Evaluate Your Power source’s Low Present With A Shunt

Designers typically use a recent shunt to measure low current currently being sourced from their energy provide given that a lot of of now!|s power supplies do not supply satisfactory accuracy from their built-in measurement methods. Making use of a shunt is easy. Just decide on a shunt resistor of a identified resistance, measure the voltage drop across the shunt, and use Ohm!|s law to calculate the current.


In practice, this implies you have to accurately know the resistance from the shunt, take into account any drift during the resistance worth, and be cautious about sizing the shunt so you don!|t have also much burden voltage (the voltage drop across the shunt that takes far from the ultimate voltage that seems on the device under test, or DUT).



The Alternatives

For static present or present that modifications above a little variety (10:one), this process will work fine. But for dynamic currents, working with a shunt gets to be much more demanding. In case you size the shunt for your minimal present, the burden voltage gets as well big at higher recent. Should you dimension the shunt for high current, you may not get adequate measureable voltage at reduced latest for an correct recent measurement.


This could force you to consider using multiple shunts, which can operate very well if you can manage when the existing is flowing. If you're able to manage your DUT as well as your test sequence which means you know you happen to be drawing higher existing for a single test and low existing for an additional, then you definitely can use relays to switch in between the shunts to attain broad dynamic selection of measurement. But switching shunts could consider numerous milliseconds and adds complexity, so it may not be an appropriate answer for high-throughput automated test products (ATE).


You may need to try out working with a single shunt to cover the whole dynamic variety of current, though measuring the two large and minimal latest in the similar shunt delivers its own challenges. As the dynamic choice of latest to be measured will get more substantial, to lessen burden voltage at large present, you will be forced to make use of a smaller sized shunt by using a smaller voltage drop at very low present.



Let!|s say your DUT necessitates ten A at full operation and ten mA in standby mode. This is a 1000:one dynamic variety of latest. If you choose a 10-m￡[ shunt to help keep your burden voltage lower, you will possess a 100-mV drop at 10 A along with a 100-￡gV drop at ten mA. You might assume this should be quick to have an accurate measurement, but there are a number of sensible considerations (see the table). So, it isn!|t so simple to accurately measure very low recent applying a milliohm shunt, as there are many factors to take into account.



A different technique should be to decide on a energy provide that has the built-in present measurement that you just need to have. Within this case, the test equipment vendor has fully engineered the option, accounting for the concerns within the table, leading to traceable, documented specs. Such a power provide will be a lot more pricey than a much less capable supply plus a current shunt. But when accurately measuring present is your goal, the additional expense of your electrical power provide lowers complexity and considerably raises test self-assurance and test throughput.


The Agilent State-of-the-art Electrical power Technique N7900 Series Dynamic dc power supply is particularly built for measuring wide dynamic latest (see the figure). It features rapid, accurate measurement of a 5000:1 dynamic range of recent, which include measuring from milliamps to dozens of amps.