Measuring current through a sense resistor with high supply voltage

[providev]

Hi,

I have a query on a DUT test requirement in which we are having a high supply voltage (500V). This voltage is supplied through the C and E terminals of a MOSFET. The gate is gradually powered on (0-15V) which switches on the device and current starts to flow through the sense resistor (100mOhm). The voltage accross the sense resistor is measured and current calculated. We would need to measure the gate voltage when a particular current starts to flow. The maximum current that will flow through the sense resistor is 5A.

My query is whether we are able to use a Multifunction DAQ in differential measurement mode to measure this signal without damaging the card or the connected PXI chassis. In theory, the voltage accross the DAQ terminals would not exceed 0.5V but I'm not too confident since there are high voltages and currents present within the test.

Any advise or feedback from someone with experience in this is really appreciated.

Cheers!

[JÞB]

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@sanka wrote:

Hi,

I have a query on a DUT test requirement in which we are having a high supply voltage (500V). This voltage is supplied through the C and E terminals of a MOSFET. The gate is gradually powered on (0-15V) which switches on the device and current starts to flow through the sense resistor (100mOhm). The voltage accross the sense resistor is measured and current calculated. We would need to measure the gate voltage when a particular current starts to flow. The maximum current that will flow through the sense resistor is 5A.

My query is whether we are able to use a Multifunction DAQ in differential measurement mode to measure this signal without damaging the card or the connected PXI chassis. In theory, the voltage accross the DAQ terminals would not exceed 0.5V but I'm not too confident since there are high voltages and currents present within the test.

Any advise or feedback from someone with experience in this is really appreciated.

Cheers!

---

This is one of those time you do not want to use an un-isolated input channel.  In a PXI form you will not get more than a +/-42Vdc range (going differential you still cannot exceed 42V with respect to ground without damaging the input protection of the MFD

Actually 3 possible solutions.  use a DMM (they have a 600DVC range), use an optical isolator.  or put your shunt in the return path.

[providev]

Hi Jeff,
Thank you very much for your reply. Is my understanding correct that even though across the DAQ channel +ve and -ve there's only a small voltage; the high voltage with respect to ground will damage the system?

Could you explain where I should put the shunt in the 'return path' method?

Thanks again!

Cheers!

[JÞB]

1. Yes, the DAQ device will be dmaged if any terminal is presented with a potential greater than the rated voltage with respect to the DAQ ground.  The DAQ device is refeanced to "earth ground" though the PC it is driven from.

the DAQ sees no more than - I * Rsns Volts with repect to GND (the supply cannot be ground refeanced or this shorts Rsns) but most power supply outputs are not ground referanced except by external connection.

[Henrik_Volkers]

Depending on the input impedance of your DAQ (or add a low bias current buffer) you can add 2 R as current limiter and Z-diodes to for clamping or use a crowbar for protection

http://www.radio-electronics.com/info/circuits/scr_overvoltage_crowbar/scr_over_voltage_crowbar.php

(But look at the leakage current)