What's the best way to translate current in a DC loop into voltage referenced to

[cdev]

ground on my scope. Neither side of the controller is groundable, and my scope cannot float,

Theres something being sent down it once a second that I'm assuming is data...  There also may be some higher frequency stuff or it may just be RFI

( I cant just use a resistor because neither side of the controller is grounded.)

I am going to try a simple toroid with two windings - a transformer, and see what I get doing that.  I don't have the time to build an opto isolator today.

Does this look like anything people are familiar with? The controller board is made by Kanalflakt.
.

[yashrk]

First of all share more info on your circuit, what exactly you want to do.

By the information you have provided guess you want to measure current in a circuit. Why not just put a low value resistor in the loop and measure the voltage of it and let ohm's law do it's magic ?

[cdev]

Neither side of the loop is groundable/referenced to ground..

Its an expensive fan/heat exchanging ventilation controller that ventilates based on several presets which are fairly limited. I want to be able to emulate the controller eventually so a small computer with sensors can control it based on rules.

 By and large I do want to have it running, most of the time, just sometimes less and sometimes more. certain things outdoors make me need to turn it off.

I have sinusitis- long story.

Generally I sleep better when there is a small amount of ventilation all the time. But currently it doesnt have all the settings I need.

Initially I dont want to change anything, I just want to see whats being sent so I can figure out which of the two devices is speaking when and what is being said, and why.

Eventually  I want to be able to have a small SBC talk to the fan box in its own protocol if I can figure it out beyond a shadow of a doubt. I dont want to damage anything.

Does this sound like its potentially any known protocol? When there is no activity occurring (when the fan is off) this signal is sent once a second. Thats probably establishes the timing and when the controller wants the fan to do something it probably sends some variation back.

[AndersJ]

Use a currrent probe.

[cdev]

what is in a current probe, an electromagnet and a hall effect sensor?

Can I build one?

[AndersJ]

https://en.m.wikipedia.org/wiki/Current_clamp
It is difficult to build.

Or use the previously mentioned resistor,
and measure differentially with two oscilloscope probes.

[cdev]

DOH!

thank you that may work.

[AndersJ]

Yes, invert and add channel 2.

[suicidaleggroll]

Could you elaborate on this, do you mean use the two channels inputs on both sides of the resistor and invert[/b] one?

Most (all?) scopes will let you plot the difference between the two channels, should be in the math menu or similar.

[yashrk]

As suggested putting a resistor in the loop is the simplest way of doing it, also you can use hall effect sensor ACS712 which will give a sine signal with its center at VCC/2. Depending upon the current, the peak voltage will increase or decrease.
Check this link for more info <http://www.instructables.com/id/How-to-Measure-AC-Current-using-Hall-Effect-Sensor/>

[Brutte]

Isolated hall sensor is not a bad choice.
And with lower common mode voltage, if you need bandwidth then you can use a raw opamp. Of course reference has to be tied to scope's GND.

[cdev]

I need something more suited to milliamps or perhaps even microamps (data) (also faster- there may be higher frequency signals )
I don't need raw current measurement in amps..

I do have a bunch of older opamps in my junk box.. don't know if they are still good.

[Brutte]

I need something more suited to milliamps or perhaps even microamps (data) (also faster- there may be higher frequency signals )

You need to know the bandwidth, the differential mode and common mode voltages first.
If it is 1uV Ethernet frame on a 10kVAC then obviously it can be more challenging than a 1V 9600_8N1 frame on a 24VDC.

[cdev]

Maybe I can tell something from the audio domain..