Products > Test Equipment
Hantek CC-65 AC/DC Current Probe Teardown and Testing
CDaniel:
I don't have a Fluke DC clampmeter , but from videos I see that the behaviour could be better , but in essence the same , drift , magnetising effect if you pass a large current . That's why there is a zero offset button .
Yea , the idea of using 2 sensors should cancel out common magnetic fields like earth magnetic field ... for some reason not all core magnetising can be canceled this way
If the sensors are not balanced right than + and - currents are not the same ( putting the wire in the opposite way through the clamp ) , so if you adjust the pots for this than should be good for any common mode fields .
jrf:
Any magnetising of the core, within the core, will pass a common mode field through both sensors! ie a field circulating around within the core.
Hence the zero offset.
A large external field passes through the core & sensors with minimal circulating effect, passing through the sensors in differential mode, & hopefully being cancelled out. ie like parallel lines through the whole area of the clamp.
If the Hall sensors are not flat in the same plane then this field will give different signals from the sensors & be more difficult if not impossible to 'balance' out.
Small fields like a magnet close to the clamp will read more in one sensor than the other, hence giving a reading.
The description of the orientation of the sensors in my previous essay above may not be 100% but the logic & function is correct.
I avoid using the zero Push Button as it will result in reading drift with time. My USB oscilloscope has zero offset ability on its inputs so I use that.
If the offset >5mv I can just trim VR3 to zero it. A magnetised head will result in large zero offsets & significant drift if the Push button is used to 'correct' it. The push button is a poor replacement for the potentiometer, especially as it hides a magnetised head unless you short the 470uF cap or leave the unit for many hours to discharge completely. When I switch mine on it is usually within 2mV of zero. It changes 2-3mV depending on orientation & proximity to metal.
Cheers,
john.
CDaniel:
Doesn't seems logic at all to replace a pot with a push button for some bias voltage and achive good results ... more likely the voltage/current small pulse when you press the button and the output of the op amp is "high" is somehow reseting the sensors ;D ( not very logic either way )
The cap is discharged anyway by the 10M resistor and internal leakage relatively fast , so the output of the amp should go slowly back to 0V . Why would you want this effect if this a bias
dcac:
The "sample and hold" circuit used for zeroing has rather strange time constants and a I don't think I've seen an electrolytic timing capacitor before, usually you'd use a low leakage film capacitor for that.
When Zero button is pressed C4 is charged through R24, so 3.3K x 470u = 1.55s - so that could explain needing to hold the button a few seconds to achieve zero - remember you need 5x that time to charge the capacitor to >99%.
And the discharge time through R269, 10M x 470u = 4700s or about 1.5h, so a (slow) offset drift is unavoidable - unless you were lucky and C4 was charged to 0 volts. But then leakage and dielectric absorption in C4 can still cause offset drift.
CDaniel:
OK , that OUT on the inverting input is like ground and fooled me ;)
Should be a better way of compensating the drift of this amplifier
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