Simple 230V monitoring using PC817 opto coupler
11 Oct 2022 - tsp
Last update 11 Oct 2022
3 mins
Warning

So first off this is an article that requires an extensive warning:
- The circuit described in this article are operating from line power.
Even though this being the safer side of these projects working with line power
equipment can result in deadly shock. Be sure to disconnect everything before
working with line voltages.
- Only do such stuff when you know what you are doing - working with line voltages
is not a beginners project even if it looks simple.
- Make sure you know your local regulations. In most countries one is not permitted
of operating custom built line powered equipment on the public power network.
- Make sure you know how building such equipment effects your insurances.
The schematic
Sometimes one needs to monitor the 230V line that equipment is attached to. For
example when ones experiment is running on an USV that offers no useful interface (neither
via serial port nor via Ethernet) but one wants to gracefully shut down equipment in
case a longer power outage happens (and the USV will most likely run out of
capacity earlier or later). To do this one can use a really simple circuit
built out of an cheap opto coupler, a diode and
some $\frac{1}{4} W$ resistors.
The most simple circuit that one can build consists exactly of that:

Keep in mind that the 230V are effective voltage so the peak voltage is
around $230V * \sqrt{2} \approx 325V$. This means that the current flowing over
a $235 k\Omega$ resistor would be around $1.38 mA$. This doesnβt sound much
but converted into power $P = U*I = 325 V * 0.00138 A \approx 0.4485 W$. This is
too much for most quarter Watt resistors one has lying in ones drawer usually.
This is the reason for the two $470 k\Omega$ resistors in parallel - so they would
be perfectly capable of up to $0.5 W$ peak.
Since the forward voltage of the PC817 opto coupler is 1.25V the voltage dropping
over the two resistors can be estimated to be around 325V. Note that this is not a
hard limit - the line voltage drifts up and down so the voltage might be a few tens of
volts higher or lower at any time. The forward current at 325V would be at most $1.4 mA$
which is sufficient to be detected on the secondary side.
The PCB that this prototype has been built has been cut clear below the opto coupler and
the two 230V lines are spaced for at least $2.5 mm$ without any copper in between them.
The diode is necessary to prevent the 230VAC being active across the opto couplers
terminals - the maximum reverse voltage allowed is $6 V$. Since the diode conducts
whenever the opto coupler does not the voltage over the opto coupler is also limited
to the forward voltage of the diode - in case of the 1N5364BG used in this circuit (thatβs
a Zener diode but in this case used as ordinary diode in forward configuration) that would
be $1.2 V$ thus way below the $6 V$ allowed by the opto coupler.
As one can see supplying the circuit with $5 V$ and recording the waveform using
an oscilloscope one can recover the half-wave of the 230V signal:

This article is tagged: Tutorial, DIY, Electronics, Physics, Basics, Measurement