Electronics > Repair
PV inverter - Voltage reading circuit
fzabkar:
--- Quote from: webgiorgio on January 12, 2019, 12:37:41 am ---If the V+ equals V-, then it is like we have the 10 k connected together.
The voltage divider looks like 10M 10M 10k - 10k 10M 10M.
In theory the voltage I should see on the scope across the 20 k is:
V_LN*2*sqrt(2) * (20k / 20020 k) = 650 mVpp
--- End quote ---
Shouldn't that be …
V_LN*2*sqrt(2) * (20k / 40040 k) = 325 mVpp
webgiorgio:
--- Quote from: fzabkar on January 12, 2019, 01:49:16 am ---Shouldn't that be …
V_LN*2*sqrt(2) * (20k / 40040 k) = 325 mVpp
--- End quote ---
you are right, I was mistaken. Then the O-amp is not doing its job to keep V+ equal V- ! However the voltage indicated in the display of the inverter for V_LN is correct. mmmh
The "unkown" ST 24I EZX 6111 is used for the V_LN which is measured correctly and (if my guess on the pinout is correct) its input resistance is 1.13 Mohm. I call it U1.
Above it, there are U2 and U3 that are OPA2277U K.
U3 is used to measure V_L1-L2 (which is measured wrongly).
I measured the input resistance on U2 and U3, and found it different! Same chip, different input resistance.
U2: 312 kohm
U3: 4.12 Mohm
Shouldn't they be equal? I know I am measuring in circuit, but...
The datasheed say 100 Mohm 3 pF. :palm:
I expected something like 400 k (like the OPA270 from Analog (not on the board) that is also a precision op-amp).
U2 has differential input configuration, with two 10k resistors, and the feedback is 12k (instead of 75 k).
I promise you the schematic (so far) today
webgiorgio:
Schematic so far attached.
While checking the various connection with the continuity test, I also measured resistors (because is more comfortable than reading the small numbers).
Two resistors don't match the markings!
There are several 10kohm, but there are two that measure 75 k, despite being marked like other resistors :horse:
Still I can't figure out much of the lowest chip (U1), but at least I found out the supply pins thanks to the pinout I've got for U2 and U3.
fzabkar:
FWIW, here is a quad op-amp with a similar marking.
TS924ID, STMicroelectronics, RRIO current quad op-amp, SO14, marking 924I:
https://www.st.com/resource/en/datasheet/ts924.pdf
Note that its Vcc+ and Vcc- pins are swapped when compared with your circuit diagram.
webgiorgio:
Thanks.
So, I replaced those two resistors with 10k resistors. The old resistors read 100k measured out of the circuit.
The grid voltage (L - N) is still measured correctly, with U1.
The inverter voltage (L1 - L2) is still measured wrong, with U3. (Hence I get E031 error).
So, the problem might be U3 (it cost 8€).
Before going trough the effort of de-soldering it, I would like to see its output.
I want to see the voltage at:
- the output of U1, top left pin (I guess)
- the output of U3, top left pin (I have the pinout)
I could float the scope, and attach the ground clip to the 1.65 V pad.
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