Arduino Forum

Using Arduino => General Electronics => Topic started by: talg89 on May 07, 2016, 08:39 pm

Title: Checking current with a multimeter
Post by: talg89 on May 07, 2016, 08:39 pm
Hello,

I have a question about the current measuring with a DMM.

I know that if I expect higher current than 200mA, I should use the 10AMPS plug otherwise, it might burn the multimeter, but would happen if I connect correctly to 10AMPS and choose in the dial the wrong range (200mA for example), it will only show "Over" or a damage might happen?

Thanks
Title: Re: Checking current with a multimeter
Post by: larryd on May 07, 2016, 09:04 pm
It will show an over range,but do you not have a 2 amp range?
Show us a good image of the DMM.
There should be an internal fuse inside the meter.

.
Title: Re: Checking current with a multimeter
Post by: talg89 on May 07, 2016, 09:23 pm
Its a uni-t ut136b.

What I meant to ask is if my only concern should be if I connect the wires to the right plugs?
range will always be only a matter of "over range"?

And another thing, what is the right order of connecting: first choose function, then connect the wires the the measured object or the opposite?

Thanks a lot.
Title: Re: Checking current with a multimeter
Post by: Paul__B on May 08, 2016, 12:58 am
The 10 Amp range terminals connect directly to a heavy-duty shunt, no fuse is involved, and no switching.  It would require considerably more than 10 Amps to burn it out.

This means that if you have your meter leads connected to the 10 Amp terminals, no other switch position will give any (meaningful) reading; it is as if you had nothing connected for those ranges.

More to the point, if you then connect the probes to any circuit, it will effectively be a dead short and may cause damage to the circuit.  I made such a blunder with my car interior light circuit - now I have to figure out how to repair the computer module.  And first, I have to find it! :smiley-roll:
Title: Re: Checking current with a multimeter
Post by: talg89 on May 08, 2016, 06:28 am
Oh, thanks and good luck with your car!
Title: Re: Checking current with a multimeter
Post by: weedpharma on May 08, 2016, 11:51 am
The 10 Amp range terminals connect directly to a heavy-duty shunt, no fuse is involved........

This is a bit of a generalisation. My Fluke has a 10A fuse (and they are expensive to buy!)

Weedpharma
Title: Re: Checking current with a multimeter
Post by: MarkT on May 08, 2016, 12:49 pm
Hello,

I have a question about the current measuring with a DMM.

I know that if I expect higher current than 200mA, I should use the 10AMPS plug otherwise, it might burn the multimeter, but would happen if I connect correctly to 10AMPS and choose in the dial the wrong range (200mA for example), it will only show "Over" or a damage might happen?

Thanks
No, the current is going through the high-current shunt which is a thick piece of wire, so the max voltage it
can generate is typically 100mV or 200mV which cannot hurt the rest of the circuitry.

Always leave a multimeter with the probes plugged into the voltage input and set to a voltage
reading or off, then you can't accidentally damage anything with it.
Title: Re: Checking current with a multimeter
Post by: Paul__B on May 08, 2016, 02:45 pm
Always leave a multimeter with the probes plugged into the voltage input and set to a voltage
reading or off, then you can't accidentally damage anything with it.
Yeah!  Exactly!

  In the case of a digital voltmeter, leave it off when not in use.

Yes, I know Flukes have an auto-off, but if you don't do it for a business you may not use a Fluke all the time.

Title: Re: Checking current with a multimeter
Post by: JohnLincoln on May 08, 2016, 04:06 pm
Some multimeters can even detect which input you have got the leads plugged into, and give you a warning if you have selected a current range and the leads are in the voltage position and vice versa
Title: Re: Checking current with a multimeter
Post by: MorganS on May 08, 2016, 06:36 pm
This is a bit of a generalisation. My Fluke has a 10A fuse (and they are expensive to buy!)

Weedpharma
And difficult to find (small metric size, strange sand-filled construction) and sometimes even difficult to install in the meter. I've blown many current fuses in many meters myself. I keep a stock of fuses for my current current meter.
Title: Re: Checking current with a multimeter
Post by: talg89 on May 08, 2016, 06:37 pm
I am sorry guys, but I lost you a bit :)

What I am trying to figure out is if connecting the wires to the correct places (10AMPS PLUG+correct setup with the circuit) = no harm to the device/fuse, no matter if I choose mA and I actually checking 6Amps for example.

Plus, If I choose in the dial - Amps measure, and my cable is actually connected to the volts plug, would it be unacative or might damage the fuse/DMM (same for the opposite case of choosing volts in the dial but connecting the cable to the amps plug)
otherwise I dont get the point of seperating the amps connection and the mV connection in my DMM

Thanks for the answers.
Title: Re: Checking current with a multimeter
Post by: MorganS on May 08, 2016, 06:41 pm
You can damage the meter (more likely the fuse) if you connect the leads to the 10A sockets and try to measure a battery voltage.

No other combination will damage the meter.
Title: Re: Checking current with a multimeter
Post by: MarkT on May 08, 2016, 07:20 pm
Actually you will fry the leads.  There is no fuse on the 10A sockets, and the shunt is a thick piece of
wire, thicker than the leads.
Title: Re: Checking current with a multimeter
Post by: talg89 on May 08, 2016, 07:29 pm
Wouldn't it make more sense if the designers would make the Volts plug unactive in case of choosing amps in the dial (or make the amps plug unactive in case of choosing volts in the dial)?
Title: Re: Checking current with a multimeter
Post by: MarkT on May 08, 2016, 07:37 pm
Wouldn't it make more sense if the designers would make the Volts plug unactive in case of choosing amps in the dial (or make the amps plug unactive in case of choosing volts in the dial)?

But that would mean having to have two sets of precision resistor divider chains, rather than one.
Title: Re: Checking current with a multimeter
Post by: talg89 on May 08, 2016, 07:49 pm
I see.

So the Low impedance/High impedance modes are set by the dial or by the connection (10amps,mA,Volt)?
Title: Re: Checking current with a multimeter
Post by: Paul__B on May 09, 2016, 12:09 am
Actually you will fry the leads.  There is no fuse on the 10A sockets, and the shunt is a thick piece of
wire, thicker than the leads.
I said that, but was howled down.  See reply #5. :smiley-roll:

But that would mean having to have two sets of precision resistor divider chains, rather than one.
More to the point, it would imply using the selector switch to interrupt the 10 Amp shunt.

Ain't gonna happen!  :smiley-eek:
Title: Re: Checking current with a multimeter
Post by: weedpharma on May 09, 2016, 12:26 pm
And I think backed up by #9 re fuses in 10A circuit.

Good quality meters do have fuses. $5 specials from China probably not.

Weedpharma