Friday, September 26, 2014

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Simple Over Current Indicator

This circuit eventually surfaced while pondering over the design of a current indicator for a small power supply. Fortunately, it proved possible to employ the supply voltage as a reference by dividing it down with the aid of R1 and R2. C1 is an essential capacitor to suppress noise and surges. The half supply voltage level is applied to the non-inverting pin of opamp IC1. The value of the R3 determines the trip level of the indicator, according to

R3 = 0.4 × (desired voltage drop) / I trip

Actually this is high side sensing but the method can be used as low side sensing, too! The desired voltage or sense voltage can be any value between 0.35 V and 0.47 V. If currents greater than about 1A are envisaged, you should not forget to calculate R3’s dissipation on penalty of smoke & smells.

Another voltage divider network, R4, R5 and P1 divide the voltage between supply voltage and desired oltage. This divided voltage, filtered by C2, is fed to the inverting input of IC1 to compare levels. The result causes D1 to light or remain off. Turn P1 to the end of R4 to hold off D1. Then connect a load causing over current and adjust P1 towards the end of R5 until D1 lights. The accuracy of the circuit depends entirely on the tolerances of the resistors used - high stability types are recommended.

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