Active Dummy Load
Category: Analog Circuits
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is designed and simulated in Proteus 7.10. This is a simple circuit for making low cost discrete solution as active dummy load by using power transistor and zener diode. Before understanding this circuit, it is recommended that you should see the previous post "Zener Diode as Dummy Load". The working of this circuit is given below. 1. In this circuit, you will learn two concepts: a) How a capacitor is used to divide mains AC voltage (220Vac rms or 110Vac rms). b) How a zener diode and a power transistor is used as to make actuve dummy load to test a power supply. 2. For selection of capacitor you should know at least the input voltage and the current. 3. Then find its resistance (reactace) X = V/I, V can be 110V or 220V. 4. Value of capacitor can be calculated by using this formula C = 1/(2*pi*f*X) f can be 50Hz or 60Hz 5. When the mains power is applied to the circuit, the voltage drops across the capacitor by V = IX. 6. Which means there should be almost constant load (current) and constant mains voltage, otherwise the output voltage will vary with respect to the load or mains input voltage. 8. Use series circuit (a 100W bulb is placed in series with the mains) for safety to test this circuit. 8. Use this discrete active load as your dummy load, because by adding power transistor, this active dummy load has the ability to withstand with fluctuations in voltage and current as compared to just zener diode. 9. The wattage and Vce voltage of power transistor should be considered according to the current and voltage fluctuations. 10. Once you make this circuit and tested successfully with active load, then remove series lamp and active load and place your real load across resistor R2. 11. The R1 (1M ohm 2W) and R2 (100K ohm 0.5W) are placed for safety, to discharge the capacitors C1 and C2 when the circuit is OFF. 12. There are many types of capacitors, use polyester x class capacitor of 400V. 13. The power transistor should be placed on heat sink. 14. The wattage of resistor R4 should also be considered according to the current and power dissipation across R4. 15. The impedance of zener diode is few ohms and current through it is very small as shown in previous circuit "Zener Diode as Dummy Load". Therefore, a transistor is added to reduce the impedance by its (HFE) gain. 16. Sumarizing the concept of this circuit is; the impedance of active dummy load circuit (across R2) is equal to the impedance of Zener diode divided by transistor gain. Application: 1. This circuit can be used to derive 12VDC fan from mains voltage. 2. This circuit can also be used to derive 12V LED buld or LED strip or a single power LED of 12V. Precautions: 1. THIS CIRCUIT IS DANGEROUS, BECAUSE THERE ARE LIVE VOLTAGES AT THE CAPACITOR, SO BE EXTRA CAREFUL WHILE IMPLEMENTING THIS CIRCUIT. 2. USE SAFETY GLOVES BEFORE USING OR TURN ON THIS CIRCUIT. 3. DON'T MAKE AND RUN THIS CIRCUIT ALONE. THERE SHOULD BE SOMEONE NEAR TO OBSERVE YOU WHILE YOU ARE WORKING ON THIS CIRCUIT. Before practical you can download these files and perform simulation experiments for more understanding.