Measuring Current

[Badar]

I have asked something like that in my some previous post but i want to be cleared about one point.
I have a source (of power,Battery etc).If i connect the leads or probes or whatsoever of my DMM directly to the terminals of the battery i can measure Voltage it can provide.The battery is not of so much power.Can i set my DMM on 10A Fused or simply the ammeter directly (in parallel) to know the maximum current passing through it's Shunt (as i know that current will not be high enough to blow the shunt)

 

[ThermalRunaway]

If you connect the probes of your multimeter directly across the battery then yes you will measure current. What you'll actually measure is the short circuit current output of the battery, and I wouldn't advise this!!!

If you want to measure the current through a circuit then you definitely can't connect your meter in parallel, you'll need to break the circuit at a suitable point and connect it in series.

Brian

 

[Badar]

If you connect the probes of your multimeter directly across the battery then yes you will measure current. What you'll actually measure is the short circuit current output of the battery, and I wouldn't advise this!!!

Brian

Does this short circuit current means the maximum current that a battery can provide

 

[ThermalRunaway]

Yes. You acheive the maximum current of any source when you place a short circuit across it's output, which is effectively what you're doing (aside from a very small resistance) when you connect a meter in current-measuring-mode across it.

Generally, it is not adviseable to short circuit a source. They normally have maximum output limits which will be exceeded if you try to short circuit the output. Damage can result, and batteries can actually explode if you short circuit them so I wouldn't advise it at all.

Brian

 

[stevez]

Badar - it is my understanding that the current delivered by a battery has limits and the internal resistance is one of those limiting factors. If I wanted to estimate the internal resistance of a battery I'd make several current measurements at moderate currents and the change in voltage across the battery terminals reveals the internal resistance. Once you know the internal resistance you can estimate the short circuit current.

If you put your DMM (on 10 A) across the battery terminals and the battery is small enough you would see a current reading but it won't be the short circuit current - it will be the current flowing thru the shunt in the meter as well as the internal resistance of the battery. If the battery is large enough you'll either blow the fuse or ruin your meter - possibly injure yourself.

 

[Nigel Goodwin]

Does this short circuit current means the maximum current that a battery can provide

It's a tried and tested technique for testing NON-RECHARGEABLE small batteries, as it heavily loads (in fact overloads) the battery - a VERY brief short across an AA or AAA should result in a couple of amps for a good battery. The self-test on a Duracell battery does a fairly similar thing!.

You must NEVER do this on NiCd, NiMh or Lead acid batteries - their internal resistance is MUCH too low.

 

[bananasiong]

Is there any good way recommended for measuring the maximum output current of the circuit? For example, if I don't look at the datasheet, how should I test for the maximum output current of the PIC? Should I test with different load resistance, and reduce to get the max current?

Thanks

 

[Nigel Goodwin]

There's a BIG difference between maximum output current and maximum SAFE output current - there's no fixed method, you need to understand how each circuit works and what it's capabilities are - or test lots of examples to destruction?.

 

[Badar]

Is there any good way recommended for measuring the maximum output current of the circuit? For example, if I don't look at the datasheet, how should I test for the maximum output current of the PIC? Should I test with different load resistance, and reduce to get the max current?

Thanks

That is what i wanted to know.
Once i inserted the probes of DMM in my university lab in the outlet of the mains and meter was set on ammpers.The current it read was 110 Amp at 240 V.I don't know how much accurate it was but i still wonder that it was giving reading in amperes i.e. all this current was passing throgh shunt (almost no resistance) and yet it didn't blow the fuse or trip the circuit breaker(I am not sure what was installed).Anyway all the lights were still on after that.Any ideas or comments?

 

[Nigel Goodwin]

That is what i wanted to know.
Once i inserted the probes of DMM in my university lab in the outlet of the mains and meter was set on ammpers.The current it read was 110 Amp at 240 V.I don't know how much accurate it was but i still wonder that it was giving reading in amperes i.e. all this current was passing throgh shunt (almost no resistance) and yet it didn't blow the fuse or trip the circuit breaker(I am not sure what was installed).Anyway all the lights were still on after that.Any ideas or comments?

You obviously did something wrong, does the meter even read to 110A?, it would be a rare meter that did, and need some VERY thick leads. Also, I suspect you already know that you won't get 110A out of a mains socket!.

Either the meter is faulty, you had it connected or set wrong, or the meter/mains supply was damaged by your suicide attempt!.

 

[Sceadwian]

That's over 26kilowatts. There's no way that reading you got was correct. It takes wireing over a half inch thick to carry that much power without vaporizing.

 

[audioguru]

Don't short-circuit the output of a PIC. Its output transistor might vapourize becuse its max allowed current is only 25mA. It is high-speed Cmos which is able to provide about 60ma which is way higher than its max rating.

 

[gramo]

That is what i wanted to know.
Once i inserted the probes of DMM in my university lab in the outlet of the mains and meter was set on ammpers.The current it read was 110 Amp at 240 V.I don't know how much accurate it was but i still wonder that it was giving reading in amperes i.e. all this current was passing throgh shunt (almost no resistance) and yet it didn't blow the fuse or trip the circuit breaker(I am not sure what was installed).Anyway all the lights were still on after that.Any ideas or comments?

Your playing with fire, there's no "pause/restart" button in real life

 

[bananasiong]

Don't short-circuit the output of a PIC. Its output transistor might vapourize becuse its max allowed current is only 25mA. It is high-speed Cmos which is able to provide about 60ma which is way higher than its max rating.

For PIC, we can read frm the datasheet, but some type of circuit, the output current is not mentioned. For example, some battery charger circuit that I've seen. The output of the voltage regulator connected via a diode to the battery and a comparator to compare the voltage. If we don't know the output current, so we won't know how long does it take to charge the battery.

That is what i wanted to know.
Once i inserted the probes of DMM in my university lab in the outlet of the mains and meter was set on ammpers.The current it read was 110 Amp at 240 V.I don't know how much accurate it was but i still wonder that it was giving reading in amperes i.e. all this current was passing throgh shunt (almost no resistance) and yet it didn't blow the fuse or trip the circuit breaker(I am not sure what was installed).Anyway all the lights were still on after that.Any ideas or comments?

Maybe that is 110 V?

 

[TheVictim]

I'd like to see someone try that with a large pack of NiCads. They can dump an obscene amount of current if shorted. SOMETHING is going to explode...

 

[Badar]

I suspect too that meter was not accurate. Or the reason may be that i don't remeber whether i inserted the red probe on Ampere or at V , ohm but i am sure the meter was giving me reading in amperes as the dial was set on Ammeter.
And this was not my only suicise attempt.I remember that when i was less than 10 years old the main wires of 11 KV was too close to my roof to touch one of them easily.
I did that and after so much time that has left i still wonder why that wire didn't throw me on the road while i was bare footed and was fully grounded.

 

[Hardwire]

That's over 26kilowatts.

1.21 jigowatts!

 

[ThermalRunaway]

That's Gigawatts to you!!!

A quote from Back to the Future, I believe...

Brian

 

[ericgibbs]

thats 1.21 jigawatts for only 15 banana secs.