Reactance measurment and current sources

Hi,

I have build 32 constant current sources controlled by a DAC. Each source can provide upto +/- 1mA peak to peak with +/- 15volts compliance voltage. The sources had some DC current leakage problem which I solved using 6H inductors at the output of each source. The output becomes dependent on frequency and reactance. I could not go back and try to null the offset voltage thats why I had to use inductors. The load impedance is between 1 to 10kohm. I need to know that how much AC current is flowing through the load and how much current is flowing through the inductor practically at 10kHz. The inductor impedance at 10kHz is 370 kohm. What method should I use to measure the current? I looked into the oscilloscope current probes and they do not do that low current mesurement. I used the multimeter but it did not show me anything else except zero.. plus how can measure the reactance of the inductor at 10KHz?

Please advice!

John

Reply to
john
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hai u use shunt in which line u want measure the current across the shunt there will be voltage . the shunt rating example

10:1 in this 10A current flow 1 volt is across it . by htis method r use hall device

anand

Reply to
velans81

Shunting excess current with an inductor WILL lower the 'leakage', but it also decalibrates your current sources. Either reprogram the DAC inputs so as to compensate the current offset, or find a way to make your load less sensitive to the 'problem'. I presume the DAC updates sample/hold amplifiers for each of the outputs?

A multimeter takes tens to hundreds of milliseconds to complete a measurement, so it's unlikely to capture a current before the inductor conducts and the current disappears.

Reply to
whit3rd

Thirty-two 6H inductors to ground? Arrgghh!!

OK, trying to make do with a horrible configuration.

"Ground" each of your inductors with a "virtual ground" made from the summing junction of an inverting opamp stage, instead of the real ground they're connected to now. A voltage will be developed across the opamp's feedback resistor, at the opamp output, that you can measure to determine the inductor current.

Ahem! Have you accounted for the 6H inductor's winding capacitance? What if it's as high as 50pF? What then, grasshopper?

This winding capacitance, BTW, may force you to add a little capacitance across the opamp's feedback resistor to stabilize the feedback loop.

Reply to
Winfield

???? How in the he*** does one solve a "Current leakage" with an

*inductor* ???? Those six henry inductors must take up a *lot* of space!
Reply to
Robert Baer

Hi,

Why do I need an opamp. Can I not just resistor in series with the inductor is enough?

Regards John

Reply to
john

John, the opamp is to make a virtual ground, so the inductor will appear to be grounded, just as it is now, shorting out your current source at DC (I assume that's what you're doing now?). The resistor is in the feedback path of the opamp, carrying the current path, and the opamp output voltage lets you monitor the current without disturbing the inductor.

You can learn about this stuff using Google. Or you could get a copy of our book, The Art of Electronics.

:-)

Are you really using 6H inductors? Where did you get them? What's their internal resistance and ac loss? Did you know their ac resistance is much higher than their DC resistance? That's the whole poor Q thing.

Also, did you know you can make simulated inductors from opamps? As for me, I like to make auto-zero servo loops when I have to kill a DC offset error.

Reply to
Winfield Hill

Hi,

Yes, I am using 6H inductors. I got them from soem comapny in california . I do not remember the name. The DC resistance is 2.5kohm. I do not know the AC loss. But the plan was to get high impedancw from them at 10kHZ. So, 2 x pi x f x L = 370Kohm, the intended load is between 1 to

10kohm , so most of the current will go to the load not inductors. Now, I need to measure the current in both branches.

Can you advice in detail that how to do it efficiently?

Thanks John

Reply to
john

Reply to
john

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