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Hantek CC-65 AC/DC Current Probe Teardown and Testing
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toli:
Thanks, that now matches my calculations much better :)

BTW, don't forget the current consumption of the sensor bias. This can be a few mA per sensor (the spread is quite wide and sensitive to temperature) and we have 2 of them.
dcac:
Yeah it was your calculation that made me check the simulation again.

The simulation only includes active elements U3-1-2 and U4-1 to do the Noise and Frequency response, so none of the Hall, DC control or Regulator current is accounted for.

dcac:
I measure CC-65 total current draw to 10mA at 9.6V. So upgrading U3 really shouldn't make that much difference.

A rechargeable battery really is preferable in any case - else it will kinda ruin your day if you forgot to switch the probe off.
toli:
My probe arrived. I'm yet to make any significant measurements on it, and no mods yet (will need to order parts first). However, I've probed inside of it for a few minutes just to fill some gaps I've had.
First, as far as the LDO stability is concerned, I don't see any oscillations at the output. Nor do I see any oscillations at the bias of the sensors which U1-2 regulates. This is good, although I still don't like this structure of the bias circuit, it has 2 poles which can cause instability or at the very least noise peaking which might still be inside the BW of the amplifier after it. I will try to remember to probe this with the audio measurement setup, to observe the spectrum there, might reveal something more interesting that way.

The current draw of my unit is ~11.4mA, which is higher than the 10mA you've measured, but its expected there would be variation among units, especially with the value of the sensors resistance varying so much. Still, not too impressive life time out of it.

What I found interesting is the value of the DC rails. The positive rail I've measured at 3.22V, so ok for a 3.3V LDO with slightly lower voltage than nominal (I see the schematic marks it as 3.2 nominal, in which case its almost spot on). The negative rail though is -3.8V, so D4 might be a 3.9V variant (I see the schematic you've posted is marked with 4.2V, also possible as these have a fairly soft knee and a few percent tolerance - I admit I was too lazy to check). Why do they plan for such a significant voltage for the negative rail? The bias of the sensors, and as a result the CM voltage at the inputs of the op-amps are derived from positive rail, so I'd expect them to shift the rails in favor of the positive rail if anything, not the negative rail. 3.2V+3.8V = 7V, so if we leave a bit of headroom the battery must have >7.2V for normal operation, seems like inefficient use of battery energy.
Additionally, because of the way the low battery threshold is sensed, the positive rails needs to drop quite a bit for this indicator to light up. So this only happened at 6.8V in my unit at which point the positive rails was already 0.3V lower than nominal at 2.95V. This means that performance might degrade even before the low battery indicator lights up.

Might be of value to replace D4 with a lower voltage shunt regulator (just so its sharper than a zener, but a zener is ok too), perhaps 2.5V for instance. Then move the voltage sensing for low battery to 3.5V instead of 3V. Then 3.5V+2.5V would allow operation with a 6V supply instead of 7.2V.

It'll probably take a little while before I order and get the parts for mods, but I will post if I see anything interesting before any changes are made to it.

BTW, what I found quite funny is that if the battery voltage drops slightly lower than the value designed as the limit, the LED turns off. Probably because U6 has very limited ability to drive positive output voltage, so that combined with the LED drop is too much. So at 6.8V it lights up, but at 6.5V you can barely see it  :-DD
toli:

--- Quote from: dcac on April 03, 2021, 09:19:15 pm ---I replaced C4 with a 1000uF/10V high quality cap

--- End quote ---
Did you use a bipolar cap or still a polarized cap?

I see the original capacitor is polarized, which means the negative voltage range is very limited if we want leakage to remain low. However, if we go to a bipolar cap, we can live with an increased voltage over this cap,
which means we can reduce its weight in the circuit (increase value of R23). This will make it less sensitive to leakage of this capacitor. There's still the effort of finding a cap with low leakage though.
Edit: its possible to do the same trick with a polarized cap obviously, but then there a risk of the reverse voltage being large enough to cause increased current.
If we go too far we might need to increase R269 too, and then there's the question of how the leakage of the cap depends on the voltage over it when the applied voltage is so much lower than rated voltage :)
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