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Low current measurement with high voltage

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dweinho1
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Low current measurement with high voltage

Post by dweinho1 » May 6th, 2014, 8:45 am

Hello, I own a Keithley 6485 - and am looking to measure a plasma discharge current in vacuum. I need to supply -4kV DC to the plasma discharge. Is it possible to use a Keithley Model 248 power supply to do this? If so, how would I connect it? I am not very experienced with this type of thing, and I don't want to damage the 6485.

Thanks.

Vince W
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Re: Low current measurement with high voltage

Post by Vince W » May 6th, 2014, 12:26 pm

This is a very difficult problem. The 248 power supply will provide 4kV, but you do risk damaging the 6485 if it is exposed to that voltage, even if it is connected to the low side of the power supply. The maximum input voltage the 6485 can tolerate is 220VDC. Also, we cannot recommend a way to connect both of these instruments to make this measurement, since neither has a safety interlock mechanism.

I recommend reading our application note "Breakdown and Leakage Current Measurements on High Voltage Semiconductor Devices"
http://www.keithley.com/data?asset=57867

This explains how to perform high voltage measurements and protect a sensitive leakage current measurement instrument safely using a 10kV protection module and instruments with interlocks. The instruments of choice are the 2290-PM-200 Protection Module, a Series 2290 Power Supply, and a 2634B SourceMeter.
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dweinho1
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Re: Low current measurement with high voltage

Post by dweinho1 » May 6th, 2014, 12:48 pm

So if I didn't need the interlock feature, would it be possible to use the 248 and 6485 with a 2290-PM-200 SMU Protection Module?

Vince W
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Re: Low current measurement with high voltage

Post by Vince W » May 6th, 2014, 4:46 pm

That application of the 2290-PM-200 SMU is not something we recommend. You would be entirely on your own responsibility using the 2290=PM-200 that way. As I mentioned earlier, the 248 and 6485 do not have safety interlock mechanisms so we cannot recommend them for this high voltage application.
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dweinho1
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Re: Low current measurement with high voltage

Post by dweinho1 » May 7th, 2014, 5:31 am

So it should work it just won't be safe? We have other safety precautions, so I am not concerned with the absence of an interlock, but I do not want to damage either units.

Vince W
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Re: Low current measurement with high voltage

Post by Vince W » May 7th, 2014, 7:10 am

Please understand that Keithley/Tektronix is bound to safety regulations which were not in place for the 248. Since the 6485 does not source high voltages, there is no need for an interlock, just protection. We take safety measures very seriously. Our position is that its a bad idea to risk exposure to lethal voltages. We want you to be around to buy some of our newer instruments. :)
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jehugo
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Re: Low current measurement with high voltage

Post by jehugo » July 23rd, 2014, 8:29 am

Hello,

I have a bench test wich performs current measurements under high voltage. The high voltage supply is a FuG which goes up to 35kV and the current is measured from a 30m length miniature cable with the 6514 electrometer. I had some results but i need some help to prevent damages on my electrometer, which i had to return twice already for repair. My main concern is about the circuit protection, should i need one and how would it be (resistance value and what kind of diode)? Is there any already-made circuit protection for the 6514? Which cable is more appropiate for the measurement, the triax cable or the crocodile ?
Thanks for your help,
Regards.

Vince W
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Re: Low current measurement with high voltage

Post by Vince W » July 31st, 2014, 5:03 pm

Hello jehugo, sorry for the delay.

We do not make a protection module which is rated for up to 35kV, or one specifically designed for the 6514. Our 2290-PM-200 protection module is designed for our 10kV power supply 2290-10 or our 5kV power supply 2290-5. The 2290-PM-200 protection module is designed to protect an SMU like the 2636A which will tolerate up to 200V.

Since the 6514 Triax input can tolerate up to 250V, it should work with the 2290-PM-200 as a current measurement instrument for a low-side connection to your device which could be supplied on the high-side by our 5kV or 10kV supplies. You would connect the 6514 INPUT to the 2290-PM-200 HI IN connection using a triax cable. Then connect the LO IN connection to the COMMON using a 237-BAN cable. Connect one of the 2290-PM-200 OUT connections to your device, depending on whether you are using the 5kV or 10kV supply. Here is the manual for the 2290-PM-200: http://www.keithley.com/support/data?asset=57843

The 2290-PM-200 is not rated to work with your 35kV supply.
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LGSilveira
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Re: Low current measurement with high voltage

Post by LGSilveira » December 16th, 2014, 10:03 am

Greetings,

I am using a 6514 electrometer to measure the resistivity vs temperature of a insulator ceramic sample inside a cryostat. Using the ohmmeter function I could successfully measure the resistance of the sample from room temperature to 100 K. Bellow 100 K, the resistance is higher than 200 Gohms, the upper limit of the electrometer. As an alternative, I am trying to use an external DC voltage source (SRS P375 - up to 20kV) and measure the current with the electrometer. The measurement setup is as follows: voltage source HI terminal is connected to one electrode on the sample by a coaxial cable, another coaxial cable connects the second electrode to the center conductor of a triaxial connector. The inner shield of the connector is connected to the LO terminal of the voltage source. These connection are made through a shielded circuit, in contact with the carcass of the triaxial connector. The triaxial connector is connect to the electrometer by a triaxial cable. Until this moment, I applied 250 V and could measure the current between 100 and 65 K. Bellow 65 K the current is too low and the measurement gets too noisy. I would like to know if I can apply over 250 V without risking to damage the electrometer.

Thanks

Dale C
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Re: Low current measurement with high voltage

Post by Dale C » December 16th, 2014, 10:43 am

The Model 6514 can measure to 0.1fA(1e-16A) with about 3.5fA of offset current.
So raising the voltage is a possibility, but not sure you need to do so.
The input of the Model 6514 can handle up to 250V.
250V / 250G ohms is 1nA.
Model 6514 can certainly measure lower current that this and be stable.
You should be able to do 1,000 times this resistance at 250V with no trouble.
250V / 250T ohm = 1pA.
Model 6514 should be able to measure this with the proper shielding, cabling and time.

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