Measuring Current Directly

[muashr]

Hi,

Is it safe (without causing any damage to the source or multimeter) to measure the current produced by a supply or current source directly using an ammeter connected across the source?

If not, why?

And how can it be done safely?

Note: Here, I want to know about DC current measurement. However, does there exist any difference when it comes to measuring AC current?

 

[audioguru]

If you connect a current meter across a power source then it is shorting the power source. The power source and/or the meter will be destroyed!
Instead, current is measured by connecting a current meter in series with the load. Same for measuring AC current.

Maybe you want to see how much current a power source can produce when it begins to smoke? Then you overload it with a huge resistor, measure the voltage across the resistor and use Ohm's Law to calculate the current. Not good.
A power source is supposed to have a maximum allowed continuous output current rating printed on it or in its datasheet. Look for it.

 

[KeepItSimpleStupid]

It makes a big difference if your trying to measure pA or 1000's of Amps and at what voltage and the error you can live with. Clamp on meters exist as well that don;t require breaking the circuit.

 

[crutschow]

There is no essential difference between the way the meter is connected to the circuit whether you are measuring AC or DC current (or voltage). In either case it must be in series with the load.

 

[KeepItSimpleStupid]

A clamp-on meter isn't quite series. It just needs to include only one wire.

 

[fezder]

If you connect a current meter across a power source then it is shorting the power source. The power source and/or the meter will be destroyed!
Instead, current is measured by connecting a current meter in series with the load. Same for measuring AC current.

Maybe you want to see how much current a power source can produce when it begins to smoke? Then you overload it with a huge resistor, measure the voltage across the resistor and use Ohm's Law to calculate the current. Not good.
A power source is supposed to have a maximum allowed continuous output current rating printed on it or in its datasheet. Look for it.

i remember when my friend almost measured mains voltage on current setting, and our meters didn't have fuse on 20A side!

 

[Ian Rogers]

i remember when my friend almost measured mains voltage on current setting, and our meters didn't have fuse on 20A side!

Past tense..... I see!!!... Have your new meters

 

[Paul_L]

To measure DC current with a DMM you will get negative amps on your reading. No big deal. If you do this with an analog meter it will damage your movement.

 

[audioguru]

I used a plug with a short circuit on it to "identify" circuit breakers in my home because they were not marked logically.
The only time I measured mains current was when I adjusted the pulley on the furnace blower so it blew maximum air flow at its rated current.

 

[Reloadron]

When measuring current the meter is placed in series with the load to measure the load current. When measuring voltage the meter is placed in parallel with the load. The cartoons look like this:

Also, for what is worth or not worth placing a current meter across a current source (power supply) will not, in every case, destroy the meter. If for example a meter having a 10 Amp current range is placed across a supply only capable of delivering 2 amps or <10 amps the meter will read only what the supply is limited to. If the supply (source) is capable of or limited to 2 Amps for example the meter should read about 2 Amps.

Ron