Show All ItemsStep 1: Things that you will need... Things that you will need to make this power supply is...Piece of veroboardFour 1N4001 diodesLM7812 regulatorTransformer that has an output of 14v - 35v AC with an output current between 100mA to 1A, depending how much power you will need. (I found a 16v 200mA transformer in a broken alarm clock.)1000uF - 4700uF capacitor1uF capacitorTwo 100nF capacitorsJumper wires (I used some plain wire as jumper wires)Heatsink (optional)You should be able you get most (maybe all) of the parts at Radio Shack or Maplin.« Power transformers may be obtained from old radios, which can usually be obtained from a thrift store for a few dollars (or less!). The radio would also provide the power cord and plug necessary for this project. Line cord switches may be obtained from a hardware store. If you want to be absolutely sure what kind of transformer you’re getting, though, you should purchase one from an electronics supply store. If you decide to equip your power supply with a fuse, be sure to get a slow-acting, or slow-blow fuse.

Transformers may draw high “surge” currents when initially connected to an AC source, and these transient currents will blow a fast-acting fuse. Determine the proper current rating of the fuse by dividing the transformer’s “VA” rating by 120 volts: in other words, calculate the full allowable primary winding current and size the fuse accordingly. Lessons In Electric Circuits, Volume 2, chapter 1: “Basic AC Theory” Lessons In Electric Circuits, Volume 2, chapter 9: “Transformers”This project involves the use of dangerous voltages. You must make sure all high-voltage (120 volt household power) conductors are safely insulated from accidental contact. No bare wires should be seen anywhere on the “primary” side of the transformer circuit. Be sure to solder all wire connections so that they’re secure, and use real electrical tape (not duct tape, scotch tape, packing tape, or any other kind!) to insulate your soldered connections. If you wish to enclose the transformer inside of a box, you may use an electrical “junction” box, obtained from a hardware store or electrical supply house.

If the enclosure used is metal rather than plastic, a three-prong plug should be used, with the “ground” prong (the longest one on the plug) connected directly to the metal case for maximum safety.window ac unit for walk in cooler Before plugging the plug into a wall socket, do a safety check with an ohmmeter. carrier ac unit costsWith the line switch in the “on” position, measure resistance between either plug prong and the transformer case. split unit air conditioning maintenanceThere should be infinite (maximum) resistance. If the meter registers continuity (some resistance value less than infinity), then you have a “short” between one of the power conductors and the case, which is dangerous! Next, check the transformer windings themselves for continuity.

With the line switch in the “on” position, there should be a small amount of resistance between the two plug prongs. When the switch is turned “off,” the resistance indication should increase to infinity (open circuit—no continuity). Measure resistance between pairs of wires on the secondary side. These secondary windings should register much lower resistances than the primary. Plug the cord into a wall socket and turn the switch on. You should be able to measure AC voltage at the secondary side of the transformer, between pairs of terminals. Between two of these terminals, you should measure about 12 volts. Between either of these two terminals and the third terminal, you should measure half that. This third wire is the “center-tap” wire of the secondary winding. It would be advisable to keep this project assembled for use in powering other experiments shown in this book. From here on, I will designate this “low-voltage AC power supply” using this illustration:

Schematic with SPICE node numbers: Netlist (make a text file containing the following text, verbatim): Here's another take on the transformerless AC line power supply, which finds use in some well-insulated, low-power devices. Our technical reviewer pointed out that Cac should be an X-rated safety type, and I think we'd both feel better if the ground symbol wasn't there! SMPS circuits offer an efficient way to reduce AC from a mains source to any desired level for powering low-voltage circuits, though this comes at the cost of components such as control ICs, switching transistors, inductors, etc. Figure 1 shows a simple way you can use more common components to step-down and regulate the AC mains to the desired low DC voltage. The reduction in AC voltage is obtained by dropping the unwanted extra voltage across a capacitor (impedance Z=1/ωC) Cac of suitable value and voltage rating. The remaining AC is drawn as rectified output through a diode bridge. So even though DC flows through the output circuit of the bridge, the voltage dropping series capacitor Cac sees an AC flowing in its remaining part of the circuit.

The value of the capacitor determines the current output at reduced voltage. A larger capacitor is required for larger output currents. A bleeder resistor (1MΩ) is set in parallel with Cac to discharge it when the AC is disconnected. The DC after rectification and filtering is shunted by the Q1-based regulation circuit, which basically tries to maintain the output voltage within certain limits. 1 This step-down converter drops AC mains voltage across Cac to produce a lower DC voltage. is optional and is related to safety issues – choose higher wattage if is the unregulated output which can be further regulated using chips like 7805/12 etc or a simple zener-transistor regulator. chosen according to power requirement – essentially it should be able to drain the unused current when the load is absent. of fuse are indicative – choose it according to your design/need. The circuit has two LEDs. The red LED indicates whether the power from AC is being used, or bled as waste through Q1.

The green LED indicates the power availability at the output where further regulating devices can be added. R1, R2, and RB (R1,R2 » RB) form a voltage divider network which essentially monitors the residual rectified AC from the bridge. Their values are chosen such that when the current is flowing through the load (not shown), Q1 is switched off and hence little current flows through the second bleeder resistor RB, limited by the large values of R1 and R2. The voltage drop across RB is not sufficient to turn on the red LED. At this point, we say the current flowing through R1, R2, and RB is the housekeeping current which constantly flows apart from the maximum load current. Q1 is biased by the voltage drop across R2, which should be at least 0.6V for Q1 to turn on. During normal operation (current flowing through the load), the values of R1, R2, and RB are chosen such that this voltage is less than 0.6V. However, if the load is disconnected (no output current is being drawn) then the voltage after the diode bridge will increase, which in turn will increase the voltage drop across R2 until the transistor turns on and draws current through RB.

This stops the voltage from increasing further, simultaneously increasing the current through the red LED. A glowing red LED indicates power wastage. However, the green LED always glows when power is available at the output. RB should be chosen such that the rise in voltage at the bridge output during no load condition is within the upper limit of any final regulator connected to +VU. The required capacitor Cac is calculated as: where IL is the maximum load current, Vrms is the RMS AC voltage, VE (~VU+1.2) is the residual expected voltage at the bridge input, which is taken to be the sum of VU and 1.2V across the bridge diodes. ΔIL is the house keeping current apart from the maximum load current. A rough estimate of Cac is IL / (ωVrms) (where ω = 2πfAC) as can be seen from the formula by neglecting ΔIL and VE, which are small compared to IL and Vrms respectively. This circuit offers an alternative to a bulky, noisy, vibration/magnetic field/heat producing transformer.

However, the advantage of the transformer is in the isolation it offers from live AC. The danger with the proposed circuit is when Cac shorts out. Precautions should be taken to see that the fuse blows out before the voltage across the output rises to a hazardous level. Under increased voltage conditions, extra current paths are offered by the zener VUZ and the filter capacitor CF . Additional cheap neon lamps can be added across the bridge input to salvage the output regulating circuit and the load. In a test set up which was designed to obtain a zener-transistor regulated 4.8V supply at 5mA current: Cac=0.068μF, R1=10kΩ, R2=470Ω, RB=470Ω, CF=470μF, and Q1=BC549, with a 240V AC line. This power supply could power a 555 oscillator driving an LED. Another test used Cac=0.22μF, R1=10kΩ, R2=470Ω, RB=470Ω, CF=470μF, and Q1=BC549. This could be used to build a zener-transistor regulated power supply of 4.8V at 15mA current. It could easily power a 555 astable, a flashing LED, and a CD4518 counter driving eight LEDs.