How simple measure current on 300kHz?

[asrock70]

I need to measure the current in the circuit, it may not be entirely accurate, it is rather to determine when the current has reached its maximum.
Parameters
Voltage +- 50V
Current 1-20mA
Frequency 300kHz

 

[WimRFP]

How much voltage drop can you accept across your current meter?
galvanic isolation required?
sinusoidal or square wave?
what will be the readout device (ADC, just volt or current meter, etc)?
accuracy?

 

[asrock70]

Sinusoidal, no galvanic isolation , readout device, Agilent 34401.
What is it
Picture shows a simplified equivalent circuit.
C3 + L1 form a series resonant circuit
C2 is parasitic and L2 is tune inductor.
The goal is
Xc2=XL2 and frequencies is resonant frequencies of L1+C3.
At that moment, current will be approximate maximum.
I'm looking for a simple way to be able to track the progress of current when tuning frequency and L2

 

[schmitt trigger]

What is the purpose of R2 and R4?

I understand R1 is the inductor's equivalent series resistance...which by the way, at that frequency will be different from the DC resistance.

 

[WimRFP]

Very likely the frequency range of the DVM doesn't go to 300 kHz.

I would go for a (shielded) current probe and an oscilloscope. If you don't have an oscilloscope you could use a current probe with a two diode (voltage doubling) rectifier, but that requires serious design as you have low current level.

You can make a current probe yourself from a high permeability ferrite with sufficient secondary windings that go to a 50 Ohms input of an oscilloscope. Of course you can also load the current transformer with a resistor and use a high impedance probe. You can't make the current ratio too small, otherwise your signal may dissapear in the noise.

Don't take for granted expensive looking current probes. When the impedance level at the measurement position is relatively high, the current probe may respond to voltage also. That is why I prefer shielded current probes (own construction) for probing high impedance circuits. If you have access to the return (ground) wire, you may put the current probe around the return wire reducing respons due to voltage.

 

[chuckey]

I was going to say put a one ohm resistor in series with the earthy end of L2 and look at the volt drop across it with a CRO.
Frank

 

[firewhale]

Or with a serise resistor & rms2dc convertor ic

 

[carpenter]

Agilent 34401 can measure AC voltage up to 300kHz. So you need to convert current to voltage, I thing 1 or 10 ohm resistor.

 

[WimRFP]

@Carpenter: Is this true for all measurement ranges (included mV range)? I had some meters in the past that didn't measure correctly at the lowest voltage range (and it was mentioned in the manual).

@asrock70: If you add a series resistor somewhere in the circuit, be aware of differential to common mode conversion issues. You may meausure something that is actualy not across the current sensing resistor.

 

[carpenter]

Accuracy Specifications for 34401A (6.5 digit) say True RMS AC voltage for 100mV range and frequency 100kHz-300kHz have error 4%+0.5% (% of reading + % of range).