How To Measure Current With Your Scope
1. Your scope as a voltmeter
Your oscilloscope is a voltmeter but also a special type of voltmeter. It is a voltmeter that not only allows you to measure the voltage but also allows you to see how this voltage varies in time. When my mechanic friends complain that they cannot understand electricity because they cannot see it I would reply that, yes they can see it, but they need special goggles for this, the oscilloscope.
Your oscilloscope can measure voltages just like and better than your multi-meter but there is a huge and important difference: Your scope is not fully insulated like your multi-meter is and, because of this, the common of the probes (the little alligator clip) is permanently connected to the Ground! I am not talking about portable, battery operated scopes of course. Oscilloscopes usually have two or more channels. Each channel can be considered as another voltmeter, but with one particularity: The common leads of all those voltmeters are connected together and connected to the ground! The disadvantage of this common ground is that when you measure different signals on different channels those signals must have a common ground. But no panic, there are ways around!
The scope’s ground connection has two reasons for being, one for safety and the other for noise immunity. Scopes are very sensitive instruments and can easily capture electro-magnetic interferences. Shielding and grounding the cabinet is an effective way of reducing the effect of those interferences.
If you measure isolated and low voltage signals this is generally not a problem. Most signals are measured with the “– lead” (common) as a reference and the fact that you connect a ground to this common (often already connected to the ground anyway!) when effecting the measurement has no consequence. But you should be aware that whenever you connect the little alligator clip of your scope probe to any point of a circuit you are connecting this circuit’s point to the ground… You must be sure that it is acceptable and safe to do so!
Things start to become hairy when you want to measure voltages related to the mains, your home 110V or 240V supply. And the answer is a big NO! You should not directly measure the mains voltage of your house with an oscilloscope. Even if your oscilloscope probe and input tells you that you can measure up to 250V or more the answer is still NO! Because the problem is the common (this little alligator clip) which is hard wired to the ground!
If you connect the little alligator clip to the live there will be a big Bang!
If you connect the little alligator clip to the neutral it will more likely trip your house ELCB (Earth Leakage Circuit Breaker). The neutral is not the ground. The neutral is usually connected to the ground at the switchboard but due to voltage drops caused by currents circulating into the neutral lines for various appliances the neutral could raise to potentials that could even be dangerous!
Some people suggest connecting your scope through an isolation transformer in order to make it “floating”. This is not a good idea either as you will bring the cabinet of your oscilloscope to either live voltage or neutral voltage depending where you choose to connect the little alligator clip.
The safest way is to use the oscilloscope as it was designed for, an instrument to measure signals with the ground as a reference. However there are situations where you need to do measurements that are not referred to ground. Consider the following example where we want to measure the voltage between point A and point B on the amplifier:
If you connect the little alligator clip (Ground) to either point A or point B you will create a short that will prevent the amplifier to function normally. One way around is to use both channels of the oscilloscope and set them to display the difference between CH1 and CH2. Most scopes have this function. To use this method, both probes must be identical and calibrated the same way. This method also had two disadvantages:
- We are using two channels for one measurement
- The accuracy is said to decrease at higher frequencies
Another method is to use a differential probe that will give you exactly that: the difference between both signals. There are various types of differential probes, many suitable for high voltages. Unfortunately they are quite expensive… But if you have a lot of high voltage measurements to do; there is no price for safety!
The figure below shows a differential probe.
2. Measuring Current
Remember, your oscilloscope is a voltmeter and the only one way to measure a current with a Voltmeter is to create a voltage that is directly proportional to the current you want to measure. And there is a component that is doing exactly that, and only that: the resistor!
Open the circuit (yes, currents are measured in series!) where the current has to be measured and insert a resistor. Connect your Voltmeter across the resistor and bingo, apply the Ohms Law, I = V/R. Simple, isn’t it?
But there are two things to consider:
a) The value of the resistor should be much smaller than the load. If the resistance is too high it will affect the circuit under measurement and modify its characteristics by limiting the current. If the resistance is too low the voltage across the resistor will be very small and noisy, hence difficult to measure accurately. So the value of the resistor must be a compromise between both situations. Don’t forget the resistor’s power rating,
b) Our voltmeter is an oscilloscope so we must be careful where to insert the resistor in order not to create a short with the, now famous, little crocodile clip… this is why we usually insert the resistor in the return path, so one side of it will be the ground and our little alligator clip will be happy!
To illustrate the above let’s build a small circuit to demonstrate the relation between voltage and current in a capacitor.
When we close the switch, we charge a capacitor through a 10k resistor. To measure the current we insert a 150 Ohms resistor in the return path. The voltage across the 150 Ohm resistor represents the current and is measured on CH2. The voltage across the capacitor is measured on CH1. It will include the voltage drop in the 150 Ohm resistor but this is not going to affect the demonstration as it is very small compared to the 12V.
We can see that when we close the switch the current goes from 0 to its maximum value almost instantaneously as the discharged capacitor is equivalent to a short circuit. The current maximum value is 12V / (10000 + 150) = 0.00118 A = 1.18 mA.
Our measurement shows a voltage drop of 181.176 mV across the 150 Ohm resistor. Calculated 181.176 mV / 150 Ohm = 1.2 mA (this is good enough!)
As the capacitor charges the voltage increases and the current decreases just like it was written in our good old basic electronics book! We successfully measured the current with a reasonable accuracy! Now, just for fun, let’s remove the switch and replace the 12V DC with an AC Voltage:
We can see that, when the current is at its maximum, the voltage is zero. When the voltage is at its maximum, the current is zero. We successfully demonstrated that the current and voltage in a capacitor are shifted by 90 degrees!
It is important to note that we could have measured the voltage and current with a multi-meter and getting the correct values. But we wouldn’t know that they are shifted by 90 degrees. This makes a whole world of differences and justifies why it is often useful to measure the current with an oscilloscope. In AC the relation between voltages and currents is rarely “1 to 1”. Besides the phase shift demonstrated above there are many situations where the shape of the current is totally different from the shape of the voltage that created this current, as we will demonstrate soon.
3. Current transformers
So far we measured isolated and low voltage circuits and it was safe to connect the grounded little alligator clip to the circuit’s ground or where we wanted our reference to be. For higher voltages and non-isolated circuits there is a much safer way to measure the current: using a current transformer!
The current transformer works like a normal transformer. However, the primary winding has only one or very few turns and is designed to be inserted in series where the current has to be measured. The secondary winding has many turns and is perfectly isolated from the primary. The secondary can be connected to an Ampere meter to measure the current. Alternatively we can connect the secondary to a resistor (called Burden Resistor) and measure the voltage across this resistor. The Ohms Law will give us the current, which will have to be adjusted according to the ratio of turns of the transformer!
Another type of current transformer is the clamp type, also called noninvasive current sensor. It works like a clamp ammeter where the primary is the wire in which circulates the current to be measured. It is not necessary to open the circuit to measure the current as the clamp will open then close around the conductor. The secondary will be connected to an ammeter or to a burden resistor and voltmeter just as seen before. The beauty of this kind is that you can loop the current carrying wire several times to increase the sensitivity. In the figure below we looped the current carrying wire (the red one) twice, thus doubling the sensitivity.
The model above can be purchased for a few dollars on Ebay. It is very popular amongst hobbyists who want to build energy monitors. You can find a number of projects on the Internet, using this sensor together with some kind of microcontroller such as Arduino or others.
In the example below we want to measure the output ripple and the rectifier’s input current of a linear power supply. As the negative output is connected to the ground it will be no problem to hook the little alligator clip there in order to measure the ripple. However, for the rectifiers input current, there is no way to connect the scope directly without creating an undesirable (and catastrophic) short circuit. This is where the current transformer comes handy:
This example also illustrates that the shape of the input current (CH2) is very different from the shape of the voltage that creates this current, as we know the input voltage is a perfect sine wave (not shown here). This comes from the fact that the input only supplies current during the charging times of the capacitor as we can see on the ripple waveform. One might be surprised to see that our well known linear power supply doesn’t present a linear load to the input voltage!
The relation between current and voltage, although straight forward in DC, can be quite complex in AC circuits. It varies from simple phase shifts to totally distorted shapes and the best way to know is by using your scope. Although your scope seems to have been designed to measure and observe voltages it can be easily converted into a current measuring instrument with just one resistor or better, a current transformer. Looking at both, voltage and current, helps us to a better understanding of the basic AC circuits.
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