it's a lot more complex if you start trying to explain internal resistance of a transformer or battery. I always simply considered "internal resistance" as a construct to explain that batteries and transformers only have so much capacity.

The important thing for the non-electronic types to take away from this discussion is that you can't measure a circuit without affecting it, and a high amp circuit is more affected by in-line resistance than a low amp one.

[webenda]

Skyray,

You have the right idea. The measurement is a little more complex than in your example.

Actually, the current shunt in my meter is 0.01 Ohms. The 15 VAC from the transformer drops to 0.7 VAC when the ammeter is connected. This means that the transformer has 1.1 ohms internal impedance. The meter leads have about 0.04 ohms resistance. So the total resistance is 1.1 + 0.04 + 0.01 = 1.15 ohms. Plugging this into the formula I = V/R = 15/1.15 = 13 Amps.

Wayne

[Anonymous]

I went back and reread the thread, and it looks like the 13+ amps was using the Multi-meter to dead short the terminals of the transformer. What you get then is the maximum current capability of the transformer into whatever resistance the meter has. If I am correct in my assumption, it looks like his meter uses the same circuit I described to measure high amperage. Fifteen volts at 13 amps is pretty close to one ohm. Many meters use a shunt circuit to measure amps--the electricity runs through a high current capable conductor like a copper bar, and the meter measures the voltage drop across the bar and reports it as current. Electricity is fascinating, isn't it?

[FJ and G]

thanks, Doug

[Anonymous]

shouldn't hurt train performance too much.

Assuming that you are running a layout like mine where I never see more than three amps and the voltage drop across the resistor would never be more than three volts. If you are running mega layouts like someone here was talking about 13 amps, you need to use a much lower value resistor like maybe one tenth of an ohm (that will keep the calculation easy, 1.3 v = 13 amps. Most transformers are capable of 20-25 volts, and if you lost 13 (13 amps = 13 volts @ 1 ohm) across the resistor, there wouldn't be enough left to run the railroad.

[Anonymous]

There's another place that tells you how to measure current. It comes with a warning to turn off the current before measuring it.

One of the major problems with outsourcing electronics equipment. Turn it off, and I can pretty much guarantee that the current will measure zero.

Something else they don't tell you is that to measure voltage, you use the leads from point to point, and it measures voltage drop across whatever load is between the points. To measure current, you have to break the circuit and attach the meter so that the whole current flow goes through the meter. Normally if the expected current is more than one amp, that means there is a special lead outlet that will measure (usually) up to ten amps. The clamp on amp meter is safer, and if they have one for fifteen dollars, grab it.

Not to get into a basic electronics class here, but if you have volts and ohms on your meter (most do) you can calculate current. You need a point in the circuit that has fixed resistance (locomotive resistances usually vary with motor function), then you measure the voltage drop across the resistance and calculate the current with E=IR where E is voltage, R is resistance, and I is current. A one ohm wirewound (needs to have high current capacity) from Radio Shack placed between the transformer and the layout will directly read out current as the voltage drop across the resistor, and shouldn't hurt train performance too much.

[FJ and G]

Wayne,

That is some great help.

BTW, I had to laugh reading the manual where it warned to always turn off the meter after use. In another section of the manual, it says that after 30 min of nonuse, it shuts off automatically.

There's another place that tells you how to measure current. It comes with a warning to turn off the current before measuring it.

[Anonymous]

Flukes(brand name) are some of the best multimeters around. I have a nice one I bought at a pawn shop for 70.00 dollars. It retails for $270.00 so I got a good deal. Any multimeter will work as long as it will measure at least 10amps or preferably 20 Amps. The problem with cheaper brands is that they are not as precise as expensive brands. That is for example if the true voltage is 12.524 the less expensive brand might read 12.5 +- .2 volts. The expensive meters also come with a certificate of calibration. Have fun with your multimeter; it is fine.

[webenda]

Hi FJ and G,

I do not have the 22-811, but I went to radioshack.com and looked at the manual.

You said, "The instructions are TERRIBLE..."
That is common. Writing manuals is difficult to do well.

The on-line manual has an error in the section titled, "MEASURING DC/AC CURRENT."

IS:
6. Apply power and read the current.
If the measurement is less than 400mA,
connect the red test lead to + 10A MAX,
and proceed to Step 7.

SHOULD BE:
6. Apply power and read the current.
If the measurement is less than 400mA,
proceed to Step 7.

You state, "I cannot offhand think of anything that would be over 10 amps."

I just applied my Radio Shack RS-232C to the terminals of my Lionel 1015, 45 Watt, 8-15 VAC transformer. The reading was 13.01 Amp AC. (Measuring transformer terminals with meter set-up for AC Amps measures the transformer's short circuit current - not a good idea. Don't try this yourself. Heed wrmcclellan's warning.)

My Lionel 5366 SD-9 measures .15-.22 Amp AC in neutral (just lights on) 1.20 Amps AC running light @ 8 VAC and 2.7 Amps AC stalled @ 15 VAC.

Ten amps should work fine for measuring current in individual items.

The on-line manual does not give limitations on how long you can measure ten amps. If the area around the current terminals gets hot, use good sense and limit how long you measure maximum amps. Warm is normal.

SPFan said, "Very few multimeters I am aware of can measure AC current." The on-line manual states that yours can.

Regarding fusing, the on-line manual states, "+ V.Ohm.mA is fuse-protected." Note that the ten amp range is not included in this statement. No fuse is good and bad news. Good if you measure the short circuit current capability of a 45 watt transformer, because there is no fuse to replace afterwards. Bad if you are not fast at reading the 30% over current and smoke the 10 amp current shunt.

Personally I believe you made a good choice.

Wayne

[FJ and G]

Gee,

I thought that by throwing money at the best top of line meter, it can do it all. Just goes to show that the most expensive gizmo can't always do the job. :-(

[ben10ben]

The clamp-on meter is on sale at Radio Shack right now for $14.99! Get 'em while they're hot!

[SPFan]

David,
Very few multimeters I am aware of can measure AC current. at all let alone 10 Amps. The Current range is generally limited to DC current. As Ben points out you can add a clamp-on accessory that will plug into your meter and give AC readings.


Pete

[ben10ben]

Dave,
I have the more expensive $90($70 when on sale) Radio Shack meter, and one of the precautions that it specifies in the instructions is not to measure in the 10A range for more than 30 seconds every 10 minutes. This really isn't much use for train applications, so I have never had an occasion to use this function of the meter. I am guessing that your meter probably has the same precaution, although it may very well not.

Rather, when I need to measure amp, I use my Radio Shack analog clamp-on meter. It's $20 normally, but can be had for $15 on sale pretty frequently. The clamp-on can measure up to 30 amps of AC, and, even better, doesn't require any sort of dismantling of the circuit to take a reading. All you need is to slip it over one wire, and you have your reading.

[FJ and G]

Roy,

Glad to hear it's a good unit. I didn't want to get a voltmeter today and later decide I need an amp meter.

How can you measure current when you are not in a voltage range. For example, if I want to measure the voltage on my layout, wouldn't I dial in AC?

I'll read my manual at least 3 or 4 times before doing anything. Heck there's 2 pages just on how to install the battery!

[wrmcclellan]

Dave,

1. 10 amps is typical limit for amperage on handheld multimeters.
2. The unit you have is autoranging which is a nice feature. This means you do not have to select 0-1, 1-10, 10-100 volt range - moving the dial each time you go out of the range you initially selected.
3. So the unit you purchased is the easiest to use.
4. Biggest issue is to NOT accidently measure current when in a voltage or ohm range. Usually the worst thing that happens is an internal protection fuse is blown which can be changed.

Regards,
Roy

[FJ and G]

$50 Radio Shack digital multimeter (no model # just Cat No. 22-811).

I'm wondering now whether or not should have bought it.

The instructions are TERRIBLE and it recommends buying a Radio Shack book. I'm just paranoid that I might somehow damage it by dialing up the wrong thing.

It measures up to 10 amps. Is that adequate for all toy train applications? I cannot offhand think of anything that would be over 10 amps.