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| Need help: spikes in current measurement signal conditioning circuitry |
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| balage:
Hi All, Background: I am developing an industry 4.0 power meter. I can measure the power consumption of the machine on one phase, it can calculate everything based on the measurements, and send everything to the server for further analysis. Basics: I have done the field tests, and have found serious problems. You can see the problematic part of the schematic down below. S1 is a LEM HAIS50-P type hall-effect current transducer. U5 and the surrounding resistors form a subtractor amplifier, so the resulting voltage will be in the range of -0,635V - 0,635V (in case a -50A to +50A input current) and relative to the GND1 ground. This subtractor amp is required because the transducer outputs the voltage on a 2,5V reference potential. Problem: I have connected the circuitry to the machine, one of the phases go through the current transducer. The whole device is powered from the line (L1 and neutral). Once the machine, that draws around 5 amps, is turned on, spikes generated between TP1 and TP2 test points. It seems that the transducer produces them. The problem is that they seems to be pretty huge, more than 20Vpp, while the power meter can tolerate 250mVpp. The IC seems have survived until now, but it will not live long I guess. Here is one of those spike zoomed: Question: How can I suppress these spikes? What kind of filter circuitry should I put to avoid them? Have you ever faced a problem like this? I should have visited the field earlier to investigate the behavior. Thank you very much for your time. |
| balage:
I was thinking on this problem. I cannot understand how can I even decide if these spikes are coming from the current transducer or coming from the ACDC brick. I does not answer if I measure at different point as the disturbances are everywhere. |
| SiliconWizard:
First you'd have to make sure the spikes are really there and do not come from the probing itself. Then we haven't seen your layout either... |
| duak:
I have used the LEM current sensors that produce a current output rather than a voltage output like this one. The LEM sensors are very good at rejecting noise so I suspect another cause. I agree with SiliconWizard that the voltage pulse is a measurement artifact caused by the transient current conducted by the oscilloscope probe. The transient current is converted into a differential voltage that is shown on the oscilloscope. Try making a common mode choke by looping the oscilloscope probe cable through a high mu core as in the picture. The small snap on cores will help but for a problem like this you will need are larger core with higher permeability (> 500 H/m) to present enough common mode impedance to limit the current into the probe. If you have more cores, you can try adding them to other probes and to the AC line cord. Is the power meter IC on this board or do the output signals go to another assembly? |
| balage:
Thanks for your replies. I have attached the layout. U1 is the power meter IC type CS5463 from Cirrus Logic. You can see the wirings of the 3 phases. It may be also problematic that the high current wires run close to the electronics. I have repeated the measurements in my home lab, so the conditions are not the same as on the field. Now I can only test with a one phase load, there is an heating resistor dummy load, that draws 5 amps, and there is a hairdrier. The scope probe is not connected directly, but with a 20 cm long RG174 wire to mitigate the pickup. See the pic. When the whole device was plugged in the following transient was generated. Otherwise the output signal of the whole current meas circuit is like the following. The signal looks basically the same with the resistive load, but with higher amplitude. No change in noise. But here comes the spikes: transients are generated when I turn the either the resistive load or the hairdrier. E.g. here is a transient by the resistive load. I'm tired like hell, tomorrow I will solder another coax cables to different point of the PCB to see if the spikes are the same in time and in amplitude everywhere. I will also have some ferrite cores as duak suggested. |
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