I was commissioned to solve a problem with an off the grid house’s hot water system. The house is fed from solar power with a diesel generator for backup / peak demand use. The diesel generator is manually started on a timer during the night to boost the hot water. Unfortunately on days where the house is unattended the diesel generator keeps running even after the water heater has switched off wasting fuel and increasing motor wear.
I was asked to create a circuit which monitors the current draw of the heater and triggers a relay a couple of seconds after the current drops. Oh and it had to be cheap so into the parts bin I dug.
The schematic is quite simple. A current transformer provides a small voltage across the input of the OPAMP U1. U1 applies a gain of 100 despite what the comment in the schematic says and is referenced to GND which effectively rectifies the incoming sine voltage. U2 is a comparator which compares the incoming signal to a threshold voltage set by the ratio of R5/R6. Towards the peaks of the sine wave U2 will output +24V. The bigger the voltage (clipped by the high gain of U1) the larger the portion of the sinewave above the threshold voltage. The output of U2 is effectively 24V for a large signal, 0V for no signal, and a PWM of values in between.
D1, R7 and C1 form an envelope detector. Once the PWM signal is large enough the voltage at the base of the Q1/Q2 darlington pair begins to rise until the current starts flowing through the relay coil and it reaches its turn-on voltage. When the incoming signal drops C1 is slowly drained through R8 until the turn-off voltage of the relay is reached several seconds later.
The circuit is built on a perfboard and glued into a small flange mounted enclosure. It’s not pretty but it is functional. What’s even less prettier is the home made current transformer. A current transformer has a single cable pass through the middle of the core (effectively the primary) and a secondary coil wound around the transformer and terminated into a load transistor. In this case a doner transformer had its core cut out and replaced with ~150 turns of wire. The active cable going to the water heater is passed through the same gap as the wire turns. In theory this works well, in practice this DIY job is atrocious. Poor magnetic coupling, poor choice of transformer design, a miscount in the number of turns needed, and it’s a wonder why it works at all. But it does work which is what is important. Even better the above can be considered a feature as due to the poor magnetic coupling it is almost impossible to arc over the secondaries when the transformer disconnected open-circuited (something HV power engineers would say is “Sub-optimal” ™.
Ugly but functional describes the circuit. It does its job and it does it well behaving almost exactly as simulated above. When the current drops the output current through the relay steadily drops until it is unable to hold in any longer and the relay is opened in turn stopping the generator and saving a lot of diesel in the process.