Practical case: Basic rectifier filtering

Basic rectifier filtering prototype (Maker Style)

Level: Basic. Demonstrate how a capacitor smoothes ripple in a half-wave rectified signal.

Objective and use case

In this practical case, you will build a half-wave rectifier circuit and observe the effect of adding a filter capacitor in parallel with the load.
* Why it is useful:
* Essential for converting Alternating Current (AC) from the mains into Direct Current (DC) for powering electronics.
* Used in simple battery chargers.
* Fundamental concept for audio signal demodulation (envelope detectors).
* Demonstrates energy storage properties of capacitors in power supplies.
* Expected outcome:
* Input: A pure AC sine wave (swinging positive and negative).
* Step 1 Output: A pulsing positive-only signal (half-wave rectification).
* Step 2 Output: A steady DC voltage with slight variation (ripple) after connecting the capacitor.
* Target audience and level: Students and hobbyists understanding basic AC/DC conversion.

Materials

  • V1: 10 V (peak), 50 Hz sine wave source, function: AC power input.
  • D1: 1N4007 diode, function: rectifies AC to pulsating DC.
  • R1: 1 kΩ resistor, function: acts as the electrical load.
  • C1: 100 µF electrolytic capacitor, function: filters voltage ripple (stores energy).
  • GND: Ground reference (0 V).

Wiring guide

Construct the circuit following these node connections:

  • V1 (Source): Connect the positive terminal to node VAC and the negative terminal to node 0 (GND).
  • D1 (Rectifier): Connect the Anode to node VAC and the Cathode to node VOUT.
  • R1 (Load): Connect between node VOUT and node 0 (GND).
  • C1 (Filter): Connect the positive terminal to node VOUT and the negative terminal to node 0 (GND). Note: Initially leave C1 disconnected to observe the unfiltered signal, then connect it.

Conceptual block diagram

Conceptual block diagram — LM7805 Half-Wave Rectifier w/ Filter
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ AC SOURCE ]            [ RECTIFICATION ]             [ OUTPUT STAGE ]

                                                          +--> [ C1 Filter ] --> GND
                                                          |    (100 uF)
    [ V1 Source ] --(VAC)--> [ D1 Diode ] --(VOUT Node)-->+
    (10 V, 50Hz)              (1N4007)                     |
                                                          +--> [ R1 Load ]   --> GND
                                                               (1 kOhm)
Schematic (ASCII)

Measurements and tests

Perform the following steps using an oscilloscope or a multimeter:

  1. Input Verification:
    • Connect the probe to VAC.
    • Verify a sine wave oscillating between +10 V and -10 V.
  2. Unfiltered Rectification (C1 Disconnected):
    • Remove C1 temporarily.
    • Measure VOUT. You should see only the positive half-cycles of the sine wave (approx. 0 V to 9.3 V due to diode drop). The voltage drops to zero between peaks.
  3. Filtered Rectification (C1 Connected):
    • Connect C1 across R1.
    • Measure VOUT. The signal should now be a DC voltage near the peak value (approx. 9 V) that does not drop to zero.
    • Vripple Measurement: Set the oscilloscope to AC coupling to zoom in on the small voltage fluctuation («sawtooth» shape) on top of the DC line.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Basic rectifier filtering

* --- Components ---

* V1: 10 V (peak), 50 Hz sine wave source
* Connected: Positive -> VAC, Negative -> 0 (GND)
V1 VAC 0 SIN(0 10 50)

* D1: 1N4007 diode (Rectifier)
* Connected: Anode -> VAC, Cathode -> VOUT
D1 VAC VOUT 1N4007

* R1: 1 kΩ resistor (Load)
* Connected: Between VOUT and 0 (GND)
R1 VOUT 0 1k

* C1: 100 µF electrolytic capacitor (Filter)
* Connected: Positive -> VOUT, Negative -> 0 (GND)
* Note: Included to demonstrate the filtering effect described in the case.
C1 VOUT 0 100u
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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* Basic rectifier filtering

* --- Components ---

* V1: 10 V (peak), 50 Hz sine wave source
* Connected: Positive -> VAC, Negative -> 0 (GND)
V1 VAC 0 SIN(0 10 50)

* D1: 1N4007 diode (Rectifier)
* Connected: Anode -> VAC, Cathode -> VOUT
D1 VAC VOUT 1N4007

* R1: 1 kΩ resistor (Load)
* Connected: Between VOUT and 0 (GND)
R1 VOUT 0 1k

* C1: 100 µF electrolytic capacitor (Filter)
* Connected: Positive -> VOUT, Negative -> 0 (GND)
* Note: Included to demonstrate the filtering effect described in the case.
C1 VOUT 0 100u

* --- Models ---

* Standard silicon rectifier diode model approximation for 1N4007
.model 1N4007 D(IS=7.03n RS=0.04 N=1.85 CJO=10p VJ=1 M=0.5 BV=1000 IBV=10u TT=5u)

* --- Analysis Directives ---

* Transient analysis: 100ms duration (5 cycles of 50Hz) with 0.1ms step
.tran 0.1ms 100ms

* Operating point analysis
.op

* Print directives for simulation logging
.print tran V(VAC) V(VOUT)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Show raw data table (1017 rows) 
Index   time            v(vac)          v(vout)
0	0.000000e+00	0.000000e+00	-2.77024e-22
1	1.000000e-06	3.141593e-03	3.430255e-10
2	2.000000e-06	6.283185e-03	6.932562e-10
3	4.000000e-06	1.256637e-02	1.411758e-09
4	8.000000e-06	2.513271e-02	2.956960e-09
5	1.600000e-05	5.026527e-02	6.646271e-09
6	3.200000e-05	1.005293e-01	1.882015e-08
7	5.304087e-05	1.666251e-01	6.310202e-08
8	7.565486e-05	2.376544e-01	2.484107e-07
9	1.009625e-04	3.171298e-01	1.270798e-06
10	1.280850e-04	4.022822e-01	7.576310e-06
11	1.570209e-04	4.930958e-01	5.140208e-05
12	1.876236e-04	5.890955e-01	3.869871e-04
13	2.197798e-04	6.899101e-01	3.065854e-03
14	2.535671e-04	7.957622e-01	2.015809e-02
15	2.900907e-04	9.100857e-01	7.787813e-02
16	3.269176e-04	1.025237e+00	1.740794e-01
17	3.659101e-04	1.147010e+00	2.922342e-01
18	4.156771e-04	1.302180e+00	4.470469e-01
19	4.731074e-04	1.480844e+00	6.257990e-01
20	5.731074e-04	1.790758e+00	9.360689e-01
21	6.731074e-04	2.098905e+00	1.244455e+00
22	7.731074e-04	2.404980e+00	1.550935e+00
23	8.731074e-04	2.708681e+00	1.855020e+00
... (993 more rows) ...

Common mistakes and how to avoid them

  1. Reversing Capacitor Polarity:
    • Error: Connecting the negative leg of an electrolytic capacitor to the positive voltage node.
    • Solution: Always ensure the stripe (negative side) of the capacitor connects to Ground (0). Reverse polarity can cause the capacitor to explode.
  2. Load Resistance Too Low:
    • Error: Using a very small resistor (e.g., 10 Ω) with a small capacitor.
    • Solution: If the load draws too much current, the capacitor discharges too quickly, causing massive ripple. Increase C1 or R1.
  3. Ignoring Diode Voltage Drop:
    • Error: Expecting exactly 10 V DC from a 10 V AC peak source.
    • Solution: Account for the ~0.7 V drop across the silicon diode. Expect around 9.3 V peak.

Troubleshooting

  • Symptom: Output is identical to Input (AC sine wave).
    • Cause: Diode is shorted internally.
    • Fix: Replace D1.
  • Symptom: Output is 0 V.
    • Cause: Diode is open or connected backward (blocking positive cycle).
    • Fix: Check diode orientation (anode to source).
  • Symptom: Ripple is very high (voltage drops deeply between peaks).
    • Cause: Capacitor value is too low for the frequency or load.
    • Fix: Increase C1 to 470 µF or 1000 µF.

Possible improvements and extensions

  1. Full-Wave Rectification: Replace the single diode with a bridge rectifier (4 diodes) to utilize the negative half-cycle, doubling the ripple frequency and improving efficiency.
  2. Voltage Regulator: Add a Zener diode or a linear regulator (like an LM7805) after the capacitor to create a fixed, stable DC output regardless of ripple.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary function of the 1N4007 diode (D1) in this circuit?




Question 2: What is the role of the capacitor C1 in the circuit?




Question 3: Before adding the capacitor, what does the output signal look like after passing through the diode?




Question 4: Which component acts as the electrical load in this specific circuit?




Question 5: What is the expected output after connecting the capacitor to the circuit?




Question 6: To which node should the Anode of the diode D1 be connected in a standard half-wave rectifier configuration?




Question 7: What is the frequency of the AC sine wave source (V1) specified for this experiment?




Question 8: Why is this circuit useful for powering electronics?




Question 9: Where should the negative terminal of the capacitor C1 be connected?




Question 10: Besides power supplies, what is another application mentioned for this fundamental concept?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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