Methods to measure floating voltage - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 16, 2010, 08:03:10 pm
content: "Differential voltage probe" vs "LEM LV25-P voltage transducer" LV25-P provides isolation voltage measurement. I assume "(High-voltage) Differential voltage probe" (e.g.Tektronix product) does NOT provide isolation voltage measurement. Please correct me if I'm wrong. If "Differential voltage probe" is safe for high-voltage floating voltage measurement, then why do we need "isolation voltage" measurement like LV25-P?Thanks.

author: Unknown
author_rank: Unknown
timestamp: September 16, 2010, 08:30:24 pm
content: Quote from: onemilimeter on September 16, 2010, 08:03:10 pmThese are very different products, the LEM transducer is for mounting on a PCB for inclusion into a product, the Tek probe is for attaching to a scope.Quote from: onemilimeter on September 16, 2010, 08:03:10 pmIsolation voltage sounds like a Hipot test, neither of the products will do that. I assume you mean isolated voltage measurement (isolating the measurement device from the voltage under test)?Quote from: onemilimeter on September 16, 2010, 08:03:10 pmBoth of the probes (banana leads) of the differential probes are isolated up to the rated voltage. Neither of them is grounded (hence differential), and can be connected to any potential within the specified limits.Quote from: onemilimeter on September 16, 2010, 08:03:10 pmYou can't really integrate a Tek probe into your product, they're probably much more expensive and much larger. But a PCB mounted device isn't very convenient for measurements on power electronics. The bandwidth of the LEM device is probably lower, and so is the rated voltage I think (can't find a spec of differential mode voltage).

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 18, 2010, 10:37:58 pm
content: Hi alm...Thanks. I'm aware that the "differential voltage probe" and "LEM LV25-P voltage transducer" are two different things.Here's my application. I got a 3-phase AC motor. I wish to measure the phase voltages and these phase voltages will be fed to a controller. The motor is controlled using SVPWM (space vector PWM at 20kHz) method. At first I think of using isolated voltage measurement method like LEM LV25-P. However, LEM LV25-P has limited bandwidth, i.e. less than 10kHz, and hence it can not capture correctly the 20kHz switching voltage waveform. We have a "differential voltage probe" and it can capture the high frequency switching voltage waveform correctly. I do not know much about the circuit insides the "differential voltage probe". Does it using the same "isolated" technology like the LEM LV25-P? If it's not how does it provide "isolated" or "floating" voltage measurement?Thanks.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 18, 2010, 11:59:43 pm
content: Found this IC:QuoteIf a difference amplifier can do the job, what do we need a "galvanic isolation"? Please share if you know an application that need "galvanic isolation" and a difference amplifier can not be used?Or, the CMRR of a difference amplifier is not high enough for some applications?Thanks.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 19, 2010, 01:03:09 am
content: Please refer to the specifications of a differential voltage probe given below. What does "Common Mode Voltage DC + pk AC: 6500V" mean?I'm interested to learn how to achieve such design specifications (e.g. 6500V, DC-50MHz). I found several difference amplifiers, e.g. INA117 and AD629, but their bandwidths are only 200kHz and 500kHz, respectively, and they are designed to use in application with the presence of common-mode signals up to ±270VDC only.

author: Unknown
author_rank: Unknown
timestamp: September 19, 2010, 01:46:11 pm
content: Differential voltage is the voltage between the two HV inputs. Common mode voltage is the voltage between either of the HV inputs and the low-voltage side (BNC connector).Don't expect the commercial probes to use a canned solution. Since these are differential probes, I don't think they're galvanically isolated, since the impedance looking into a differential amplifier is very high anyway as long as you're within the common mode range. I believe they use a matched (for CMRR) attenuator to bring the voltage down to reasonable levels, and use a differential video buffer to convert the signal to a single-ended signal. Building even the attenuator that's flat from 0-50MHz or so and matched within say 1% over the frequency range (for a lousy CMRR of 100:1) will probably be quite a challenge.An older device that was used for similar work is the Tektronix A6902B. It was an isolation amplifier (so it was really isolated, CMRR was not as good as current products, don't think it was specced) of 0-20MHz or so. With the correct (attenuating?) probes, it allows up to 3kV common mode voltage and 6kV differential mode voltage.Here is the service manual for the A6902B, large PDF with full schematics and theory of operations. It uses optical isolation for low-frequency signals (HF transformers don't work well for LF) and a HF transformer with proper drivers and compensation for high-frequency signals. I believe the TPS2000 series of isolated scopes uses the same technique. I don't think you'll be able to clone this (probably uses various custom parts and critical layout and construction), but it may give you some ideas.