EURORACK V/mA
3
HOW TO USE
VOLTAGE (V)
Eurorack modules are powered from three DC voltage rails: +5 V, +12 V, and −12 V. Not all modules use all
three rails. The +5 rail is considered optional: it is meant to power digital circuitry, and not all power
supplies deliver it.
Test 3 allows you to check the voltage rails of a power supply. Simply set it to V mode, and cycle through
the rails using the rail button. The measured voltages should be within about 10 % of their nominal values.
If a power supply is overloaded, the voltages on one or more rails will start to drop, until eventually the
overcurrent protection engages and one or more rails are switched off. If only one rail goes out, this can
damage some modules. Test 3 allows you to monitor the rails, so you can switch off an overloaded system
before damage occurs.
CURRENT (mA)
The current draw of Eurorack modules can vary greatly. Analogue modules tend to draw constant
currents, changing slightly with knob settings. Digital modules often have irregular current draws,
changing greatly depending on the selected operation mode. LEDs and displays also have a large impact
on the current draw.
It’s important to determine the maximum current draws of a module on the different rails. The Eurorack
system’s power supply should have enough capacity to source these currents, at all times. These are the
figures that should be published, not the ‘typical’ values! Once the maximum current draws of all modules
in a system are known, calculating the current draws of the entire system is simply a matter of adding up
the figures of the separate rails.
In the regular mA mode, Test 3 will show the real-time current draw. Whenever a maximum value is
reached, it is stored to the mA peak. To measure a module’s maximum current draws, power it from the
Test 3 meter output header and use it in various ways: try different knob settings, patch inputs and
outputs, and make as many LEDs light up as possible. Once you’re done, use the mode button to enter
the mA peak mode, and write down the values of the different rails.
Modules will often draw a large current spike when powered up. A power supply should be able to handle
these short spikes, which can also vary greatly, so you typically don’t need to measure these. It’s
recommended to reset the mA peak measurements (by holding down the mode button) after plugging
in a module. This removes the start-up measurements, allowing you to get the maximum current draws
during actual use.
If you are building your own DIY modules, Test 3 can be a very useful troubleshooting tool. After building
a module, first check if there aren’t any short-circuits on the power lines using a standard multimeter.
Then power it from Test 3 and monitor the current draw(s). If these figures are wildly different from the
expected values, this indicates that there’s an issue with the module.
Test 3 is also an excellent aid during Eurorack module development. Use it to power breadboards and
prototypes, and track how the module’s current draws change as the design progresses. As design
problems often cause excessive current draws, this can speed up the development cycle.
