I'm new to electronics so I purchased the Electronics for Dummies book. My first project is with a solderless breadboard and a 6 volt battery (4 D batteries) a 2.2k ohm resistor and an led. Putting it together the led lights up. Checking the volts i had a reading of 6.4 for the battery. 4.8v through the resistor and 1.9 volts through the Led. But when measuring the amps I was only getting a reading of .34 milliamps or .0034 amps. Using Ohms law I should be getting a reading of around 2.9 milliamps. I tried a different resistor, led and battery source but came up with the same value. What am I doing wrong. I have the multimeter set to milliamps and DC. Any suggestions?

 

Hello,

What kind of meter are you using?
When adding the resistor voltage and the led voltage, they do not match the battery voltage.

Bertus

 

Hello,

What kind of meter are you using?
When adding the resistor voltage and the led voltage, they do not match the battery voltage.

Bertus

I'm using a Klein multimeter. Yes the resistor and led voltages do add up to the battery voltage

 

Hello,

When I add 4.8 Volts + 1.9 Volts, that is 6.7 Volts and not the 6.4 Volts you mention for the battery voltage.
What are the specfications of your Klein multimeter?
Wich one do you have?
https://www.kleintools.com/catalog/test-measurement/multimeters

Bertus

 

getting a reading of .34 milliamps or .0034 amps. Using Ohms law I should be getting a reading of around 2.9 milliamps.

.0034A is 3.4mA not .34mA.

 

Hello,

When I add 4.8 Volts + 1.9 Volts, that is 6.7 Volts and not the 6.4 Volts you mention for the battery voltage.
What are the specfications of your Klein multimeter?

Bertus

I just tested them again and I got 4.5 and 1.8. It's a MM600 model. Current Auto ranging is from .1 micro amps to 10 amps. Not sure if that is what you were looking for.

 

.0034A is 3.4mA not .34mA.

Sorry, that's what I meant

 

So I'm getting .036 amps with 6.4 volt battery and a 2.2 k resistor. Does that make sense?

 

The Led will not light up when I test the current with my multimeter. When I put the selector over to 10A I will get a reading of .034 amps. But when I turn the selector to mA I will get a reading of .34 mA, doesn't make much sense to me. I'm new to this multimeter but I can't think of what I'm doing wrong.

 

On some meters on a mA range, have a fairly high series resistance. R ??

 

When measuring the current, did you move the positive meter lead from the meter V/Ω socket to the uA/mA socket?

 

On some meters on a mA range, have a fairly high series resistance. R ??

Ok. Since it wasn't reading when plugged into that jack I put the lead in the ohms, volts, continuity jack but it will read 0.034 amps when selector is on 10A and it will read .34 when selector is on mA. Very odd

When measuring the current, did you move the positive meter lead from the meter V/Ω socket to the uA/mA socket?

i did. And it wasn't giving me any reading at all and that was set to DC current. It would only read in the ohms/volts jack but the reading were off by a lot.

 

So I'm getting .036 amps with 6.4 volt battery and a 2.2 k resistor. Does that make sense?

Which is it? First it was either 0.34 mA or 3.4 mA. Now it's 36 mA.

You need to pay more attention to detail or you are going to have nothing but problems.

If you batteries are actually 6.4 V and the LED is dropping 1.9 V, then you would have 4.5 V across the resistor. If it is 2.2 kΩ then you would expect 2.05 mA.

Remember that Ohm's Law requires that you use the voltage across THAT resistor and the current through THAT resistor.

In your first post you said that the resistor had 4.8 V across it (note that the term is "across" and not "through"). You would then expect about 2.2 mA of current through it.

How are you measuring the current?

 

i did. And it wasn't giving me any reading at all and that was set to DC current. It would only read in the ohms/volts jack but the reading were off by a lot.

Check the fuse. Most meters put a fuse in series with the current sensing circuitry on the lower ranges -- that's why they usually have a separate jack for the 10 A range. The volt and ohms ranges will still work with a blown fuse.

 

Ok. Since it wasn't reading when plugged into that jack I put the lead in the ohms, volts, continuity jack but it will read 0.034 amps when selector is on 10A and it will read .34 when selector is on mA. Very odd

i did. And it wasn't giving me any reading at all and that was set to DC current. It would only read in the ohms/volts jack but the reading were off by a lot.

In that case your meter current current ranges are not working. Commonly that's because the fuse has blown. I believe your meter has two: 10A and 500mA. You need to check/replace those fuses.

 

Which is it? First it was either 0.34 mA or 3.4 mA. Now it's 36 mA.

You need to pay more attention to detail or you are going to have nothing but problems.

If you batteries are actually 6.4 V and the LED is dropping 1.9 V, then you would have 4.5 V across the resistor. If it is 2.2 kΩ then you would expect 2.05 mA.

Remember that Ohm's Law requires that you use the voltage across THAT resistor and the current through THAT resistor.

In your first post you said that the resistor had 4.8 V across it (note that the term is "across" and not "through"). You would then expect about 2.2 mA of current through it.

How are you measuring the current?[/QUOT

In that case your meter current current ranges are not working. Commonly that's because the fuse has blown. I believe your meter has two: 10A and 500mA. You need to check/replace those fuses.

that would make sense. I pulled both fuses out then checked the continuity of both fuses and it displayed OL on the screen so I assume both fuses are blown.

 

So what was happening is when I was plugging the red lead into the mA/micro amp jack I would get a reading of 0.00. Same thing when plugging it into the 10A jack. But when I plugged it into the ohms/volts/continuity/temp/%/temp jack I would get a current reading of .34 milliamps when selector is turned to mA DC and .034 when selelector is turned to 10A DC. And when selector is turned to Micro amp DC it would give a reading of 3.4. But I just received another post saying it's probably the fuses are blown for both the 500mA and 10A jacks. Which would make sense.

 

You cannot measure current (milliamps) by inserting a multimeter in series with the component(s) being measured because the multimeter has a resistance between the probes that reduces the current by a small amount.
The only way to get an accurate measurement is to take a reading of a voltage across a resistor and work out the current by using Ohm's LAW.

 

You cannot measure current (milliamps) by inserting a multimeter in series with the component(s) being measured because the multimeter has a resistance between the probes that reduces the current by a small amount.
The only way to get an accurate measurement is to take a reading of a voltage across a resistor and work out the current by using Ohm's LAW.

Well, the current is always correct, but you cannot use the same meter to make the measurements. i.e. A circuit with and without the meter.

There should be a burden voltage specified for all ranges at full scale. It's usually below 0.6 V at full scale.

If you have another meter, put the Klein and measure current and use another meter to check all of the drops. battery, resistor, meter, LED. Also measure the 2.2K resistor.

In the real world, tolerances matter too. The LED burden voltage will be different for every LED. In this circuit contact and lead resistances are negligible.

Remember that you need to measure the current through the device and the voltage across it. If the meter has a 10 M-ohm input Z, that can sometimes influence your measurement. i.e. No use in measuring the voltage across a 10 Meg ohm resistor.

There are feedback ammeters which have nearly constant and extremely low burden voltages < 1 mV.