# Source: https://learn.adafruit.com/multimeters/current.md

# Multimeters

## Current

# What is current? &nbsp;
Current is the rate of electricity flow in a circuit. &nbsp;Using the same water analogy as before, higher pressure (voltage) and a bigger pipe (lower resistance)&nbsp;means a greater volume of water per second (current) will flow. &nbsp;This simple relationship&nbsp;is represented by the equation known as "Ohm's Law":  
  

## &nbsp; &nbsp; I = V/R
  
Where: 'I' is current, 'V' is voltage and 'R' is resistance.  
  
Another important equation is the one for power.  
  

## &nbsp; &nbsp;&nbsp;P = I x&nbsp;V
  
Where 'P' is power (measured in Watts), 'I' is current and&nbsp;'V' is voltage.  
  
Watts is a measure of work, or the conversion of electrical energy into some other form such as&nbsp;heat, light or motion. &nbsp;As the equation implies, it takes both voltage and current to do work.  
  

# Why Measure Current?
  
If there is not enough current, your circuit may not be able to do the work it was&nbsp;designed&nbsp;to do. &nbsp;Logic circuits&nbsp;may not function reliably, displays may be dim,&nbsp;motors&nbsp;may stall. &nbsp;  
  
On the other hand, if there is too much current, things will heat up and&nbsp;components may be damaged. In extreme cases there may even be&nbsp;smoke or flames.  
  
Reasons for measuring current in a circuit include:  

- Determining circuit power requirements  
- Verifying correct circuit operation  
- Testing power supply performance
- Verify that batteries are charging or discharging at a safe rate  
- Estimating battery life or recharge time  
- Diagnosing circuit problems  

![](https://cdn-learn.adafruit.com/assets/assets/000/001/696/medium800/instruments_Wavetek.jpg?1396775437)

# Pick a safe range.
Most meters have several current measuring ranges. &nbsp;Choose one that is good for AT LEAST the maximum current you expect to be measuring. &nbsp;If in doubt, choose the next higher range. &nbsp;There is usually overlap between the ranges and you can always go back to a lower one after you have verified that it is safe to do so. &nbsp;  
  
Info: 

![](https://cdn-learn.adafruit.com/assets/assets/000/001/697/medium800/instruments_wavetek_dial.jpg?1396775442)

This meter has 4 ranges from 200 microamps to 200 milliamps. &nbsp;In addition, the 20 milliamp setting can be used to measure up to 20 amps when used with a special high-current probe jack.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/699/medium800/instruments_Extech_dial.jpg?1396775454)

This is an 'auto-ranging' meter and has just 3 very wide range&nbsp;settings. &nbsp;It will automatically adjust the range to give you the best precision measurement within those settings:

- microamps (uA)  
- milliamps (mA)  
- amps (A)  

# Choose the right connections.
  
Multimeters contain sensitive circuits capable of precision measurements of tiny currents and voltage. &nbsp;These circuits can be damaged or destroyed by high current flow.&nbsp; That is why most meters have a separate jack for high current measurements. &nbsp;This jack is fused for safety. &nbsp;If you are using the high-current setting, be sure to use the right jack. &nbsp;  
Info: 

Info: 

![](https://cdn-learn.adafruit.com/assets/assets/000/001/701/medium800/instruments_Wavetek_jacks.jpg?1396775475)

This meter has a separate jack for measuring voltage and resistance. &nbsp;And two jacks for different ranges of&nbsp;current measurement. &nbsp;One&nbsp;current measurement jack is safe for currents up to 200mA. &nbsp;The other can be used to measure currents up to 20 amps.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/705/medium800/instruments_Extech_jacks.jpg?1396775514)

![](https://cdn-learn.adafruit.com/assets/assets/000/001/704/medium800/instruments_Mastech_jacks.jpg?1396775508)

These two meters use the same jack for all measurements except high current measurements.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/703/medium800/instruments_Wavetek_low.jpg?1396775498)

Here we are using the&nbsp;0-200mA jack to make a measurement in the 2mA range.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/706/medium800/instruments_2012_08_12_IMG_0423.jpg?1396775543)

This is the correct jack and range to use for measurements above 200mA on this meter.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/710/medium800/instruments_Extech_high.jpg?1396775503)

On this meter, the correct jack and range selections&nbsp;for high-current measurement are clearly indicated in yellow. &nbsp;  
  
Also note the warning labels indicating maximum safe levels for each jack.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/711/medium800/instruments_Mastech_High.jpg?1396775568)

But with all meters, use care when choosing range and connections to avoid damage to the meter. &nbsp;Most meters have internal fuses to protect the circuitry, but they are not always&nbsp;readily accessible for easy replacement.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/716/medium800/instruments_Fuse.jpg?1396775605)

# Get in the loop!
To measure the current, you have to make it flow through the meter. &nbsp;To do that you need to make your meter part of the circuit. &nbsp;You need to break the circuit at the point where you want to measure the current and insert your meter in the middle. &nbsp;Before connecting your meter to the circuit, double check your range and make sure you have the leads plugged into the right jacks.&nbsp;  
  
For this example, we are going to measure the battery supply current going into a MintyBoost:![](https://cdn-learn.adafruit.com/assets/assets/000/001/712/medium800/instruments_MintyBoost.jpg?1396775576)

## Breaking the circuit:
  
First we need to break the circuit so we can insert our meter. &nbsp;In this case we will just unsolder the battery wire from the Mintyboost circuit board  
![](https://cdn-learn.adafruit.com/assets/assets/000/001/713/medium800/instruments_Disconnect_Lead.jpg?1396775584)

Then we will attach a temporary lead to make it easier to connect the meter.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/714/medium800/instruments_Add_Jumper.jpg?1396775591)

## Closing the loop
Now we connect the meter between the battery lead and our temporary lead. &nbsp;We'll use some alligator clips to hold it all together.  
  
The positive meter probe is connected to the positive battery lead. &nbsp;The negative probe connects to the&nbsp;temporary&nbsp;lead we soldered to the Mintyboost circuit board. &nbsp;This closes the loop and makes the battery current flow through the meter.![](https://cdn-learn.adafruit.com/assets/assets/000/001/719/medium800/instruments_Connections.jpg?1396775627)

We'll start with the high-current range and probe connection to be safe:

![](https://cdn-learn.adafruit.com/assets/assets/000/001/717/medium800/instruments_HiRange.jpg?1396775612)

We see that the current is 0.23A (230mA). &nbsp;This is well within the 400mA safe limit for the low-current probe jack, so we set the meter to the milliamp range and use the low current jack for a more precise measurement.

![](https://cdn-learn.adafruit.com/assets/assets/000/001/715/medium800/instruments_Current_MEasurement.jpg?1396775599)

Using the milliamp range, we can see that there is 226.9 mA of current going from the battery to the mintyboost.

- [Previous Page](https://learn.adafruit.com/multimeters/voltage.md)

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