Burden voltage when measuring current is the voltage your meter develops across its own internal shunt while the current flows through it. It is small, it is unavoidable, and on low-current measurements it can be large enough to change the behavior of the circuit under test. A reading that looks stable and precise can still be describing a circuit that only exists because the meter is in it.
Where the drop comes from
An ammeter does not measure current directly. It passes the current through a known resistance and measures the voltage that appears across it, then applies Ohm’s law. That resistance is the shunt, and the voltage across it is the burden. Designers make the shunt small to keep the burden small, but they cannot make it zero, and they must also make it large enough to produce a signal the meter’s front end can resolve. On the highest current range the shunt may be a few milliohms; on the microamp range it may be hundreds of ohms or more, because a tiny current needs a large resistance to develop a usable signal. That is why the burden problem grows worse, not better, as the current being measured gets smaller.
Why low-power circuits suffer the most
Consider a sleeping microcontroller running from a coin cell and drawing a handful of microamps. Put a meter in series on its lowest range and the burden voltage may be an appreciable fraction of the supply. The device now sees a lower rail than it would in service. It may draw less, it may draw more, it may reset, or it may drop into a brownout mode that consumes a completely different amount of current. The number on the display is a genuine measurement of what flowed, but what flowed was the behavior of a circuit with an extra resistor in it. Battery-life estimates built on that reading can be wrong by a wide margin, and the error usually flatters the design rather than exposing it.
Range switching changes the answer
Because each current range has its own shunt, changing range changes the burden. This produces a confusing symptom: a circuit reads one current on a low range and a different current on the next range up, and neither reading is a mistake. Autoranging makes this worse by doing it without announcement, and it makes it worse again on circuits whose current swings between sleep and active states, because the meter may hunt between ranges while the load itself changes. Some instruments briefly open or greatly increase the input impedance while switching ranges, which can interrupt the supply to a sensitive load. Checking the manual for the burden voltage specification of each range is the only way to know what you are adding.
Ways to measure without starving the circuit
Several approaches reduce or eliminate the problem. Using a higher current range accepts more quantization noise in exchange for less burden, which is often the right trade when you only need a rough figure. An external shunt of your own choosing, sized deliberately for the current range of interest and measured with a voltmeter, lets you pick the burden rather than inherit it. Dedicated low-burden current measurement instruments actively hold the voltage across their input near zero using a feedback loop, which is the cleanest answer for battery-powered work. A current probe or clamp adds no series resistance at all, though the practical clamp types have their own lower limits on how small a current they can resolve.
Sanity checks that catch the error
The simplest test is to measure the supply voltage at the device while the current measurement is running. If it is meaningfully lower than the supply with the meter removed, burden voltage is affecting the result. A second useful check is to repeat the measurement on two different ranges and compare. Agreement between ranges suggests the burden is not dominating; disagreement says it is. Where the circuit has a defined minimum operating voltage, compare that figure against the supply minus the burden to see whether the device is being pushed below its specification. None of this requires exotic equipment, only the willingness to treat the measurement setup as part of the circuit rather than as an invisible observer.
What to check before you act
- Look up the burden voltage specification for the range you are using.
- Measure the supply at the load while the ammeter is in circuit.
- Repeat the reading on a second range and compare the two.
- Prefer a clamp or low-burden instrument for battery-life work.
Common questions
Does burden voltage matter on a mains-powered load?
Usually not much, because a few hundred millivolts is negligible next to line voltage. It matters most on low-voltage, low-current circuits.
Can I just subtract the burden voltage afterward?
You can correct the voltage, but not the circuit’s response to it. If the load changed its behavior, arithmetic cannot recover the original current.
The practical takeaway
Every series ammeter adds a resistor to the circuit under test, and the smaller the current, the bigger that resistor tends to be. Check the burden specification, verify the supply at the load, and move to a clamp or a low-burden instrument when the circuit cannot afford the drop.
References and further reading
- Department of Energy: estimating device energy use from measured draw
- ENERGY STAR: standby power, where very small currents dominate the result
- CPSC: general home electrical safety guidance
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
