Quick question on Multi meters.

Thread Starter

Teljkon

Joined Jan 24, 2019
265
Hello,

I have multi meter fairly standard one its a commercial Electric brand multi meter. It seems to work fine. My question is fairly simple the amprage meter goes up to 200m amps and then I get OL on the screen. When I switch over to the 10A fuse I get nothing. My expectation would be that the 10A fuse would pick up where the the the 200mA fuse left off. model MMM-8301S.

Is this incorrect, or user error? Why is this not working the way one would expect?

Thanks in advance.
 

Ya’akov

Joined Jan 27, 2019
5,657
How are you switching? On that meter you need to move the red lead to the 10A jack and switch the meter range to 10A.

Also, how is the meter wired into the circuit under test?

Finally, check for a blown fuse.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
How are you switching? On that meter you need to move the red lead to the 10A jack and switch the meter range to 10A.

Also, how is the meter wired into the circuit under test?

Finally, check for a blown fuse.
I am switching right taking the red lead out of the 200ma jack putting it into the 10a one then moving the dial the the 10a spot. I will check for a fuse but I doubt that is it. I mean Ive never used this thing on more than 120v and I never measured amps with it.

Ok i poped it and a visual inspection of the fuse indicated no problems.
 

Ya’akov

Joined Jan 27, 2019
5,657
I am switching right taking the red lead out of the 200ma jack putting it into the 10a one then moving the dial the the 10a spot. I will check for a fuse but I doubt that is it. I mean Ive never used this thing on more than 120v and I never measured amps with it.

Ok i poped it and a visual inspection of the fuse indicated no problems.
How are you measuring the current? How is the meter in the circuit?
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
How are you measuring the current? How is the meter in the circuit?
Standard probes that come with the multi meter. I am sticking them into 14awg SAE connectors. Red probe red wire black probe black wire. Simple no.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
In series with the load?
Some times, the load is a battery, and Ill test at the clips. Some times there is no load and I am putting right in the connector. No matter where I test it nothing appears on the meter when its in the 10 amp slot.
 

djsfantasi

Joined Apr 11, 2010
8,323
If your probes are in parallel with the power source, you’re creating a near short. That is not how you measure current. The power leads to the circuit need to be broken on one side and the probes placed so the meter completes the circuit. This is what Yaakov is getting to.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
Ok, I don't know how to be more clear about this. I disconnect the 14AWG SAE connector and I put the probes into the connectors as per the color coding red in red black in black. If you need me to take a picture I can.

So yes the meter would be completing the circuit. I have mesured at the battery as you describe just touching it to the clips while the circuit is completed by the battery.
 

Ya’akov

Joined Jan 27, 2019
5,657
What do you think you are measuring when you do that? Do you understand how an ammeter works?

To measure voltage, we measure across a load, to measure current, we measure in series with the load. If you put your meter across the output of a power supply, you are effectively shorting it.

The way the meter does the measurement is to put a very low resistance shunt in series with the circuit under test. That shunt has a known resistance and by measuring the voltage across it the meter can provide a number using Ohm's Law. The voltage drop measured across the shunt will be proportional to the current passing through it.

That voltage drop is called the burden voltage and it is why shunts are kept to very low resistances—to avoid interfering with the circuit under test. It is also why there are fuses in the meter.

The numbers you get by putting an ammeter across a power source aren't really useful for anything. You need to have a load that allows that power source to act as it would actually powering something. You also need to double check your fuse, it might look OK, but hooking your DMM in 10A range directly to a battery is just the kind of thing to blow it.
 

Ya’akov

Joined Jan 27, 2019
5,657
Ok, I don't know how to be more clear about this. I disconnect the 14AWG SAE connector and I put the probes into the connectors as per the color coding red in red black in black. If you need me to take a picture I can.

So yes the meter would be completing the circuit. I have mesured at the battery as you describe just touching it to the clips while the circuit is completed by the battery.
To be clear, a battery is not a load, and there is no sensible reason to measure current across its terminals. Current measurements need to be made in the context of a load, that is something the power source will be powering. There is something called open terminal voltage which you can approximately measure with your DMM because in the V range it presents a very high resistance, almost no current flows.

There is no equivalent 'open terminal current". The ammeter might as well be like putting a screwdriver across the terminals. It tells you nothing useful.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
To be clear, a battery is not a load, and there is no sensible reason to measure current across its terminals. Current measurements need to be made in the context of a load, that is something the power source will be powering. There is something called open terminal voltage which you can approximately measure with your DMM because in the V range it presents a very high resistance, almost no current flows.

There is no equivalent 'open terminal current". The ammeter might as well be like putting a screwdriver across the terminals. It tells you nothing useful.
Ok so operator error is a factor. What I still don't understand is why when I leave the battery out of the circuit it still does not give me a reading on the 10a fuse.

If I am getting a reading that is OL on the DMM on the 200ma fuse and switching to the 10A fuse and I get nothing is it 100% a bad fuse?
 

Ya’akov

Joined Jan 27, 2019
5,657
Ok so operator error is a factor. What I still don't understand is why when I leave the battery out of the circuit it still does not give me a reading on the 10a fuse.

If I am getting a reading that is OL on the DMM on the 200ma fuse and switching to the 10A fuse and I get nothing is it 100% a bad fuse?
The meter could be defective, but it is usually the fuse. It's very common to make the mistake of level the probes in the amp jack and frying the fuse. Better meters even detect the mistake and alert the user.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
Ok, I see that. Makes sense basically the Load is also functioning as a resistor protecting the DMM from to many amps.

Annoying truth is I cant really remember doing A at any point in time. Would this also be true say with a solar panel. Also The OL seems to work really well on the 200ma fuse. I am fairly sure the fuse is a 400ma fuse on that one. So, why not just do a 20a fuse on the 10a circuit and do the same thing. Just musing out load. :D
This also brings up another question how the hell do you test how many amps something like the battery I have makes. It is unmarked, basically salvage, might actually be good btw seems to be holding 9V. Do I just stick a resistor in series or do I have to stick an LED in there and have a "load" on the circuit.
 

MrChips

Joined Oct 2, 2009
26,084
Voltage sources output voltage.
The amperage is dependent on the load for which you need to apply Ohm’s Law.

I = V / R

Yes, you need resistance in series with the ammeter.

You can also wire a known resistance across the voltage source and measure the voltage across the resistor. Use Ohm’s Law to calculate the current.
 

Thread Starter

Teljkon

Joined Jan 24, 2019
265
Voltage sources output voltage.
I kinda take issue with that statement because a battery can do both. A solar panel ok fine but a battery takes charge then dispates wash repeat. Not trying to be nit picky electronics are confusing enough though eh.
 
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