Products > Test Equipment
Hantek CC-65 AC/DC Current Probe Teardown and Testing
jrf:
R24 is just a surge current limit to protect the op-amp U4-2. Good work on calculating why we need a 10sec press to get a good zero.
The 10M R269 may be another 'commissioning' part. It may be a 'bypass' leakage current resistor for the op-amp & without it the capacitor may actually change. Size & location do not make sense.
Regardless if the zero offset is only +/-5mv then the drift is slow & workable, once the capacitor has been charged for a time. Unfortunately it discharges exponentially, so will drift faster initially, & not good for larger offsets. ie when magnetised.
A better way to Zero is as mentioned on my Schematic & as used on all original & many new clamps. ie a potentiometer.
VR7 was probably the original zero adjuster before the PB capacitor was added. ie simply add a wire from the wiper of VR7, replaced with a potentiometer with shaft, to the U1-1 side of the Push Button, & remove/disable the PB. It may be possible with resistors to range adjust the new VR7 to be single turn.
john.
CDaniel:
In theory with the cap discharged if you can tweek the pots for zero output and measuring right positive and negative currents then you have the basic adjustments right .
So far not so easy in my case because the 2 hall sensors are not perfectly equals
dcac:
It might be an idea to experiment with the value of R24 (3.3K). I’m pretty sure it can be lowered to perhaps 1K or so for a faster setting time.
But it seems this circuit was mostly intended short time measurements. If you want stability over a longer period you probably, as already mentioned, need to remove/replace C4 and rely on the trim pots. And when measuring with a DMM you can then also use REL to clear the offset.
CDaniel:
Not very sure how the pot could work without some modifications , since the circuit is made to zero the output with no input , that's why is momentarily connected . With the button pressed you can't measure , it is just showing zero , when released and the wire removed from clamp it will show that last current value as the new zero level .
If you don't want that function just remove R23 and use REL all the time .
jrf:
CDaniel,
When balancing the halls, VR5-6, the idea is not to zero the output but to nullify the difference as the clamp is rotated about a post.
Any change within 2-5mv is probably reasonable.
Once adjusted then Zero, Calibrate the clamp.
Replace VR7 with a pot mounted for shaft access externally. Connect wiper to output of PB going to Zero Circuit Cap. Done!
As I mentioned you may use balancing resistors to change the voltage to/from the pot to achieve a good central zero position on average using a single turn pot. (actually one could just solder wires to either side of VR7 & a third, wiper, to the 470uF side of the PB & connect to an external 10K pot to test. Do not use the PB as will not work! Range +/-0.25V I think giving +/-25mv zero adjust.)
From memory the voltage across the Cap is 10x the zero offset voltage required. ie 10mv zero requires 100mv on cap.
VR7 ranges from +/-0.5V, by my calcs.
John.
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