Source Measure Unit

Source measure units are similar to bench-top power supplies only in their basic operating principle; source measure units output a controlled voltage and measure the current that flows. SMUs are orders or magnitude more precise, are fully programmable, and allow the user to sweep the voltage over a defined range.

With a bench-top power supply you usually use a dial to select the voltage you want to generate, and then look at the display to read off how much current is flowing through your circuit. Usually a bench-top power supply will output a voltage from zero to 12 or 24 volts and measure the current to the nearest milliamp (1 thousandth of an amp) or so. This is great when measuring the current used by motors or light bulbs or high power devices. However, if your want to make precise scientific measurements, then a milliamp is actually a huge amount of current — very often it is necessary to have a precision of microamps (1 millionth of an amp) or nanoamps (1 billionth of an amp) to characterize many electronic devices.

It is important to separate the function of a source measure unit from a regular multimeter; both are very useful but have different purposes. A standard multimeter can measure voltage and it can also measure current — but not at the same time. It also doesn't output a voltage. A good handheld multimeter will be able to measure voltage with an accuracy of a few hundred micro-volts and current to an accuracy of a micro-amp or so — much better than a bench-top power supply. As such, you could also build a source-measure unit with moderate accuracy by using a bench-top power supply to output a voltage/current and two good quality multimeters — one to measure voltage and the other to measure the current. However, this still wouldn't be programmable and also wouldn't allow negative voltages to be measured very easily (both of which are important for many applications).

For some applications it might not be important to have a programmable instrument — you may just want to read off the value once or a small number of times. However, in many cases you might want to collect lots of data so that you can plot a graph or measure the performance of something over time, or link several pieces of apparatus together. However doing this manually is time consuming and difficult. There are also lots of different experiments that require automated data collection to get faster or more accurate measurements, or to take measurements over a long time-scale (months or even years). Here, you will certainly need a computer to collect your data and export it to a spreadsheet or database for analysis.

Not all experiments will need negative voltages — and in some cases, you can avoid this. However, many different types of devices work differently if a positive or a negative voltage is applied. To fully understand how such devices work, we need to be able to change the sign of the voltage applied.

As an example, consider a diode — a device that only allows electricity to pass through it in one direction. In order to evaluate if a diode is working, we need to see if it can pass electricity in both directions. We can do this in one of two ways. We can measure the diode in one direction, then manually turn it around and measure it the other direction, and then 'stitch' the data sets together. More simply however, we can just measure current flow when we apply either a positive or negative voltage. In fact, this technique is so useful it is used to characterize many types of devices that have diode-like behaviour — solar cells and light emitting diodes are both very good examples of this.