Electrical current is the flow of electric charge from positive voltage to
negative voltage. We now know that electrons move through conductors from
negative voltages to positive voltage. We had a 50⁄50 chance of guessing
“right”, so, yeah.
Electrical current is the flow of charge per time, which is measured in
units of ampere (A), often called an “amp”. One amp is one coloumb of charge
flowing per second. One coloumb is approximately 6.242 x 1018
A device that measures current is called an ammeter.
Directly measuring current is difficult. Counting individual electrons
is not usually an option for electric circuits. Therefore, the two primary
techniques use “side-effects” of current. First, moving charged particles
create a magnetic field (Ampère’s Law), and charged particles moving through
resistance create a voltage (Ohm’s Law). Both of these techniques can now be
derived from Maxwell’s equations.
Current creates a magnetic field that was first discovered by Ørsted in 1820
using a compass. This technique was later refined into the modern galvanometer.
Most modern galvanometers have a permanent magnet and a coil of wire.
When current flows through the coil, it pushes towards or away from the
The coil is attached to a needle and torsion spring. If you have ever
seen analog multimeters or vintage stereo equipment,
you have likely seen a galvanometer.
The magnetic field can also be measured using hall-effect sensors. Changing
magnetic fields (AC) can use a sense coil through inductance.
As the current gets smaller (under 1 mA), measuring the magnetic field with
sufficient precision becomes difficult.
When a resistor is placed in the path of current,
it produces a voltage according to Ohms’s Law:
V = I * R
Solving for current:
I = V / R
If the resistance is known and we measure voltage, we can compute current.
Most modern ammeters use shunt resistors. The best part about this approach
is that we can select a shunt resistor value that gives us a suitable voltage
A shunt resistor is also called a “current sense resistor”, or simply
By design, shunt resistors cause a voltage drop, also called burden voltage or
insertion loss. If this voltage is too large, it affects the load. The
additional resistance also changes the source impedance as seen by the load,
which can cause some load circuits to behave differently. Ideally, the shunt
resistance would be so small that it would not affect the target circuit.
Practically, the shunt resistance has to create a measurable voltage.
Measuring a large current range is difficult for a single shunt resistor.
The voltmeter has a fixed range. To expand the range, most ammeters
use multiple shunt resistors, each with different resistances.
However, if the current changes over time, a shunt resistor that
is too large can cause an excessive voltage drop that affects the behavior
of the target circuit.
If the shunt resistor is too small, it cannot accurately measure the current.
Most multimeters can measure current. These multimeters have
internal shunt resistors, and they actually measure voltage!
When you change multimeter ranges, you are selecting different shunt resistors,
usually with two different voltage gain settings.
Multimeters are well-suited for measuring currents that are constant, either
as direct current or “constant” RMS alternating current. Multimeters cannot
easily measure currents that vary rapidly or that change dramatically over time.
Some multimeters also support current clamps. These clamps fit over wires
to measure the current through the emitted magnetic field.
DC current clamps often have a hall-effect
sensor. AC current clamps often use magnetic coils. Some current clamps
support both DC and AC measurements. Current clamps are practically limited
to resolutions around 1 mA, which is not sufficient for many applications.
Oscilloscopes measure voltages at regular intervals, often over a million times
per second, to construct a voltage waveform. Oscilloscopes then
display a graph showing changes in voltage over time. By measuring the voltage
over an external shunt resistor, oscilloscopes can effectively display changes
in current over time.
However, this approach has two primary challenges. First, the shunt resistor
measurement technique has the dynamic range issues associated with shunt
resistors. Oscilloscopes usually trade-off speed for limited dynamic range,
and typically have just 10 or 12 bits of dynamic range.
Second, oscilloscopes are usually earth ground referenced. The oscilloscope
measures the voltage difference between earth ground and the signal. However,
we want the differential measurement across the shunt resistor. Introducing
shunt resistance into the ground path often causes signal integrity
issues. We often want “high-side” shunt resistors on the positive power supply.
However, if the test circuit is also
earth ground referenced, we cannot use the oscilloscope’s standard probe to
measure the voltage difference across the shunt resistors. We can either use
two oscilloscope probes and use a mathematical subtract feature, which
introduces additional measurement error, or we can use a differential oscilloscope
probe, which are often quite expensive. Either way, we are still left
with the dynamic range issue.
Oscilloscope manufacturers also provide current probes, which are usually just
a combined shunt resistor and differential probe. These probes also allow
the oscilloscope to get the units right so that you don’t have to do Ohm’s law
calculations every time you measure current. However, you still have limited
Oscilloscopes also have current clamp probes, which are limited to around 1 mA
A variety of other equipment exists that can measure current, sometimes while
sourcing or sinking current. Equipment of this type includes electrometers,
picoammeters, and Source Measurement Units (SMUs). These products are specially
designed to overcome some of the standard multimeter drawbacks, and many
do have lower burden voltages and lower input bias currents. However,
the primary drawback is cost. These devices often employ multiple,
more complicated, active feedback ammeter sensing methods.
How do we overcome these limitations without breaking the bank?
What if we had a device that automatically and instantaneously selected the
shunt resistor to keep the voltage in range? This approach maintains a
maximum burden voltage while also accurately measuring current. Sounds great!
Until the introduction of Joulescope, this type of dynamically switching
equipment was either too slow (introduced too large of a dynamic burden voltage)
or very expensive.
This approach does have two drawbacks, but unlike with the other approaches,
we can mitigate these drawbacks.
The first drawback is the shunt resistor switching
time, especially when the current goes over-range for the current shunt
resistor value. If the resistor value does not switch quickly enough, then the
burden voltage becomes excessive and affects the target device. The required
switching time can be calculated. A simplified equation, suitable for
many practical applications, is:
t = C * ΔV / ΔI
Let’s take a typical example. The target system takes 3.3V and can tolerate
a temporary 3% voltage glitch on a 1 amp change. If the system has 10 μF of
capacitance, the required shunt resistor switching time is:
10 μF * 3.3 V * 0.03 / 1 A = 1 μs
The second drawback is that this approach presents a variable impedance to the
target circuit. Some circuits may exhibit unusual behavior to changing
supply impedance. However, we can mitigate this susceptibility by
adding decoupling capacitors, which effectively lower the input impedance at
the higher frequencies of interest. Most modern electronics already require
bypass capacitors, so this drawback is often not a concern when measuring
current to target devices.
Specialized equipment designers must also account for Johnson-Nyquist noise,
the noise generated by any resistor. This noise, the voltage measurement
accuracy, and bandwidth are the critical design factors.
Joulescope includes the most affordable, fast auto-ranging shunt ammeter.
Joulescope switches shunt resistors in approximately 1 μs on over-range to keep
your target device running correctly. It maintains a maximum burden voltage of
20 mV across the shunt resistor, for any current up to 2 A. Joulescope is
electrically isolated to avoid any grounding and ground loop concerns.
In addition to being an ammeter, Joulescope simultaneously measures voltage
so that it can compute power: