Multimeter Use Question

Usually you have to put the multimeter inline such that the current does flow through the meter. First you need to make sure that your meter can handle this current. Alternatively and possibly the better solution would be to use a clamp on multimeter. This type of multimeter doesn't have any electrical contact with what you are measuring, you simply clamp a ring around one of the wires you are measuring and the meter can determine the amperage from the flux around the wire. A clamp on meter looks like this.

Is that exactly?

I need it really exactly.

Thank you very much.

I think you should be able to get a fairly accurate reading with a clamp on meter, you need to check out the meter specs before you decide one one and see if it will give you what you want. As I said you can measure the current by having the power go through a multimeter that is designed for that. Just make sure the meter you have can handle that current, many multimeters 10amps is there limit. I don't know what voltage you are measuring or if it is AC or DC. It sort of sounds like you aren't experienced with electronics, and a clamp on meter is definitely a safer way to go.

I am not exactly sure what multimeter you have at you disposal.

If you have a clamp meter you can clamp the jaws around an insulated cable but special precaution are required if you are attempting this on exposed conductors or bus bars.

If you only have a standard voltage/current/resistance multimeter, you can only measure current up to the limit of that meter. In your case 10A. This involves breaking the circuit cable which you wish to measure and putting the meter in series with the open circuit connections.

To measure higher current you need a shunt which would normally be provided by your meter supplier who will have specified the ratio of the shunt.

Say it was a 10:1 shunt, then in theory your meter could measure 10A x 10 = 100A Max

If it was a 5:1 shunt then your meter could measure 10A x 5 = 50A Max.

Basically you multiply the exact reading on your meter by the ratio of the shunt.

Again the circuit has to be broken and the shunt inserted in series with the circuit, the meter connections must be firm terminations not probes, and must be connected to the metering terminals on the shunt which will have been calibrated by the supplier.

This has to be done with the circuit dead and the meter on the correct scale, unless it is very low voltage. The connections must be proper current carrying connections

This type of measurement is mainly a thing of the past with the introduction of the clip-on ammeter which is also more versatile and quicker, but maybe not so accurate for very small values. Clip on ammeters are mainly AC - if you require to measure DC you will need a "Hall effect" clip on ammeter.

Electricity is very dangerous and you should NOT poke around if you do not understand what you are doing! Get some one who knows to show you how.

Even if you get the right equipment you can still get hurt or even killed!

Do not take any risk with a powered circuit. Even low volts but high current is dangerous. Seek good advice.

I agree with BrainWave (BTW BrainWave, I like the new avatar). Current in the range you are speaking of is most certainly lethal! Interesting deadly current link.

To answer your accuracy question, checkout this meter. It has a 1mA precision and claims a 1.3% accuracy. Will that suffice? If not just search around or call an electronics supply store, they will probably be more helpful than I!

Now I wasn't going to post but I think a voice of reason needs to be heard here.

Yes current is dangerous, yes 10A can be fatal but then again so can 75mA (if the voltage is high enough to overcome the bodies internal resistance) or even a tablespoon of water (if you inhale it).

Lets get back to basics and start from the beginning. Now, do you know the voltage level (the voltage in volts) and type (ac or dc) of the current you trying to measure? If you don't know this, could you describe the application to us (we should be able to guess). A shunt is a useful tool for measuring voltage but it is also usually not safe to touch and so I would not advise using one if you are trying to measure a dangerous voltage, and I am guessing by your experience that you do not have access to a clamp-on ammeter.

Voltage levels below around 25V are fairly safe, but you still have to be careful of the heating effect or sparks created by a supply that can deliver in excess of 10A. If the supply is from batteries, then further discussion on safety is likely required.

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"You must learn to crawl before you can run the marathon"

A shunt is a ratio device. A 10 amp 50 millivolt shunt is one that will have a DC voltage drop of 50 MV when it passes 10 AMPS DC.

Now if the DC circuit is 1000 volts to ground, both of the 50 MV terminals will be 1000 volts above ground = lethal. If it is 24 Volts = less danger.

For AC measurements you use a current transformer.

google these

Ahh, I think this may confuse him/her. Even I don't get a chance to deal with 50MV shunts in my line of work (and I work with substations).

I definitely agree that 24Vdc is less dangerous than 1000Vdc thou.

A current transformer is certainly nice, but I can't help thinking this is a very simple dc current measurement application. From the original posts I am guessing that Rager may not even know what "Ohm's Law" is .

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"K.I.S.S - Keep It Simple, Stupid" Words to live by.

Please....write correctly the units!!!!! 50 MV means 50 Mega- Volts and it can be only seen in THE LIGHTNING for less than one ms (millisecond).

I have seen a few times that somebody meant 50 milli-volts which is rated voltage drop across the shunt used in electric labs (usually old ones).

My teacher said that this typo (MV instead mV) could represent a bad salesman or... do not want to repeat the word.

I have to agree with jack of all trades.

Issue 1:

Australian/New Zealand Standards define 50Vac or 120Vdc or lower as extra low voltage as these levels are NORMALLY fairly safe. Anything over this should be treated with extreme caution as fatalities can, and have, occurred even with extensively trained personnel. Sorry, you don't sound like you are extensively trained or you would already know the solution.

As for current levels over 10A, also treat with considerable caution. Remember that many electric welders work at extra low voltage levels but you can see what their effect is! (Blinding light and lots of HEAT)

Issue 2:

You haven't given enough information. Even some of the answers above have made some assumptions about what you are measuring. Most clamp meters will not measure DC currents, some will. If you are trying to measure current in a standard multicore cable, a clamp meter also won't work. They operate only when the clamp is placed around a single conductor. Otherwise there is an equal current flowing in both directions and you will read zero!

Give more information and this group should be able to assist.

I like the hair cut rather seventies!

Rager,

The "someone" was referring to the use of a shunt which is in fact a low ohmage precision resistor. The low resistance is so as not to interfere with the flow of current significantly, and they typically are made to produce a voltage drop of 50 mV at their full rated amperage. For instance a 100 Amp, 50 mV shunt would read 20 mV across itself (they typically have sense terminals for reading the voltage across them) when conducting 40 Amps. They are placed in series with the load, but are called shunts because the mV meter or other measuring device typically provides a parallel current path, albeit a very small one.

They are very accurate, but the downside is that they have to be physically installed in the circuit, whereas a clip on type ammeter does not.

Regards,

Greg

are u professional or graduated engineer?

If you need to ask how to do this -- you should not attempt it on your own. Get someone to show you and have them explain thoroughly what the potential hazards are. If you are in a distribution cabinet with high voltage you should have the proper protective clothing and equipment. It can be VERY deadly, or at least VERY painful.

Yes you could use a shunt (say 20amp 100mv which will show 60mv for 12 amps etc.)which is connected in series with the load and then measure the millivolts dropped across it with the voltage scale of your multimeter . That is what happens internally on the 10amp scale of your meter. Shunts are more expensive than they look. You don't say if you need AC or DC, shunts will measure both, just use the appropriate voltage scale. Beware the shunt terminals are all at the supply voltage.

With the price of instruments as low as they are today I would recommend using a tong tester (clamp meter) for about A$160 you can get a 1000amp AC / DC meter which also reads other useful parameters such as voltage, resistance, frequency and temperature (effectively a multimeter in the high current ammeter package). Using this method you will not need to disturb the circuit.

Cheapest Clamp-DMM (Digital MultiMeters) in USA may be found for less than 100 USD e.g. 69.99.

Even using indirect (non interrupting circuit and on one wire only) method of measurement AC current (via magnetic field) you must be very careful.

Measure DC current with clamp, check its ability to do this. It must work via Hall Effect.

Safety is First!

I don't know how exact you need it but most common multimeters are not that exact and do not measure above 10 amps. To use a shunt then the shunt needs to be measure or spec into the measurement. Shunts of the same rating can have a measurable amp difference. The clamp around amp probes work fine. When acquiring one look at the specs. Choose one that will give you an exceptable error. Now if you mean by exact you can find bench test equipment that will have shunts with them calibrated for the specific range you want to measure.

hope this helps

Without using current shunt all current must go through Your multimeter.

Check if Your multimeter has current measuring range appropriate to measued current. If You estimate Your current to be over 10A, and Your multimeter has 20A range, You can connect it. Multimeter usually has special, separate input for current measurement. It can be marked "20A" . Switch off Your ... what You want to measure. Take off power cord from wall outlet for any case. Disconnect cable currying this current You want to measure. Set range of Your multimeter to this 20A range. Connect one end (positive) to this special current entry. Connect another cable to multimeter input marked "common" or "COM". Check sureness of connectors.. For 10A they must be sure to avoid sparks. Check if there is no short circuit of cables to case or ground. Put ON Your multimeter, and see if it reads Ampers and near zero. Without touching cables, connect power cord to power line. Switch Your device ON. See current readings of Your multimeter. If it flashes, that means, measured current is higher, than Your multimeter can measure. In this case You will need to have mentioned current shunt. Switch off Your device, take off power cord, and disconnect Your multimeter. Set back original connection.

Then contact somebody more experienced

I think you forgot to mention that if multimeter flashes you may need a new fuse. If it smokes you may need a new multimeter. If it dissappears you may need a new workbench and medical attention.

Check if Your multimeter has current measuring range appropriate to measued current.

Doesn't this mean "check if estimated value of measured current is lower than maximum current measureable by multimeter? " May be. English is not my native language.

But I value very much Your comment. Writing my post I had similar thoughts in my mind, but I didn't wrote them down.

I'm not sure what they were trying to get at specifically by citing a mV value, but there are shunt resistors with low resistance that will give a nice voltage (V=IR) per amp, so if you had a .05Ohm resistor, it would give 50mV/A. You would then set your multimeter to a voltage scale and place the terminals across each side of the shunt resistor.

fyi, you may have to put a couple 5th Ohm (0.20ohm available at radio shack) resistors in parallel to achieve low enough resistance.

In any electrical stores you will find "Tong tester" OR "Clip-On-Meter"

Its tongs are like a outside caliper and you have to encircle the wire(in case of a single core) OR a single wire if its a cable of multi cores.

The instruction manual inside its carton will tell you how to check current, OR

even the shop keeper himself will explain you how to use it.

As others have said above, we need more information. Rager may only want to measure the current draw of a couple of 100 watt driving lights on his truck or he might be checking out the output of a 6kvA generator set. Different questions, different answers.

Depending on the application a shunt is an easy way to measure large (relatively) currents. For rough and ready results it doesn't even need to be calibrated. A lump of fencing wire can serve providing you know your ohms law!

If you only have multimeters capable of only measuring a max of 10 Amps, and you expect to need to measure 20 Amps, use 3 multimeters in parallel and add your readings (current divider). But only do that if you're DEADsperate. An alternative way would be to just run your circuit through P-Spice, get an Idea of how much current you're dealing with, figure out what level of precision you need, THEN find the right measuring device that can handle up to 2x of what you expect... that is unless you're DEADsperate.

I have never met anyone who doesn't know how to wire up a multimeter in series to measure a current, but is capable of modeling a system in P-Spice.

Then again, I have come across some particular University students that would rather jump at a simple problem and model it than actual fix it using common sense.

Yeah spice is complicated and needs copious amounts of brain power to use. Sorry to original poster if this is all to complicated. We had to do it the hard way.

I'd better not give advice (in school a friend got a multimeter for his birthday and I adviced to measure the mains resistance, how could I know that he would really do this)

Why do you need to ask us? Electricity is deadly and no-one should let you work on it when they are not sure that you are capable to do it.

When you need to do something for the first time, ask the one with advice to demonstrate it.

What you can do is ask a salesman from Fluke or so to find out on site what you need, and let him decide for the best measurement tool. They should know what they do. (This in case you find no-one that is capable)

Most DMM's only go up to 10A for measurement purposes some clamp meters go much higher and are also non intrusive therefore they are much safer as this is what kills current i mean and 10A would certainly more than do the job! keep safe and get a clamp meter.

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