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A filter circuit used in a rectifier, the value of load resistance and capacitance are 200 Ω and 15 μF. Then the value of time constant is ________.

Asked in GUJCET 2025 · Rectifiers and filters

• (1) 1.33 ms
• (2) 3 ms
• (3) 7.5 ms
• (4) 0.3 μs

Step-by-step solution

Why the other options are wrong

• (1) No slip gives 1.33 ms. The digits come from dividing instead of multiplying, R/C = (200)/(15×10⁻⁶) = 1.33×10⁷, which is not a time at all. Sanity check: the time constant is the product, RC = 3×10⁻³ s.
• (3) The digits 7.5 come from C/R = (15×10⁻⁶)/(200) = 7.5×10⁻⁸, again dividing where the definition multiplies.
• (4) No single slip produces 0.3 μs; it is 10⁴ times too short. Sanity check: RC = 200 × 15×10⁻⁶ = 3×10⁻³ s, and a filter has to hold the load up for milliseconds, not microseconds, if it is to smooth mains-frequency ripple.

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