use low cost USB DAQ with high voltages

[Ron B.]

Trying to use the low-cost USB-6008 to measure at high voltage. Right now it works with laptop unplugged from the wall (batteries only). But laptop is floating at >300 VDC found by probing to any metal part on the laptop. I think everything would be fine if I could get an optically isolated USB interface cable or similar. There are some optically isolated usb hubs out there. Wondering if anyone has any experience or ideas.

Thanks,

Ron 

[Ron B.]

Clarification: I am measuring the voltage drop (of a few volts) across a resistor at 350 V or so. Using an instrumentation amp circuit with a gain of four to buffer between resistor and analog input of usb-6008. Only ground in the  system is the hv power source connected to the resistor. As a result, good noise behavior since no gnd loops, but usb interface and laptop are at 350 V or so relative to earth ground. I wish to continue using the 6008 but with optical isolation to improve safety and to permit the laptop to be connected to ac power (which it can't be now due to ground loop problems). Please suggest possible product or circuit ideas for isolating the 6008 from the laptop.

Thanks much,

Ron B.     

[K C]

Maybe you can solve this with a transformer between the DAQ and the resistor. Depends on frequency, waveform and what you want to measure.

[Ron B.]

Measurement is DC now and into kHz range later. Don't think tranformer will help. My thought is that it's easier to isolate the digital side of the A/D than the analog side.

[K C]

Yes, maybe it is easier to isolate the digital side but not realy safe.

A current clamp with a range from DC to everal KHz could also be a solution, but that will cost a bundle

[Chris_D]

Because of the way the USB-6008 is built, it gets is ground from the computer. If you used a USB hub you would have to find a way to supply a ground to the hub, else the 6008 will be floating.

Your best option may be to use one of National Instruments USB devices with Ch-Earth Ground Isolation.

[stilly32]

I would recommend you stop using this configuration immediately!* Floating your whole system 350V above ground is a recipe for disaster/huge safety risk. The 6008 has absolutely no built in isolation, which is what you really need. We have several products that have 250V isolation and while they may not meet your specs, they are at least still less of a safety hazard than what you are currently doing and if they burn up they at least don't short you to ground. If you want to look into using an optically isolated hub, Sealevel has a few that I've used in the past with our products. I didn't run into any performance issues (bandwidth, device detection, power on... I didn't test the isolation) with them. This still would not be the recommended solution (as we can't verify Sealevels isolation and the 6008 is not built to work with any potential arcs that could occur) but would at least insulate your laptop and presumably what you would be interacting with most.

I would strongly encourage you to re-evaluate this situation and ensure you have the right equipment for the measurement.

Andrew  S
NI DAQ R&D


* I realize this sounds preachy and you're probably following proper safety procedures as best you can, but this is the wrong HW for the situation and I would hate to lose a user/anyone to something like this. Dead developers don't do good repeat business

[Wayne.C]

I have to agree with Andrew, floating any piece of instrumentation is very dangerous.  Practice is outlawed in my lab because someone was nearly killed floating a scope a few years ago.  The guy survived but was never quite the same after.  That laptop was NEVER designed to isolate the user from that kind of voltage.  

Suggest you look at isolated instrumetation amps from Analog Devices.

[Ron B.]

I have ordered the optically isolated 4 port usb hub from B and B Electronics
(http://www.bb-elec.com/bb-elec/literature/UISOHUB4_1104ds.pdf).
According to the vendor it should completely isolate the laptop while also preventing ground loops. I will put the usb hub, the usb 6008 a/d unit and our interface electronics within a plexiglass box to prevent accidental touching of any high voltage spots. Laptop should then be safe and protected from damage. Only potential damage is to NI usb 6008, but even so it does not present a safety hazard. I have already successfully operated the 6008 for more than an hour while it was floating at > 500V.

Does this sound OK to everyone?

[Wayne.C]

That configuration should be much much safer.  Please keep us posted as to how the isolated hub works out.