Safety and proper usage

Avoid battery polarity mismatch (R1-R2)

This topic is already covered on the Battery & Charging page. But make sure you connect a LithiumPolymer battery with the correct polarity (as marked on the PCB) or you can damage the battery charger (MCP73831). R3 includes reverse polarity protection, so if you have the battery wired in reverse, nothing happens, the unit just won’t turn on.

Non-isolated USB

It’s also important to understand the USB connection is not isolated. If you leave the USB connected to a computer that is earth ground connected, and you also power your DUT from an earth ground referenced supply, that will permanently damage your CurrentRanger. If you want to use USB serial/connection for high speed logging/debugging or other purposes, you must use an external USB isolator such as this one on Aliexpress:


Avoid ground loops

This is a high-side current meter (ammeter). This means that power from your source must be applied at the input [+] terminal, and your DUT (device under test) to be powered from the input [LOAD] terminal.

The GROUND of the CurrentRanger circuit is NOT THE SAME as the Output negative [-] terminal. Be sure you heed the warnings below and understand proper usage of the instrument or you may risk damaging it.

To avoid damage to CurrentRanger and/or an oscilloscope connected to the output, it’s important to understand that the CurrentRanger was not designed to be used with the same ground reference from input to output (ie with the same ground connected at both I/O ends). This can cause ground loops and in certain cases can damage this instrument or your oscilloscope/test equipment.

You may have only one (1) connection to earth ground attached to the CR at any given time.

This means that it’s OK:

  • to have a floating battery powered circuit at the input and an mains/earth connected oscilloscope at the output
  • mains/earth connected powered device at the input and floating DMM or mains-isolated oscilloscope at the output
  • USB to be plugged in from a laptop that is not┬áconnected to mains/earth ground while measuring with a mains/earth connected (ie. non-isolated) oscilloscope/DMM
  • USB to be plugged in to a mains/earth PC/laptop for logging purposes, and your DUT to be floating (ie. battery powered), and nothing at the output or a floating DMM/oscilloscope at the output

But it’s not OK:

  • to measure a DUT that is mains/earth powered/connected, with an oscilloscope/DMM that is also mains/earth connected. Depending on the range and opamp biasing mode in use, this can cause ground looping and can damage the CurrentRanger
  • to measure a DUT that is mains/earth powered/connected, while USB is also connected. This is passing current through the CR, from the input terminal, to the USB earth/ground, and will completely damage the CR
  • to use the CurrentRanger with a non-isolated (directly mains-powered) oscilloscope while the CurrentRanger is plugged to mains connected uUSB

Input protection / overloading

To minimize burden voltage, there is no input protection by default. You should never connect a low impedance voltage source to the inputs. This alone can damage the internal precision shunts and/or op-amps, the most expensive BOM parts. A maximum differential of 33mV is allowed before overloading starts to occur (vBat > 3.4V).
It’s not hard to carelessly overload the inputs of the instrument. Make sure you double check your input connections before you apply power in your DUT circuit.

Some pads and pin holes are included on the bottom of the PCB where you may solder back-to-back diodes if you can accept the leakage/error these devices may add to your measurements. Essentially any protection device will either add noise or leakage leading to higher burden voltages or less than accurate readings.