07.04.2016

Dc power supply short circuit protection

A Variable DC Power Supply is one of the most useful tools on the electronics hobbyist's workbench. P1 sets the maximum output current you want to be delivered by the power supply at a given output voltage. P2 sets the output voltage and must be a logarithmic taper type, in order to obtain a more linear scale voltage indication.
You can choose the Transformer on the grounds of maximum voltage and current output needed. Q4 must be mounted on a good heatsink in order to withstand sustained output short-circuit.
In some cases the rear panel of the metal box in which you will enclose the circuit can do the job.
In Capacitor Power Supplies we use a Voltage Dropping Capacitor in series with the phase line. This is a step by step guide to modifying the disposable camera flash unit that powers the Geiger tube in the last post. The schematic diagram come from circuit: Stable Power Supply for High Voltage -100V to -1000V power supply. The circuit is use to regulation dual power supply that provides +12V and -12V from the AC mains. This Precision Power Supply is a nice addition on your workbench as primary, or in my case, a supplementary power supply.
A very simple high efficiency switching mode buckboost power supply circuit can be designed using the LTM4609 switching regulator IC. Using a single 7812 IC voltage regulator and multiple outboard pass transistors, this power supply can deliver output load currents of up to 30 amps.. I have designed this high voltage, low current power supply for various experiments in systems & synthetic biology. Using the LTC3810-5 synchronous step-down switching regulator controller can be designed a very easy and simple 12 volt switching power supply electronic project using few external electronic parts. This LM723 variable power supply circuit design is a very simple variable power supply that will provide an output voltage between 8 and 30 volt at a maximum output current of 3 ampere. This RS232 power supply circuit diagram is an simple RS-232 Line Driver Power Supply, that operates from an input voltage as low as 4. Variable Voltage Power SupplyContinuing on from our tutorial about converting an ATX PSU to a bench power supply, the question has been asked if it is possible to use this ATX PSU to produce different voltage outputs other than the fixed voltage power supply of +5 or +12 volts. With the aid of a small bit of additional circuitry added to the output of the PSU we can have a bench power supply capable of a range of fixed or variable voltages either positive or negative in nature. There are a wide variety of 3-terminal voltage regulators available in a standard TO-220 package with the most popular fixed voltage regulator being the 78xx series positive regulators which range from the very common 7805, +5V fixed voltage regulator to the 7824, +24V fixed voltage regulator.
The fixed 3-terminal regulator is useful in applications were an adjustable output is not required making the output power supply simple, but very flexible as the voltage it outputs is dependant only upon the chosen regulator. The input voltage to the regulator will be the +12v yellow wire from the PSU (or separate transformer supply), and is connected between the input and common terminals. So suppose we want an output voltage of +9 volts from our PSU bench power supply, then all we have to do is connect a +9v voltage regulator to the +12V yellow wire. These additional capacitors aid in the stability of the regulator and can be anywhere between 100nF and 330nF. These 78xx series regulators give a maximum output current of about 1.5 amps at fixed stabilised voltages of 5, 6, 8, 9, 12, 15, 18 and 24V respectively.
If we increased this pin-2 terminal voltage from 0V to 4V then the output would also rise by an additional 4 volts providing there was sufficient input voltage. Then we can use 3-terminal voltage regulators and a suitable Zener diode to produce a variety of fixed output voltages from our previous bench power supply ranging from +5V up to +12V.


The LM317T variable voltage regulator also has built in current limiting and thermal shut down capabilities which makes it short-circuit proof and ideal for any low voltage or home made bench power supply.
The output voltage of the LM317T is determined by ratio of the two feedback resistors R1 and R2 which form a potential divider network across the output terminal as shown below.
Then whatever current flows through resistor R1 also flows through resistor R2 (ignoring the very small adjustment terminal current), with the sum of the voltage drops across R1 and R2 being equal to the output voltage, Vout.
Also, the LM317T has very good load regulation providing that the minimum load current is greater than 10mA.
If we know the value of the required output voltage, Vout and the feedback resistor R1 is say 240 ohms, then we can calculate the value of resistor R2 from the above equation. Of course in practice, resistors R1 and R2 would normally be replaced by a potentiometer so as to produce a variable voltage power supply, or by several switched preset resistances if several fixed output voltages are required. But in order to reduce the math’s required in calculating the value of resistor R2 every time we want a particular voltage we can use standard resistance tables as shown below which gives us the regulators output voltage for different ratios of resistors R1 and R2 using E24 resistance values.
We can improve our basic voltage regulator circuit a little more by connecting an Ammeter and a Voltmeter to the output terminals. One of the main disadvantages of using the LM317T as part of a variable voltage power supply circuit to regulate a voltage is that as much as 2.5 volts is dropped or lost as heat across the regulator.
So to summarise, our bench power supply that we made from an old PC power supply unit in a previous tutorial can be converted to provide a variable voltage power supply by using a LM317T to regulate the voltage.
Hello Technolomaniac, The LM317 is capable of operating with a sinusoidal AC input voltage, but it would only use the positive half cycle producing a pulsed output voltage, also her 9 volts rms would need to be higher to give the correct Vaverage value for the input to give 6 volts ouput. Is there a parts list for the the last circuit on the page including useful things such as power ratings for resistors, voltage ratings and types of capacitors ? I have a 27v,10 amp power supply.What should i use to regulate this to desired voltage and current ratings? For circuits using TTL ICs the supply voltage is a great concern and a slight increase in supply from the rated 5V may damage the IC. In this circuit a crowbar scheme is used in which a triac short circuits the power supply and burns the fuse.
An ordinary capacitor should not be used in these applications because Mains Spikes may create holes in dielectric of ordinary capacitors and the capacitor will fail to work.
A power supply like this is a very essential tool on the work bench of an electronic hobbyist. With zero to 40V and 2A with adjustable current limiting it will surely gets lots of use on your bench. This symmetrical power supply is designed using a simple operational amplifier and some classic electronic components Resistances R1 and R2 form a high impedance voltage divider.
For this reason, many designs are now required to interface with existing telephones at the subscriber`s location. It adjusts from about 3v to 24v: and has the added feature that you can limit the maximum output current. This LM723 variable power supply circuit design project has a very low output ripple voltage of 0. When you proceed to the checkout page, the Seller Discount will be automatically calculated.
The 2N3055 power transistor for boost up current to 2 Amp.It suitable for general electronics work shop.
Yes we can do that either as a fixed DC output voltage of say +6V or +9V or as a variable output voltage from zero to some maximum value. In fact this is more simple than you may think as the transformer, rectification and smoothing has already been done by the PSU beforehand all we need to do is connect our additional circuit to the +12 volt yellow wire output. There is also a 79xx series of fixed negative voltage regulators which produce a complementary negative voltage from -5 to -24 volts but in this tutorial we will only use the positive 78xx types.


They are called 3-terminal voltage regulators because they only have three terminals to connect to and these are the Input, Common and Output respectively. As the PSU has already done the rectification and smoothing to the +12v output, the only additional components required are a capacitor across the input and another across the output.
The additional 100uF output capacitor helps smooth out the inherent ripple content giving it a good transient response. Then by placing a small 4 volt (nearest preferred value of 4.3V) Zener diode between pin-2 of the regulator and ground, we can make a 7805 5V regulator produce a +9 volts output voltage as shown. The feedback diode, D1 connected across the input to output terminals is for protection and prevents the regulator from being reverse biased when the input supply voltage is switched OFF while the output supply remains ON or active for a short period of time due to a large inductive load such as a solenoid or motor. But we can improve on this design by replacing the fixed voltage regulator with a variable voltage regulator such as the LM317T. By using the ratio of two resistances, one of a fixed value and the other variable (or both fixed), we can set the output voltage to the desired level with a corresponding input voltage being anywhere between 3 and 40 volts. Obviously the input voltage, Vin must be at least 2.5 volts greater than the required output voltage to power the regulator. These instruments will give a visual indication of both the current and voltage output from the variable voltage regulator. So for example, if the required output voltage is to be +9 volts, then the input voltage will need to be as much as 12 volts or more if the output voltage is to remain stable under maximum load conditions. This low dropout comes at a cost as this device is only capable of delivering 1.0 amp with a variable voltage output from 5 to 20 volts. You can use a LM317 Regulator circuit like the one shown above and set the series resistor network, for example, R1 270Ohms and R2 1KOhms, to give you the required 6 volts output. But yes you are correct that it would be better to use a rectifier first which I forgot to mention. Using fuses alone does not solve the problem because a fuse may take several milliseconds to blow off and that’s enough time for the IC to get damaged.
The burning time of the fuse is not a concern because the power supply is already shorted by the triac and the output voltage will be zero.
As you can see in the schematic circuit, this switching power supply circuit require few external electronic components.
This is invaluable when (for example) you power-up a project for the first time or soak-test a piece of equipment.
A fast-acting fuse can also be incorporated if desired in the design to provide additional short circuit protection as shown.
However, we can use this device to give an output voltage of about 11.1V, just a little lower than the input voltage.
Also due to this dropout voltage some form of heatsinking is required to keep the regulator cool. Since the switching of triac takes place within few micro seconds there will be no damage to the TTL ICs or any other such voltage sensitive components in the load circuit.
Now, the Q1 stop working because does not has the bias voltage to base pin its, The output voltage is zero or down quite a close to zero.The potentiometer VR1 acts as adjust the voltage that enter to the LM723 for the volt output by you want.
In use real, because the properties of the transistors used and values error of each device.
Mihai September 17, 2012 The power supply depicted in the first schematic(Cheap adjustable 0-30V) does not work.
Deep November 12, 2013 Will someone post the corrected schematic plz jean August 20, 2015 componetes Muhammad Atif October 29, 2015 I need pcb of 0 35 volt power supply vivek kanth February 11, 2016 S.Vivekananthan 450c,kamarajar salai SIVAKASI-626123 Tamil nadu INDIAAdd a Comment Cancel replyYour email address will not be published.



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