Latching Overvoltage Indicator Handles Large Overloads

A single resistor with suitable power and voltage ratings could be used instead of R7 to R9. But connecting three resistors from the MRS25 series (0.6-W, 250-V) in series guarantees that the parts will easily withstand a mains overload. The resistance values should be great enough to satisfy the 0.6-W power rating and ensure the zener's power dissipation is kept low. Using large values also minimizes the circuit's current draw when V_IN exceeds the zener voltage. Yet the values shouldn't be too big, otherwise, IC1's V_DD current will cause a relatively significant voltage drop. This would degrade (increase) the circuit's minimum working voltage.

When determining which values for R7 to R9 will be able to withstand mains overload, remember that D3 effectively half-wave rectifies the mains waveform. Therefore, even though the peak voltage at R7 will equal 1.414 × V_RMS, the RMS voltage at this point will only be V_RMS/2. For example, if V_RMS is 240 V, each resistor will see just 40 V_RMS. So selecting values of 3.6k will dissipate 0.44 W in each resistor. The resulting power dissipation in the 11-V zener would be 0.12 W.

With the circuit subjected to mains overload, the op amp's output briefly rises toward V_DD on each positive cycle. It then settles down to the proper, lower value required to regulate the current sink. Ordinarily, this phenomenon would result in a large RMS voltage across R11, leading to an excessive LED current and excessive power dissipation in Q2. Fortunately, this problem is easily remedied by connecting Q3 across R11 as shown. Q3 normally has no effect on the current sink. But under mains overload conditions, Q3 keeps Q2's rms collector current below 2 mA. In doing so it ensures that the power dissipation is well within the ZTX458's 1-W limit.

The trip voltage, V_T, is given by:

V_T = V_REF(R1+R2)/R2 (volts)

so: R1 = R2(V_T − V_REF)/V_REF (ohms).

R1 and R2 should be as large as possible to minimize the current drawn from V_IN and the power dissipation under overloads. The voltage ratings of R1, R2, and R3 must be able to withstand the maximum overvoltage at V_IN.

A prototype circuit was built with R1 = 300k and R2 = 100k, equivalent to a nominal V_T of 4.80 V. With V_IN = 4.70 V (circuit untripped), the total current draw was just 20.6 µA. The circuit tripped when V_IN exceeded 4.78 V. In the tripped state, the circuit's minimum working voltage (below which the LED current began to fall under 1 mA) was found to be V_IN = 2.94 V.

The LTC1541 could be replaced by the pin-compatible MAX951, if required. Note, however, that the MAX951's maximum working voltage is only 7 V (9 V absolute maximum), requiring a lower zener voltage for D4.

If narrow transients on V_IN cause nuisance tripping, a capacitor across SW1 will provide some immunity, but at the expense of response time.