The terminator is more than a 50 ohm resistor. I honestly don't know how well it'll work with only a 50 ohm. Took me a while to find a good one on eBay. I paid about $300+, probe + terminator.
An alternative is to use the Tektronix 134 Current Probe Amplifier instead of the Passive Terminator. I think they are more common on eBay and cheaper too.
An alternative is to use the Tektronix 134 Current Probe Amplifier instead of the Passive Terminator. I think they are more common on eBay and cheaper too.
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That Tek terminator just appears to be an equalizer for high frequencies. The pickup should work without all that. Years ago a friend used to take the convergence yokes off of the older color tv set sets (the ones with RGB dot trio phosphor) and use the three U-shaped ferrite assemblies with coils on that as current transformers. I'm not sure what kind of bar he used to close the top with once the conductor was inside. I think it was scribed snapped and sanded off pieces of old AM radio ferrite bars, and ferrite from tv/monitor flyback transformers. Scraps out of older PC switching supplies may work too. Besides ferrite in other transformers, some also have a small current transformer for over-current detection. What he was doing was all low frequency though. The frequency range needed is a big factor in cost and size. There has been some talk in one of the curve tracer threads about current sensing of plate and grid currents (to points generally at high voltage with respect to ground). Not much bandwidth is needed, but it should work to D.C. and it is best to be fully isolated and not to load the circuit with much capacitance.
Guys, I would like to see a bit more whats going on in my tune rectified high voltage power supply and Jac from Jacmusic/Emission Labs gave me the hint to get in any case a current probe to see what is really going on.
I seached for the Tek 6021 .... but just see offers like 600Euro/750$...wow. Is there a cheaper option which is not DIY ?
I seached for the Tek 6021 .... but just see offers like 600Euro/750$...wow. Is there a cheaper option which is not DIY ?
I appreciate this thread was resurrected by Blitz, and can point to a LEM device solution that could be of use.
I have a few semi-vintage LEM LA25NP each on a custom pcb. One is powered from a 12V battery through 7805 regulator and 0515D dc/dc with extra hf filtering that is set up to generate 1V/A output signal with a 5 turn primary (5Arms primary rated capability).
I haven't pushed testing to the higher primary current levels yet, as the 0515D peak current limit will kick in, and I would need to be mindful of the output signal level.
With the output signal in to a soundcard (emu0404) I get a -100dB noise floor below 180mArms signal, which was consistent down to a -60dB noise floor below a 1.8mArms signal from my test oscillator. The emu0404 provides 2Hz to 96kHz bandwidth, which is the limitation as the LEM module has dc-150kHz -1dB bandwidth.
With that 5-turn primary configuration, the LEM datasheet indicates the transducer is adding about 0.6uH and 6 milliohm, which is likely minor to how the transducer would be practically inserted in a current loop for test purposes.
The battery supply avoids any hum signal ingress. The primary voltage isolation ratings are acceptable for amp output stage plate current measurements, but I'd suggest making sure there was some output transformer winding over-voltage protection added such as a MOV to reduce stress if the output was overloaded and caused leakage inductance spikes, and with the caveat that it would be best to not use an expensive soundcard for that particular type of testing.
I haven't looked at LEM website for what is available nowadays, so maybe there are better solutions.
I have a few semi-vintage LEM LA25NP each on a custom pcb. One is powered from a 12V battery through 7805 regulator and 0515D dc/dc with extra hf filtering that is set up to generate 1V/A output signal with a 5 turn primary (5Arms primary rated capability).
I haven't pushed testing to the higher primary current levels yet, as the 0515D peak current limit will kick in, and I would need to be mindful of the output signal level.
With the output signal in to a soundcard (emu0404) I get a -100dB noise floor below 180mArms signal, which was consistent down to a -60dB noise floor below a 1.8mArms signal from my test oscillator. The emu0404 provides 2Hz to 96kHz bandwidth, which is the limitation as the LEM module has dc-150kHz -1dB bandwidth.
With that 5-turn primary configuration, the LEM datasheet indicates the transducer is adding about 0.6uH and 6 milliohm, which is likely minor to how the transducer would be practically inserted in a current loop for test purposes.
The battery supply avoids any hum signal ingress. The primary voltage isolation ratings are acceptable for amp output stage plate current measurements, but I'd suggest making sure there was some output transformer winding over-voltage protection added such as a MOV to reduce stress if the output was overloaded and caused leakage inductance spikes, and with the caveat that it would be best to not use an expensive soundcard for that particular type of testing.
I haven't looked at LEM website for what is available nowadays, so maybe there are better solutions.
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I just checked performance with up to 2.5Arms - no sign of distortion or other interaction from power supply to LEM.
The noise floor from the soundcard allows discrimination of isolated measurement current signal levels down to 1uArms, so a pretty neat probe tool for current measurement in audio gear.
The noise floor from the soundcard allows discrimination of isolated measurement current signal levels down to 1uArms, so a pretty neat probe tool for current measurement in audio gear.
I was just reading the Tektronix P6022 manual -- and the "duhhhh?" moment occurred -- you can increase the "implied" current into the probe by increasing the number of turns -- if you loop the wire twice instead of 10mA/V the scale is now 5mA/V.
P6022 is flat from 10kHz to 100MHz, but the chart in the manual indicates that the sensitivity decreases linearly from 10kHz to 100Hz in the 10mA range (and 1kHz in the 1mA range), so you could create a calibration curve to compensate for the falloff.
P6022 is flat from 10kHz to 100MHz, but the chart in the manual indicates that the sensitivity decreases linearly from 10kHz to 100Hz in the 10mA range (and 1kHz in the 1mA range), so you could create a calibration curve to compensate for the falloff.
good to a few hundred kHz -- probably need something with less sensitivity but greater bandwidth for measuring SMPS, linear power supply impedance/stability. fwiw, most of the problems I see are in the range of 100kHz to 3MHz.
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