Bridge Rectifier With a Missing Diode
A technician repairs a 9 V, 500 mA regulated power supply built from a 12 V RMS transformer, a four-diode bridge, a 1000 µF reservoir capacitor, and a 7809 regulator. The output is only 6.8 V average with about 3 V peak-to-peak ripple, and the ripple frequency is 50 Hz. The transformer is rated 12 V, 1 A and does not overheat. A scope shows that the ripple repeats every 20 ms, and the DC level falls badly near the valley. The load is a radio that draws 500 mA. The technician has a DMM with a diode-test mode, a spare 1000 µF capacitor, and a box of 1N4007 diodes. The workshop mains is 50 Hz. The service manual states that the 7809 needs at least 11.5 V at its input to stay in regulation. The case is plastic, and the regulator is bolted to a small heatsink.
1. Explain how one open diode produces 50 Hz ripple and a low average output. Calculate the ripple for full-wave and half-wave operation with C = 1000 µF and I = 500 mA.
Read the worked answer
Full-wave: the capacitor charges every 10 ms, so Vpp = I/(2fC) = 0.5/(2 × 50 × 0.001) = 5.0 V.
Half-wave with one diode open: the capacitor charges only every 20 ms, so Vpp = I/(fC) = 0.5/(50 × 0.001) = 10.0 V.
The peak is still about 15.6 V, one diode drop below the 16.97 V transformer peak, so the valley falls to about 5.6 V. The 7809 needs 11.5 V, so it drops out; the output collapses to about 6.8 V and the ripple appears at 50 Hz. The transformer does not overheat because the average current is lower.
2. Calculate the minimum reservoir capacitance for the 7809 to stay in regulation at 500 mA with the full-wave bridge and a 12 V RMS transformer. Use Vpeak = 16.97 V, two diode drops = 1.4 V, and Vmin = 11.5 V. Decide whether the existing 1000 µF is adequate.
Read the worked answer
Vpeak = 12√2 = 16.97 V; after the bridge, 15.57 V. For regulation: Vmin = 15.57 − Vpp ≥ 11.5 V, so Vpp ≤ 4.07 V. C ≥ I/(2fVpp) = 0.5/(2 × 50 × 4.07) = 1.23 × 10⁻³ F = 1230 µF.
The existing 1000 µF gives 5 V ripple and only 10.6 V minimum, so it is inadequate at 500 mA. Use 2200 µF, which gives 2.27 V ripple and 13.3 V minimum. At low mains (−10 %), the peak falls to 15.3 V and the minimum to about 11.6 V with 2200 µF, which is marginal; 3300 µF or a low-dropout regulator gives more margin.
3. Describe how to test the bridge in-circuit with a DMM diode-test and what replacement diode ratings are needed. Mention one safety precaution.
Read the worked answer
Power off, unplug, and discharge the 1000 µF capacitor through a 1 kΩ, 2 W resistor; verify below 5 V before touching.
In-circuit diode test is unreliable because the transformer and other diodes provide parallel paths. Lift one end of the bridge or remove it. A good diode reads 0.5–0.7 V one way and OL the other; a shorted diode reads near 0 V both ways; an open diode reads OL both ways. Test all four.
Replace with a bridge rated at least 2 A/400 V, or four 1N4007 diodes. The 1N4007 is 1 A average, 1000 V repetitive peak, and 30 A surge. It is adequate for 500 mA but has little margin for surge; a 2 A bridge is better. Check the capacitor's ESR and replace it if swollen.