Circuit Description

This circuit design is to monitor the average current in an AC line-connected load and to illuminate an LED if it exceeds a specified level.

 

 

LT6703-2 AC Line Overcurrent Indicator Schematic

µPower, Low Voltage Comparator with Reference

The basis of this design is the LT6703-2 which combines a micropower, low voltage comparator with a 400mV reference in a tiny package. The device only draws 6.5μA, making it ideal for monitoring systems.

A signal from a low-side¹ current sense resistor is passively rectified and filtered (actually the low-sided is arbitrary, it depends on where it’s grounded) and then connected to the input of the LT6703-2 comparator plus reference. If the signal exceeds the 400mV reference, the comparator turns on the LED. It’s not too complicated.

There is also an output-latch circuit that is a transistorized SCR. The LED will stay on, to “remember” that something happened. Otherwise you might miss it!

Efficient VCC Regulator with Power Factor = 0

You can “sock it to the man” i.e. the power company with the VCC supply. It’s a basically a Zener diode shunt regulator, but with a half wave bridge rectifier, and a capacitor to drop AC voltage, instead of an uncool (needlessly dissipative) 5 Watt power resistor. The current leads voltage by by 90 degrees in the capacitor, so “real” power dissipation is zero (“real” power is defined as the RMS voltage*current*cosine (90 deg.), i.e., zero). The electric company hates it (in theory) since they have to deliver current that doesn’t count toward billable kWH. In the real world it’s okay because the overall power level is so low, and the current waveform isn’t highly distorted (as compared to a pulsed waveform with a high peak value, which is probably even worse than an out -of-phase sine wave).

The main design issue is to get the right cap. A 200V ceramic cap would work, but they’re kind of expensive. A film cap is the right choice. The part indicated in the simulation works nicely on the breadboard. The 7 Ohms ESR doesn’t create any appreciable heat, carrying just a handful of milliamps.

LTspice Simulation

The load is set up as a Behavioral resistor, demonstrating an “undocumented” feature of LTspice. The resistance is inversely proportional to a pulsed CTRL voltage source with slow rise and fall times, to ramp the AC load current up and down.

Sense Resistor

Working backwards from the LT6703-2 input, with a 400mV threshold for the average signal voltage – we can divide by 0.637 to get the peak voltage (assuming a sinusoid), and add ~215mV to account for the forward drop in the Schottky diode, and also divide by 0.5 since it’s only a half-wave rectifier, to get the peak AC voltage on the sense resistor required to turn on the LED. This in turn can be related to the load current by the sense resistor value.

Breadboard Test

Below is the test setup; circuit built on copper-clad board with 75W light bulbs as load.

 

 

Line Overcurrent Indicator Test Setup

DANGER! Lethal Voltages Present. Operation by High Voltage Trained Personnel Only. Since this is connected to live AC powerline, lethal voltages are present. Don’t touch it and don’t connect a scope probe without an isolation transformer; don’t even think about it! Build and test after you are trained on high voltage operations.

Results

The image capture below shows the input voltage and current waveforms. The peak of the current waveform (bottom trace) occurs as the voltage waveform crosses zero, indicating a near-90 degree phase angle.

 

 

Line Overcurrent Indicator Results

¹ Low side, since the sense resistor is connected to the ground of the DC bias supply. It could also be called “hot side” since it’s also connected to one (either) side of the AC line.