lektor January & February 2021 67Galvanic isolation
The square wave coming from the main board is fed to the trans-
former at the right (ratio 1:1). The rectifiers give then an output voltage
of something less than 6 V. With parallel regulators this is controlled
to +/- 5 V. The windings of the transformer have an isolation value of
900 V, so the total isolation will be 1800 V peak, provided that the rest
of the construction is OK. The I2C signal will go via IC14, which has an
isolation value of 4 kV peak. So the satellite boards will be completely
floating with respect to the main board and with respect to each other.Update of the satellite board circuit
After the first prototype was built, tests proved that some correc-
tions had to be made. The crosstalk from the switching signal to the
measurement signal in the offset switches had to be limited. Therefore
C41-R102 and C40-R103 are added. The protection circuit reacted on
spikes, for example when a test lead was plugged. The sensitivity for
spikes has been decreased by adding C33. D14 and R101 also limit he
sensitivity for positive signals.When the amplifiers have the highest gain (approximately 2750x), an
offset control is needed. This control is switched off when the gain is
lower. Depending the sign of the offset a selection has to be made
with the solder jumper.will be a small current through R34, which is detected by T11 and T13.
This will trigger the thyristor circuit with T10 and T12. Once triggered
the thyristor circuit will remain in this state and will switch off T14 and
K1. Signal OC indicates to the microcontroller that the protection is
triggered. When there is no power K1 is also switched off (V-low is
disconnected), so then the circuit is safe. To restore operation the power
of the circuit must be disconnected and connected again.
The voltage divider gives 100 mV output at the maximum input voltage
of 750 V (DC), the maximum AC voltage will be about 500 V. The
voltage drop at the 6.5 mΩ current sense resistor R62 is about 100 mV
at 15 A-DC (about 10 A-AC). The amplifiers for voltage and current are
identical, so I’ll describe only the voltage amplifier. T1, T2 and T4, T6
serve as switches to connect the differential amplifier either with the
signal from the input circuit or short the input to make offset measure-
ment possible. The value of the offset can be stored and subtracted
later from the measurement to make a “clean measurement” possible.
The amplifier is a common instrumentation amplifier. T3, T5 switch
the gain of the first stage to either 1x or 25x. IC5 makes the differential
signal single ended. The last stage with IC4 can switch the gain to
either 1x or 5x. So there can be four gains: 1x, 5x, 25x or 125x. The gain
setting will be under control of the microcontroller IC13. The ampli-
fier will generate a positive or negative signal with respect to COMV.
This signal is 0.5 times VREF, which is generated by the circuit around
IC3. VREF is 4.5 V, so COMV, COMI and CINV are 2.25 V. VREF and
CINV are connected to the ADC of the PIC-controller. The ADC of
the PIC18F26K80 used here can convert to 13 bits when used in the
indicated way (12 bits plus a sign bit). For 750 V this gives a resolution
of about 200 mV per bit. For 6 V about 1.6 mV/bit. I consider this as
sufficient for this purpose.
The I2C address of a satellite board is programmed by closing or
opening SJ1 and SJ2 on the board. Closing SJ1 makes the board
channel 1, closing SJ2 makes it channel 2 and closing both solder
joints makes it channel 3.
WARNING. Working with high voltage
can be fatal. The circuit described here
is not for beginners. Do not build or use
it unless you are experienced in dealing
with high voltages!