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electronic/electrical
by Roy O on 08/22/02 at 21:35:23

I just bought a multimeter and have been playing around with it.  It has lead me into a pool of questions.

1. Why does it say 10A maximum?  The standard household outlet has 15.  So does this mean It will blow If I check It anyway? because then this thing is almost useless.

2.I checked the amperage of a AA battery, expecting something in the millamps I got a reading of 7 amps.  Thats half the amps of a AC outlet,  How is this possible?

3. I'm courious to know the amp rating for the average 12volt car battery?  I cant check since my multimeter says 10A max, and so obviously I'm assuming It would blow if I do check.


Re: electronic/electrical
by Robert Fogt on 08/23/02 at 11:53:39

You probably don't want to use your multimeter without a load. They are designed to measure current of a circuit, so they themselves have very little resistance.

Current is Volts divided by resistance.
I = V/R

So when you check the Current (I) using only the multimeter that has very little Resistance (R), you're going to have very high current (I), maybe even blow the fuse of your multimeter.

Though, it has been 12 years since I used a multimeter, so things may have changed since then. :)

P.S.
I seem to remember my multimeter had different plugs or settings for DC amps versus AC amps. Make sure you are using DC setting for batteries and AC setting for household circuits.


Re: electronic/electrical
by Noel on 08/26/02 at 23:17:14

   The '10 amp max.' describes the maximum load the meter is fused to measure. That is, if greater than 10amps is present on the circuit neing measured, the internal fuse will blow to protect the meter - and the person using the meter! Every circuit has a maximum measurement for which it is designed to safely operate. In the house, the circuits are protected by fuses or breakers to directly prevent damage to the wiring in the circuit, and indirectly protect the equipment served by the circuit. So, not every meter is meant to be used for measuring every circuit.
    For your purposes, a clamp-on ammeter - one that actually goes OVER the hot wire - would better suit your purposes. It can accurately measure the amp load without having to put the meter in series with the load, as the new meter you bought would require. That means that you are exposed to potentially lethal electrical shocks to measure the amp load using your new meter, while the clamp-on meter can measure the load directly without being exposed to the voltage.
   Scond questions answer is easier. You were trying to measure voltage in the amp scale. Two totally different measurements and two different jack on the meter, also.  The jack labeled '10 AMP MAX.' is used ONLY for amp load measurements. There is another jack on the oppositeside of the face of the meter usually marked "V+-' has an omega sign (looks like a horseshoe) is marked in a red color. This is where the red lead goes to measure voltage or resistance ohms). For both amp load and voltage/ohm measurements the black lead is always plugged into the jack marked black and labeled 'COM".
    Once the leads are in the proper jacks, then the selector switch on the front of the meter determines what you are measuring (voltage or resistance).
    Question #3 is simple, to The battery in your car is rated for 12vdc (volts dc). With the leads in the proper jacks and the meter on the proper voltage settings (AC or DC), you should see 12vdc minimum with the car not running up to approx. 13.5 vdc with the engine running.

   The meter you have is not intended to measure amp loads of 'high draw' circuits such as the starter
 or headlights.  A starter can easily draw 250 amps in warm weather, 600Adc - 800Adc in very cold weather.  There are meters that can. I have one because I am an electrician. It was approx.  $150 10 years ago and has the ability to read up to 1000A ac or dc. It clamps around the wire being tested rather than having to be put in series with the load.
    I'm not trying to crack wise on ya here, but this is an excellent example of why one needs to know what they are buying before laying out the cash. In this case, a clamp-on ammeter that ca read resistance, ac and dc voltages and ac amps can be purchase at a home improvement center for under $50. The meter you have would price between $ 10 and $35 depending on the size and ranges it is capable of measuring.
    This is also the time to get the instructions out and go over how the different measurements are made until you can do it in your sleep. You are in danger of hurting yourself and/or damaging the circuits and equipment connected to the circuit.
    Someone might say 'This is just another tradesman trying to protect his business'.  In the smallest possible way, that may be true. MY concern is with the safety and continued good health of people not familiar with electrical work.


Re: electronic/electrical
by Roy O on 08/29/02 at 02:31:12

Yes okay this was a cheaper multimeter, ($20) but when I checked the AA battery for amperage I had the leads in the proper dc amp jacks, I tried it again and I get a reading of 6 amps.  It is in the right function, and when I try it in the milliamps section I get 1 meaning its to high (also in the right millamps secton and milliamps jack).  So I try it in the ten amps and 6 is what I get.  Explain this to me.

An ammeter as you explained seems to make more sense for bigger loads, because I'm noticing that even when I check little 6 volt battery packs for amps the wires get hot. When I check for voltage it doesn't get hot which makes sense why this multimeter can measure up to 1000 VDC and only 10ADC.

I know that a 12 volt car battery is twelve volts, but what I want to know is how many amps the battery puts out.  My question is, if you took an ammeter and checked the batter for amperage, while the car was turned off, what would the reading be about.
         I'm curious about this because I want to make the comparison of an AC outlet in watts (I believe about 1800 watts) to a 12 volt DC car battery in watts.  I know that if the watts were about equall that DC is still more powerfull since it does not go up and down like AC.  Also for a 12V car battery to be equal in watts to an AC outlet that means that
1800 divided by 12 = 150amps.
But it has to be more then if there are starters that take up to 800 cold cranking amps


Re: electronic/electrical
by Niall Saunders on 09/20/02 at 06:52:39

:o

Roy -  S T O P  !!!!!

Please,

Before you KILL yourself - I am NOT joking (even though you MAY be)

You CANNOT 'measure the amperage' of a battery. A battery, or ANY other supply, including the domestic mains supply  does NOT HAVE A MEASURABLE 'AMPERAGE'.

Instead, any supply will have a value of current which it is 'rated to supply SAFELY'.

DO NOT TRY THE FOLLOWING :
Buy a clamp-on current meter and a 0.5 inch diameter length of steel rod. Put the meter around the rod, and drop this across the terminals of a brand new and fully charged vehicle battery. Just before the battery explodes (and it will !!) you might be able to observe it generating a short-circuit current of around 1000 (one-thousand !!!) amps. The rod will probably glow red- or white-hot, and will certainly weld itself to the terminals such that you would be unlikely to be able to remove it - assuming you were crazy enough to go anywhere near it again.

DO NOT TRY THE FOLLOWING EITHER:
Take your cheap multimeter, and repeat the above experiment, this time replacing the steel rod with the leads connected to your meter. IF YOU ARE LUCKY, the internal 10A fuse will blow. If not, then SOMETHING else in the meter will try to pass the same 1000A current as was generated in the first experiment. Go figure !!!

Basically you should ONLY EVER USE YOUR METER TO MEASURE CURRENT WHEN THERE IS A 'NORMAL' LOAD IN THE CIRCUIT.

An example might be if you wanted to measure the current darwn by the headlights of your vehicle. You could connect the leads in place of the feed wire to the headlights (assuming you know what you are doing). MEsuring the current drawn by a single 65W quartz-halogen headlight bulb, connected to a typical 12V car battery, should give you a current reading of around 5 AMPS. Do NOT do this for too long - the leads of your meter, and the internal connections will NOT withstand this level of current for very long !!!

When you were trying to measure the 'amperage' of the small 'torch' battery, you were effectively performing the 'second' of the two experiments I described above. You are VERY LUCKY to have survived without a major, if not fatal, injury. I have PERSONALLY observed a standard 1.5V alkaline 'D' cell explode like a hand-grenade when it was being abused. Fortunately the container in which the battery was mounted absorbed the shrapnel that was created by the explosion.

Finally, I would personally advise you to only ever connect standard domestic apparatus to your household mains outlet sockets - with your limited knowledge of things electrical, you stand a very high chance of becoming a candidate for next years Darwinian Awards for a major contribution to the gene pool of the human species !!!

Remember the simple adage :
"Volts jolt, but mils kill"

In other words, a reasonably high voltage (and 24V is enough) will cause an electrical shock, that WILL hurt. However, if you arrange for the current that flows through your body to become high enough (and the 'mils' refers to around 50 MILLI-amps , i.e. 0.05, or 50 THOUSANDTHS of an amp) YOU WILL DIE - no ifs or buts.

Normally your natural body resitance is quite high, requiring a higher voltage to cause a higher, and therefore more dangerous, current to flow. Add normal domestic mains to a hot and sweaty body, and the reduced resistance caused by the salty sweat will guarantee a slow and painful death (unless you are lucky, where death will perhaps be swift and pain-free)

YOU HAVE BEEN WARNED


Re: electronic/electrical
by Roy O on 10/28/02 at 17:05:52

Thats why I asked because I sure as hell wasn't going to buy an ammeter and try that out.  Even from previous experiences in auto mech. I saw what happened when a wrench or other tool touched both terminals by accident for just a second.  From my understanding of what you said a battery has almost an unlimeted amount of amps.
I am still unclear about it though. but  ohwell,  It obviously takes many years of electrical education to have a solid and strong understanding of all this.  And I for one quit here because I lost patients for this at this point.


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