8VDC Power Supply With Over-Voltage Protection

This 8V DC power supply was designed for use with an expensive piece of electronic equipment. It features full over-voltage protection as a precaution against regulator failure, either in the supply itself or inside the equipment it is powering. The circuit uses a conventional full-wave rectifier, followed by a 3-terminal voltage regulator (REG1) with appropriate filtering. When power is applied and switch S1 is in the “Run” position, REG1’s output is fed to the load via a 500mA fuse and Schottky diode D3.

This also lights LED2 (yellow) and LED3 (green), which respectively indicate the presence of the unregulated and regulated voltages. D3 is there to protect the circuit against external voltage sources (eg, charged capacitors). A “crowbar” circuit comprising ZD1 and SCR1 provides the over-voltage protection. It works like this: if a fault develops (eg, REG1 short circuit) which causes the output voltage to rise above 9.1V, ZD1 turns on and applies a voltage to the gate of SCR1

Circuit diagram:

Figure 1 8V DC Power Supply With Over-Voltage Protection Circuit Diagram

If the voltage then continues to rise, SCR1 turns on (at about 10V) and “blows” the fuse. Zener diode ZD2 provides emergency over-voltage protection in case the “crowbar” circuit develops a fault. Switch S1 is provided so the operator can occasionally test the “crowbar” function. When S1 is switched to the “Test” posi­tion, the load is disconnected by S1b and the unregulated supply voltage is applied by S1a to the “crowbar” circuit, thus causing it to trigger. When this happenS, LEDs 2 & 3 (green and yellow) extinguish and LED1 (red) lights to indicate that the SCR has triggered. The SCR turns off again when S1 is switched back to the “Run” position.

Author: L. Cox – Copyright: Silicon Chip Electronics

Transformerless Power Supplies

There are several ways to convert an AC voltage at a wall receptacle into the DC voltage required by a microcontroller. Traditionally, this has been done with a transformer and rectifier circuit. There are also switch- ing power supply solutions, however, in applications that involve providing a DC voltage to only the microcontroller and a few other low-current devices, transformer-based or switcher-based power supplies may not be cost effective. The reason is that the transformers in transformer-based solutions, and the inductor/MOSFET/controller in switch-based solutions, are expensive and take up a considerable amount of space. This is especially true in the appliance market, where the cost and size of the components surrounding the power supply may be significantly less than the cost of the power supply alone.

Transformerless power supplies provide a low-cost alternative to transformer-based and switcher-based power supplies. The two basic types of transformerless power supplies are resistive and capacitive. This application note will discuss both with a focus on the following:
A circuit analysis of the supply.
The advantages and disadvantages of each power supply.
Additional considerations including safety requirements and trade-offs associated with half-bridge versus full-bridge rectification.

Read Online

Power Supply For USB Devices

More and more equipment is sold that runs off internal rechargeable batteries. Although a matching charger is usually supplied in the package, there are also devices that can only be charged via a USB port. That is not surprising in the case of USB MP3 players, which have to ‘dock’ in the PC anyway for some time for the purpose of file transferring. Still, the same ‘feature’ can be a serious disadvantage, for example, on ‘computer-free’ holidays. Sometimes it makes you wonder how simple the solutions to such problems actually turn out to be. After all, if it’s just a supply voltage we’re after, then a USB port is easily imitated.

The circuit shown here is nothing but a 7805 in a dead standard configuration. The innovation, if any, might be USB connector to which the MP3 player can be connected. The 7805 comes in different flavours most devices can supply 1 A, but there are also more advanced variants that achieve up to 1.5 A. Because a USB device is never allowed to draw more than 500 mA from the port it is plugged into, the circuit shown here should be able to supply charging and/or operating current to up to two (or three) USB devices at the same time. The input voltage may be a direct voltage of anything between 7 and 24 volts, so for use at home or abroad a simple wall cube with DC output is sufficient.

Figure 1 Power Supply Circuit Diagram For USB Devices

Another useful bit to make yourself might be a cable with an inline fuse and a cigarette lighter plug so you can tap into a vehicle supply (note that this may be up to 14.4 V with a running engine). At an output current of 1 A and an input voltage of just 7 V, the 7805 already dissipates 2 watts. Assuming you’re using the most commonly seen version of the 7805, the TO-220 case with its metal tab will have a thermal resistance of about 50 °C/W. Also assuming that the ambient temperature is 20 °C, the 7805’s internal (chip) temperature will be around 120 °C. In most cases, 150 °C is the specified maximum, so ample cooling must be provided especially in a car and with relatively high input voltages.

12VDC to 9VDC Converter

12V to 9V DC Converter
Posted Apr 20, 2013 at 12:21 pm

To get a more precise output voltage, replace zener diode Z1 with 10V and R1 with a 1Kilo ohm potentiometer. A Coolrib for Q1 is optional but highly recommended. You can replace Q1 for a more robust type to get more output amps depending on your requirements. Simple circuit to power your 9 volt cassette recorder and other stuff.

Parts List:
R1 = 560 ohm
C1 = 1000uF/40V, Electrolytic
C2 = 10uF/25V, Electrolytic
C3 = 330nF, Ceramic
Z1 = 9.1V, 1watt zener
Q1 = ECG184, NTE184

Transformerless 5 Volt Power Supply

An increasing number of appliances draw a very small current from the power supply. If you need to design a mains powered device, you could generally choose between a linear and a switch-mode power supply. However, what if the appliance’s total power consumption is very small? Transformer-based power supplies are bulky, while the switchers are generally made to provide greater current output, with a significant increase in complexity, problems involving PCB layout and, inherently, reduced reliability.

Is it possible to create a simple, minimum part-count mains (230 VAC primary) power supply, without transformers or coils, capable of delivering about 100 mA at, say, 5 V A general approach could be to employ a highly inefficient stabilizer that would rectify AC and, utilizing a zener diode to provide a 5.1 V output, dissipate all the excess from 5.1 V to (230×v2) volts in a resistor. Even if the load would require only about 10 mA, the loss would be approximately 3 watts, so a significant heat dissipation would occur even for such a small power consumption.

At 100 mA, the useless dissipation would go over 30 W, making this scheme completely unacceptable. Power conversion efficiency is not a major consideration here; instead, the basic problem is how to reduce heavy dissipation and protect the components from burning out. The circuit shown here is one of the simplest ways to achieve the above goals in practice. A JVR varistor is used for overvoltage/surge protection. Voltage divider R1-R2 follows the rectified 230 V and, when it is high enough, T1 turns on and T3 cannot conduct.

Circuit diagram:

Figure 1 Transformerless 5 Volt Power Supply Circuit Diagram

When the rectified voltage drops, T1 turns off and T3 starts to conduct current into the reservoir capacitor C1. The interception point (the moment when T1 turns off) is set by P1 (usually set to about 3k3), which controls the total output current capacity of the power supply: reducing P1 makes T1 react later, stopping T3 later, so more current is supplied, but with increased heat dissipation. Components T2, R3 and C2 form a typical ‘soft start’ circuit to reduce current spikes this is necessary in order to limit C1’s charging current when the power supply is initially turned on. At a given setting of P1, the output current through R5 is constant.

Thus, load R4 takes as much current as it requires, while the rest goes through a zener diode, D5. Knowing the maximum current drawn by the load allows adjusting P1 to such a value as to provide a total current through R5 just 5 to 6 mA over the maximum required by the load. In this way, unnecessary dissipation is much reduced, with zener stabilization function preserved. Zener diode D5 also protects C1 from over voltages, thus enabling te use of low-cost 16 V electrolytics.

The current flow through R5 and D5, even when the load is disconnected, prevents T3’s gate-source voltage from rising too much and causing damage to device. In addition, T1 need not be a high-voltage transistor, but its current gain should exceed 120 (e.g. BC546B, or even BC547C can be used).

CAUTION!
The circuit is not galvanically isolated from the mains. Touching any part of the circuit (or any circuitry it supplies power to) while in operation, is dangerous and can result in an electric shock! This circuit should not be built or used by individuals without proper knowledge of mains voltage procedures.



Copyright: Elektor Electronics Magazine
Author: Srdjan Jankovic & Branko Milovanovic

12VDC Transformerless Power Supply

This circuit will supply up to about 20ma at 12 volts. It uses capacitive reactance instead of resistance; and it doesn’t generate very much heat.The circuit draws about 30ma AC. Always use a fuse and/or a fusible resistor to be on the safe side. The values given are only a guide. There should be more than enough power available for timers, light operated switches, temperature controllers etc, provided that you use an optical isolator as your circuit’s output device. (E.g. MOC 3010/3020) If a relay is unavoidable, use one with a mains voltage coil and switch the coil using the optical isolator.C1 should be of the ‘suppressor type’; made to be connected directly across the incoming Mains Supply.

They are generally covered with the logos of several different Safety Standards Authorities. If you need more current, use a larger value capacitor; or put two in parallel; but be careful of what you are doing to the Watts. The low voltage ‘AC’ is supplied by ZD1 and ZD2. The bridge rectifier can be any of the small ‘Round’, ‘In-line’, or ‘DIL’ types; or you could use four separate diodes. If you want to, you can replace R2 and ZD3 with a 78 Series regulator. The full sized ones will work; but if space is tight, there are some small 100ma versions available in TO 92 type cases. They look like a BC 547. It is also worth noting that many small circuits will work with an unregulated supply.

Figure 1  Transformerless Power Supply Circuit Diagram

You can, of course, alter any or all of the zener diodes in order to produce a different output voltage. As for the mains voltage, the suggestion regarding the 110v version is just that, a suggestion. I haven’t built it, so be prepared to experiment a little. I get a lot of emails asking if this power supply can be modified to provide currents of anything up to 50 amps. It cannot. The circuit was designed to provide a cheap compact power supply for CMOS logic circuits that require only a few milliamps. The logic circuits were then used to control mains equipment (fans, lights, heaters etc.) through an optically isolated triac.

If more than 20mA is required it is possible to increase C1 to 0.68uF or 1uF and thus obtain a current of up to about 40mA. But ‘suppressor type’ capacitors are relatively big and more expensive than regular capacitors; and increasing the current means that higher wattage resistors and zener diodes are required. If you try to produce more than about 40mA the circuit will no longer be cheap and compact, and it simply makes more sense to use a transformer. The Transformerless Power Supply Support Material provides a complete circuit description including all the calculations.

Web-masters Note:
I have had several requests for a power supply project without using a power supply. This can save the expense of buying a transformer, but presents potentially lethal voltages at the output terminals. Under no circumstances should a beginner attempt to build such a project.

Important Notice:
Electric Shock Hazard. In the UK,the neutral wire is connected to earth at the power station. If you touch the “Live” wire, then depending on how well earthed you are, you form a conductive path between Live and Neutral. DO NOT TOUCH the output of this power supply. Whilst the output of this circuit sits innocently at 12V with respect to (wrt) the other terminal, it is also 12V above earth potential. Should a component fail then either terminal will become a potential shock hazard.

MAINS ELECTRICITY IS VERY DANGEROUS.

If you are not experienced in dealing with it, then leave this project alone. Although Mains equipment can itself consume a lot of current, the circuits we build to control it, usually only require a few milliamps. Yet the low voltage power supply is frequently the largest part of the construction and a sizeable portion of the cost.

Author: Ron J – Copyright: Zen

Low Voltage Power Supply Without Transformer

Figure 1 

The circuit diagram was designed to create a power supply without utilizing any transformer circuit. This circuit illustrates the advantages as well as the safety precautions to keep in mind.

Power supplies are devices accustomed to provide electrical or other sort of energy to a load or cluster of load. A type of power supply that makes use of a transformer is the AC powered linear power supply. The voltage from the wall socket is converted by the transformer to produce a normally lower voltage. Switched-mode and AC/DC power supplies are the types that does not utilize the presence of transformers. These transformers are responsible for transmitting electrical supply from one circuit to another through its coils (windings).

Designing a transformerless power supply makes it more suitable for smaller installations, in any location, where the area may be limited. The circuit can manage the high current coming from the mains by supplying 12 Volts at 20mA. The reason behind using capacitive reactance rather than resistance is the fact the the type of current flowing into the circuit is alternating. It can also be used with fluctuating DC supply. The reactance adapts with the way the components react in the circuit in terms of frequencies. A fusible resistor can also be used to provide more safety.

As an output device, optical sensors are preferred by measuring the intensity change of light when the power is increased among other controllers like temperature controllers, light switches or timers. The capacitor C1 are connected across the mains supply to act as restrainer. These capacitors are usually tagged with safety standard measures, although they are usually more expensive type rather than ordinary capacitors. Placing two capacitors in parallel or increasing the value can give way to additional current. The two zener diodes are responsible for supplying the low voltage because these types of diodes controls the output by setting their breakdown or desired voltage, as they flow to the rectifier. The rectifier is responsible for converting the AC to DC. Opposite conversion form DC to AC uses an inverter. If the circuit would require an output higher than 40mA, transformers would be more significant to use.

The comparison between transformerless and transformer-based power supplies is not easy to identify due to the technologies that each one offers in the market. But the primary difference between the two are the physical dimension, noise, efficiency and the intensity of harmonic distortion that they produce.

Source:zen22142.zen.co.uk/Circuits/Power/tps.htm