Is it sorcery to make an ammeter with zero resistance? Not really. This kind of design has been used for ammeters in corrosion science where any voltage drop would influence the experiment.
Most commonly, current is measured by passing it across a shunt resistor, then measuring the voltage drop. You can even get specialized amplifiers for that purpose (INA210-series, INA199, ZXCT1110). But such a technique creates the necessary voltage drop. What if you want not to drop any voltage and yet measure the current? There are Hall-effect probes which measure the magnetic field created by flowing current. They are good especially for larger currents. For everything else, there's Mastercard Zero Resistance Ammeter.
The ZRA is an active device; it needs a current source equal to the current being measured. For low currents, only a single op-amp is needed for maintaining equal voltages on the two input terminals. For larger currents you can boost its capabilities by a single transistor. A second op-amp (or current shunt monitor) is needed to scale the voltage drop for external measuring device or ADC.
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| Complete schematic of the zero resistance ammeter |
My design uses a single 1.2V NiMH battery as the power source. Because the op-amp needs at least 2.2V, I put a small DC switching boost regulator to increase the voltage. A switched-capacitor type would work likewise.
The positive terminal is connected to ground. Current is sinked and for the negative terminal it is sourced from the battery. The current passes through a NPN transistor, a Schottky diode (to block reverse polarity) and a shunt resistor to the negative terminal. I used a dual op-amp, OPA2376, which has a very small input offset voltage. It would work well with a cheaper type though. One op-amp senses the voltage at the negative terminal and adjusts the current to the base of the NPN in such a way that the voltage at the negative terminal is kept zero. The NPN works in linear mode, lowering the voltage from the battery. The second op-amp only amplifies the voltage at the shunt resistor by a factor of 100 to have 1V = 1A. Capacitors C1 and C2 are needed for stability.
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| Assembled on breadboard |
In the picture above, the shunt resistor is actually 0R02 and the gain was thus only 50.
From left: Trimmer for gain, OPA2376, shunt resistor, Schottky diode SS14 (bottom), FZT853 NPN, TLV61220 boost regulator with capacitors and inductor soldered directly to adaptor board, Sanyo Eneloop 1.2V NiMH battery.
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| 1.003 V (= 1.003 A) and true zero voltage drop |
The maximum current is mostly affected by heat sinking of the NPN. With a FZT853 in a rather small SOT-223, it is good for 1A. With beefier transistor, it would go somewhere around 4A, where both the current to base and the current from the battery will limit further expansion. On breadboard, keep it under 1A. I learned the hard way that at 2A, the board sometimes melts :-)
