Hi,
(some back story) so I'm working on a 3 phase active PFC & rectifier sistem. I have gotten to the point that I need to start looking more closely at the current that is being drawn by my rectifier. For that reason I looked into getting a current probe. The first thing that I saw, was that even for the lowest end Rigol probe I would have to spend 800 euros. For something better I would have to spend a couple of 1000 euros. Like the Agilent solutions are 2-4 K euros for 1 probe and another 1K for the power supply for the probe.
As I'm starting to run out of organs to pawn, I needed something more reasonably priced. So I looked for alternatives and found Testec to have reasonably priced probes but I could not get any to test. I was offered a Pintech PT-2720 DC-150 kHz for testing. I also got a old AM503 w/ P6303 probe to test form a guy who sells old test gear that he gets form liquidation auctions. So let the
begin.
First I saw that the AM503 w/ P6303 probe was not working as it should. There was some DC offset that could not be nulled out and the range switches were crusty. The Pintech worked well right out of the box. I did some testing to see the dc accuracy. I let both probes on for 30 min to let them warm up - this goes especially for AM503 w/ P6303 that drastically improves noise and performance when it is warmed up. The AM503 get noticeably warm. I set my DP832 power supply to 0.1, 0.5, 1, 2, 3 A and look at the reading from the probes. The Pintech had the old probe beat with accuracy easily. That I had some 60 hz mains on a dummy load. The Pintech beat the old probe again with lower noise flor - tighter trace on oscilloscope. But this could partly be because of the Pintech low bandwidth of 150Kh compared to the 15MHz of the P6303 and the around 100 Mhz of the AM503 amp. Then It came a time to run some real life test with the probes. And this is what I got.
Trace 1 is the AM503 w/ P6303 trace 3 is the Pintech probe trace 4 is the PWM of the 1 transistor in phase 1. Both the probes are on the same phase in the same physical location.
Picture 1 and 2.
As you can see if on picture 2 this is where the Pintech probe lost. It oscilates bad while the old AM503 w/ P6303 is showing what I expect to see form the waveform. When the PWM is on the current increases and then slowly drops back and it dances around a perfect sin wave. Just as it should work. While the Pintech probe when the transistors start switching goes all over the place.
So I talked a bit with the guy that lend me the Pintech probe and he suggest putting it into a aluminium foil, just for test.
Picture 3 and 4.
Are showing the waveforms from the burrito probe. (note the AM503 w/ P6303 is not wraped). Some of the oscillations are gone but quite some remain. The probe oscilates even when the neighbouring phase switches.
Conclusion: It seems that the Pintech probe is a good, portable, low priced choice, that works better right out of the box and is probably a good choose for hobbyists, beginners and people who will not measure current near a 10 KW 3-phase PFC. But for me the 1976 vintage AM503 w/ P6303 looks like the only option. Too bad I did not have a Rigol or Testec probes to also test, if they suffer form the same problems. But I have a suspicion that they do, as they share the same basic design. The amplifier and the analog circuitry is in the probe and then there is a standard high impedance output to the scope. But the P6303 does not have almost any circuitry in the probe it self and the amplifier can be (was in my case) around 1.5m away form the noise and it drives a 50 ohm controlled impedance output into a 50 ohm terminated scope.
Best regards,
Marko
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