So this is the Micsig CP2100 series current probe, it’s my first piece of equipment from Micsig but I’ve been hearing good things about them so I kinda have high expectations for this product.
What I have here is the CP2100A variant which is rated for 800KHz bandwidth, there is also a B variant which is rated for up to 2MHz in the latest revision, that one is a bit more expensive and I would only recommend getting it if you really need that bandwidth, otherwise there shouldn’t be any other difference between the two models. I don’t know if I mentioned this already but obviously it can measure AC and DC, it has two ranges, 10A and 100A. There is a zero function on the module for automatic zero adjustment and you can also do manual offset adjustments with these arrow keys.
Micsig specs this as 3% ±50mA accuracy for the 10A range and 4% ±50mA for the 100A range but from what I’ve been reading on the forums, this is actually better than the spec, you can pretty much measure down to 50mA without having to worry about that ±50mA but we’ll put that to a test later. Included below you see a set of images from the teardown.
We usually use an oscilloscope for visualizing a voltage over time but sometimes it’s also useful to visualize the current waveform over time. The right way to do it is to get a current probe which can sense the current and convert that to a voltage that the oscilloscope can display however such devices are pretty expensive, they can be around $1000 even for an entry level one like the Rigol RP1001C which is only rated up to 300KHz bandwidth.
But we can improvise something for a much lower cost and it should allow us to visualize the current waveform on the oscilloscope. You’ve probably seen me use a shunt resistor when testing power supply to take a look at the current waveform. Because as you know passing a current through a resistor will generate a voltage drop.
That voltage drop is directly proportional with the passing current and with a round value resistor we can have an easy to use transformation ratio between voltage and current. All we have to do is o introduce this resistor inline between our power supply and the device under test
For example if I have a 1ohm resistor, we have a 1:1 ration, for each mA passed through that resistor we will have 1mV of voltage drop that our oscilloscope can display. Such a circuit will of course have it’s limitations, for example it won’t work very well when testing low voltage low power devices because our resistor will introduce a burden voltage, which will drop our supply voltage to the device under test. This is also not an isolated measurement so it might not be safe when connected with higher voltage circuits.
But there are still a lot of scenarios where you could use this successfully on the electronics workbench so it might be worth building something like this. I want to make this nicer by building it inside an enclosure with the required bnc connector for connecting to the oscilloscope and 4mm banana plugs for passing the current through. I picked this small aluminium enclosure which would be enough to house the resistor, actually the resistors, because there are several advantages to using multiple resistors in parallel.
Alternative to this simple shunt resistor measuring method include the Joulescopewhich is a fully featured dc energy measurement test instrument with incredibly wide dynamic range that allows you to capture the smallest currents next to a jump to a higher current. I reviewed the Joulescope in Voltlog #211.
