Hi,

I have what should be a simple circuit but it is not behaving as expected.

A power supply is connected directly to a load ( used for a MAS spectrometer). The power supply is 900V DC with a user variable current of 30-200nA. I don't know anything about the load ( so black box ). The manufacture won't tell me where in the power supply circuit the current is being measured but we suspect it is not right at the output. I would like to measure the current coming from the power supply to the unit.

I put a resistor in series between the power supply and the unit (on the low side). Using a battery powered DMM I wanted to measure the voltage across the resistor to get the current. The DMM has a resolution of 0.01mV on the 100mV range ( Keysight U1241C).

From Ohm's Law I expected that 50nA through a 1k would give: ( V=50*10^-9 * 1*10^3 = 0.05mV). I also tried 10k ( V=0.5mV ). For both I get a reading of 0.

Should I just try a 100k ( or higher) or is there something else I'm missing? Is there something in the DMM data sheet I missed? As I increase the value of the resistor, I will have to start taking into account the resistance of the meter to be able to make accurate current readings.

For mV AC the sensitivity is 15mV ( assuming it has to be higher than 15mV before it shows a reading) but there is no mention for DC.

Would there be a better way to measure the current?

Thank you!

 

Yes.
50 nA x 1k Ω = 50 μV = 0.05 mV.
You need and amplifier to increase this voltage to within the range of your DMM.
Or you can use a 24-bit ADC which can resolve lower than 1 μV.

 

Yes.
50 nA x 1k Ω = 50 μV = 0.05 mV.
You need and amplifier to increase this voltage to within the range of your DMM.
Or you can use a 24-bit ADC which can resolve lower than 1 μV.

An amplifier is a good idea.
Would the DMM with a resolution of 0.01mV not be able to show 0.05mV? (or at 10K 0.5 mV?)
Tomorrow I'll see if I have a power supply that can go that low and test the DMM, see where it cuts out.

 

If you are only taking 200 nA @ 900 V your load would be higher than 4.5 GΩ.
Your DMM would have an internal resistance of 10 MΩ.
You can increase the current sense resistor to 100 kΩ. The DMM would alter the measurement by only 1%.
At 1 MΩ the DMM would have a 10% effect.

Or get a Keithley electrometer. We have one of these in the lab.

 

With that 900 volts of common mode on your shunt resistor the amplifier will need some serious insulation indeed. That alone will be quite a source of pain. A battery powered DC amplifier with a gain of 1000 will allow using a 0-10 or 0-5 milliamp meter, in a well insulated isolated package to see what is happening. It would not be suitable for long term use, but to know what is actually happening it would work well.

 

I put a resistor in series between the power supply and the unit (on the low side).

The series resistor is on the low side, not at 900 V.

 

Often putting the shunt resistor on the low side of a power load works very well. But there are safety considerations because if the resistor becomes disconnected, then the entire "low side" connections portion can approach the high voltage level, creating a serious safety hazard.

 

Putting a 1000Ω shunt across s 1 MΩ meter shunts away 99.9% of the current that could operate the voltmeter. For a 40nA current, just put the voltmeter directly in series with the device and it will read 40mV. If it is a 10 MΩ meter it will read 400mV.

 

Once again, my big concern is the 900 volt common mode voltage, and the need to assure adequate insulation and isolation of the meter circuit wiring.At the very low load current it is a high impedance circuit and also subject to noise pickup.

 

Mass spectroscopes consist of five basic parts: a high vacuum system; a sample handling system, through which the sample to be investigated can be introduced; an ion source, in which a beam of charged particles characteristic of the sample can be produced; an analyzer, in which the beam can be separated into its components; and a detector or receiver by means of which the separated ion beams can be observed or collected.

There are instruments that may involve more than being completely familiar with the manual and beyond
the care and maintenance by the client. A factory technical diagnostic specialist and the terms of warranty.

 

Now I am wondering as to why there is an effort to measure the current.Is the connected device not functioning correctly?? or not appearing to function at all??