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Sunday, October 14, 2012

Changing Voltage 220 VAC to 24 VDC Without Transformer

The following schematic is an electronic circuit to convert voltage of 220 VAC to 24 VDC without a transformer. The price can be reduced as we do not need to purchase and install the transformer, and the circuit is also smaller size and lighter weight. Power efficiency can also be higher, because the transformer has power loss with efficiency of about 89% at full load, and about 65% at half load.

For circuit with transformer please read "Power Supply, Battery Charger, With Transformer".

The circuit above is connected directly to the high voltage 220 VAC, so we should install a fuse. I use a 0.5 ampere fuse though slightly oversize. As far as I know the smallest type of glass fuse available on the market is 0.25 amperes, but sometimes it is hard to find. This fuse protects the high voltage section only, the circuit on the left of transistor. While the low voltage circuit is not protected as it produces a very small output current about 20 milliamps, so if that output is shorted it will not brake the fuse.

In principle, this circuit is the voltage stabilization circuit by a zener diode.  The output voltage will not be affected by input voltage fluctuation, as long as the input voltage is greater then zener voltage.

Read also laptop charger can be used to recharge car battery.

Input voltage will affect maximum output current. The higher the input voltage, the greater maximum output current, so if the input voltage decreases the maximum output current decreases. If the load requires current flow under maximum level, then when the input voltage drops will not affect the load voltage, or the output voltage. So, this circuit can work at a voltage of 220 VAC and 110 VAC. For 110 VAC input voltage, if you want to produce the same maximum current as with the input voltage 220 VAC, change the resistor values ​​of RB and RC to be half the value of which are described below.

I use a diode bridge (D), because it was available already in my stock. Actually diodes must be adjusted to the capacity of current and voltage. 4 pieces of 1N4007 diode can do, which is able to transmit current up to 1 ampere and a maximum voltage of 1000 volts.

Condenser 1 (C1) is 1 microfarads capacity and should have a maximum voltage above 220 volts. I use a 350 volts maximum voltage.

Base resistor (RB) will determine current fed to the base of the transistor. I use a 390k resistor at the base, so that the emitter current of transistor will be about 20 milliamps. If that base resistor is reduced, then the emitter current will be greater. If you need bigger emitter current, make sure the transistor is not too hot. If you do not have a thermometer to measure transistor heat, there is a simple way to determine whether the transistor is not too hot, by touching transistor with your finger. Make sure the transistor is not too hot and still can be touched by finger after loaded in about a minute. Before touching the transistor with your finger, make sure the high voltage is disconnected.

Collector resistor (RC) will protect the transistor and the load if the current rise above the limit. I use 10 kilo ohm resistor to limit output or emitter current of about 20 milliamps. This resistor can be reduced to increase the current.

Zener diode (DZ) determines the output voltage of the emitter. I use a 24 volt zener with power 1/2 watt. Note that the output voltage of transistor emitor will be reduced 0.6 volts. So if you use 24 volt zener, then the output voltage will be 23.4 volts. You can change this zener diode according to your output voltage requirement. The voltage produced is quite stable.

Transistor (T) is TIP50. According to the specification can transmit up to 2 watts continuous power. This transistor is able to work with the collector-emitter voltage of 400 volts (Vceo). DC current gain (hFE) transistor in the circuit is about 39. Needs to ensure manufacturer specification of TIP50 transistor that you use. This transistor needs a heatsink. Although for a current of 20 milliamps and voltage output of 24 volts will not make this transistor hot. But since we are working with high voltage, fatal incident will be happened if something goes wrong. Transistor is susceptible to overheating and can burst or explode in less than 1 (one) second, if there is a wrong connection when assembly and causing overload or short circuit. Transistor heatsink is connected to the collector so it has high voltage when circuit is connected to the power grid.

If you need bigger current above 100 milliamps, you can use bigger high voltage transistor 2SC3527 and connected as Darlington pair. Darlington pair is two transistors with paraleled collectors, the base of the larger transistor is supplied by smaller transistor’s emitor, the output is taken from the bigger transistor’s emitter. Darlington pair is activated by supplying a small current to the smaller transistor base. In other words, the Darlington pair is smaller transistor supplying bigger transistor base to generate large current. Output voltage will be reduced by 1.2 volts from zener voltage when using a Darlington pair. Resistor RC value can be lowered.

Download: Specifications or datasheet of TIP50 from Fairchild Semiconductor transistor.

Output condenser (C2) filters output voltage fluctuations . I use a 50 volt 4.7 microfarads condensor.

Resistor at the output (RI) prevents high voltage spike induced by load which can damage the transistor, when used to supply inductive load. RI value is 10 kilos. This resistor can be omitted if not using an inductive load. This resistor can be replaced with a zener diode. If using a zener, it can protect the equipment if any increase in output voltage of transistor, and only needs a very small leakage current. The minimum voltage of output zener is equal to the zener on transistor’s base, in this case 24 volts. Maximum output zener voltage is 1 volt above the base zener, in this example is 25 volts. Zener installation is unlike normal diode, zener is installed upside down, because it just works or connects when zener voltage is exceeded. Zener cathode (negative) is connected to positive output, and zener anode (positive) is connected to the negative output. I use a resistor because it is more flexible, does not need to be replaced even though the zener on base of the transistor is changed.

WARNING: Do not touch the circuit when connected to high voltage, make sure you are using insulated electrical equipment, make sure the fuse is installed before the circuit is connected to the high voltage.

As the circuit is very simple, it can be assembled on matrix board.

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