Increasing Regulator Current PSU Circuit

Description:
An outboard pass transistor used to increase the current output of a voltage regulator IC.

Notes
Although the 78xx series of voltage regulators are available with different current outputs, you can boost the available current output with this circuit. A power transistor is used to supply extra current to the load the regulator, maintaining a constant voltage. This transistor is known as an outboard bypass transistor.

Currents below 600mA will flow through the regulator. Above 600mA the input current flowing through the 1 ohm resistor develops a voltage. As this voltage increases above 0.6V (600mA through 1 ohm) then the TIP2955 power transistor starts to conduct, supplying the extra current to the load. The 10 ohm resistor limits excessive base current. The power transistor requires an adequate heat sink as it is likely to get very hot. Suppose you use a 12v regulator, 7812. The minimum input voltage should always be a few volts higher than the regulator output voltage to allow for voltage drops.

The 1 ohm resistor needs to be rated 3 Watts for load currents up to 3 amp and rated 7 Watts for load currents of 5 amps. As the HFE of a power transistor falls with increased collector current it is not recommended to draw more than 5 amps with this circuit. The 10 ohm base resistor drops less power and a 0.5 Watt resistor can be used at all output currents.

Power Dissipation in Bypass Transistor
Assume a supply of 20 volts and that the load will draw 5amps. The power dissipation in the transistor will be Vce * Ic.
Vce = Vcc - Vreg
so
Pdiss = (20-12) * 5 = 40 Watt.

It may keep you warm in the Winter, but you will need a large heatsink with good thermal dissipation. If however the input voltage was 15V then the dissipation would be reduced to just 15 Watts. If you want to increase the output current with a negative regulator, such as the 79xx series, then the circuit is similar, but an NPN type power transistor is used instead.

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36V Output L200 Power Supply Circuit

Description
Power supply with variable voltage and fixed current regulation made using the ubiquitous L200C regulator.

Notes
The versatile 5 pin L200C regulator offers both voltage and current regulation in a single package. The IC also features thermal shutdown and input over voltage protection up to 60 Vdc. The package is also available as L200CV which has straight pins for mounting onto a PCB. The above circuit has current limiting of 1 amp, hence Rsc = 0.45 ohm. The output voltage is variable from 2.85V to 36V. For voltages up to 36V then the input voltage, Vcc must be 40V. The supply voltage must always a few volts higher than the maximum output voltage. If you wanted to make a 9 Volt current limited PSU then the input voltage should be a minimum of 12 Volts.

Maximum Power Dissipation
The L200 has internal limiting to reduce the amount of heat dissipation. This happens when the internal junction temperature reaches 150 °C. The datasheet has a graph of the safe operating area, but if drawing maximum output current of 2 amp, the input voltage minus output voltage difference must be less than 20 Volts.

Specifications:
DC Input Voltage: 40V max.
Peak Input Voltage: 60V max. for 10ms
Output Voltage Range: 2.85 to 36V
Output Current Range: 0.1 to 2A
Quiescent Current: 4.2mA
Output Noise: 80uV

The L200 regulator has many versions; L200, L200C, L200CH, L200CV. These are in fact all the same regulator. The main differences between variations are the thermal junction temperature and pinout.
For the L200, the operating junction temperature is -55 to 150 °C
The L200C operating junction temperature is -25 to 150 °C.
The V stands for vertical and is the most common case style; available as a pentawatt package, shown right hand side. The H stands for horizontal and the pins of the L200 are bent at right angles for PCB mounting. The L200T and L200CT are also available but as a TO3-4 layout, this pinout can be seen in the datasheet, link below

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Multiplier Single Pulse Voltage Circuit

Single Pulse Voltage Multiplier
Obtaining higher voltage pulses without a higher voltage power supply

Originally appeared in EDN/Reed Business Information September 18, 2003

Sometimes it's necessary to generate a higher voltage pulse from logic circuits than the circuits themselves can supply. The need to generate a programming pulse for Atmel flash controllers was one of those. In that case, I needed to generate an infrequent pulse that switched from 5 volts to 12 volts in response to a control signal from a microcontroller.

A method that relies on a pushbutton to double the voltage was used in the ATtiny12 fuse restorer, but there, the generation of the +12 volts was done by using a mechanical switch that was manually operated to generate about 18 volts, which was further regulated to +12 volts by a gated regulator. It was good as far as it went, but the generation of the +12 volts could not be triggered by the microcontroller, that is unless one wanted to use an electromechanical relay or elaborate arrangement of switching devices.

The straight-for ward approach to control by a microcontroller could be accomplished by driving a capacitor voltage multiplier (such as in the Seiko display inteface) with a pulse train from a controller's pin, then regulating and switching the resultant voltage, the drawback being that this took a lot of parts.

The circuit below accomplishes the same result. It still uses a transistor to do some switching, and it still needs a pair of diodes and capacitors, as would be used in a conventional multiplier, but it doesn't require a steady stream of pulses and the output voltage is set by a resistor divider as the 100k and 150k resistors make 2 volts that are added to the 10 volt pulse on the transistors' collector. What it doesn't need is a steady stream of pulses to keep the output voltage pumped up all the time.
The basic multiplier cell was inspired by a discussion I had with a well know laser scientist, Mr. Christoph Krah, about laser triggering circuits. After I finished this circuit, I sent it to Mr. Krah to get his opinion of how this circuit related to some of those we discussed years ago. Here is Mr. Krah's taxonomic analysis, which also includes a concise description of circuit operation which I could not improve upon:

"Your circuit seems to be a combination of a Marx and a Cockroft-Walton type multiplier. You charge capacitors in parallel (100uF @ 5V and 100uF at 2V) and then switch them in series by means of a 5V voltage step (5+5+2 = 12V) at the output of the uC. The diodes provide isolation from the power supply."

For many of applications, the diode on the left side can be omitted and the 1k resistor changed to 100k.. This simplifies the circuit at the cost of slightly increasing the rate of droop of the voltage across the first 100 uf capacitor since it will discharge into the 100k resistor as well as driving the base resistor for the transistor and driving the load and 100k/150k voltage divider.

There is no free lunch with this circuit. If the pulse is initiated before the 150k AND 100K resistors charge the 100 uf capacitor sufficiently (60k x 100 uf = 6 seconds), the output voltage will be lower than intended. The charge time of the circuit can be decreased by reducing the values of the capacitors, the output 100 uf capacitor having the most effect because of the high resistance charge path. Reducing the size of the capacitors will make the output pulse droop more quickly.

It should be noted that this circuit also makes a 0 to 10 volt pulse, which appears on the collector of the transistor. If a 0 to 10 volt pulse is desired, the circuitry to the right of the transistor's base resistor may be omitted.

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5 to 15V 400mA Regulated Power Supply

5 to 15V 400mA Regulated Power Supply

This project is a simple DC regulated power supply that has a variable DC voltage range from 5V to 15V. It can supply current up to a 400mA to power the various circuits for your electronic projects. The voltage output is varied by using the potentiometer VR1. In this circuit, the input line power supply is designed for 240VAC. If 110VAC input is used, change the ratings of the varistor to 150VAC and the transformer ratio to 110V/12V.

Fuse F1 is used as a protection in case there is any short circuit in the circuit. Varistor V1 is connected in parallel to the input of the line voltage to clamp the surge voltage from the line to a reasonable level that helps to protect the transformer and other circuitry. Once the voltage level surge to a high level beyond the ability of the varistor to absorb it, fuse F1 or varistor V1 or both will burn. If this circuit failed after a period of operation, check that the fuse and the varistor are still in good condition or else replace them.

Diodes D1, D2, D3 and D4 are used to rectify the 12VAC voltage to DC voltage. Electrolytic capacitor E1 is used as a smoothing capacitor to reduce the ripple of the DC voltage. The DC voltage is fed into the input of 7805 regulator where the output DC voltage is obtained. Changing the value of VR1 will change the output of the DC voltage. Capacitor C1 is used to filter out high frequency component from the power supply.

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2V to 25V 5A LM338 Power Supply Circuit

2V to 25V LM338 Power Supply Circuit

This project uses a LM338 adjustable 3 terminal regulator to supply a current of up to 5A over a variable output voltage of 2V to 25V DC. It will come in handy to power up many electronic circuits when you are assembling or building any electronic devices. The schematic and parts list are designed for a power supply input of 240VAC. Change the ratings of the components if 110VAC power supply input is required.

As shown in the figure above, the mains input is applied to the circuit through fuse F1. The fuse will blow if a current greater than 8A is applied to the system. Varistor V1 is used to clamp down any surge of voltage from the mains to protect the components from breakdown. Transformer T1 is used to step down the incoming voltage to 24V AC where it is rectified by the four diodes D1, D2, D3 and D4. Electrolytic capacitor E1 is used to smoothen the ripple of the rectified DC voltage.

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LM317K 3A Switching Regulator Circuit

LM317K 3A Switching Regulator Circuit

When compared to linear voltage regulators the switching voltage regulators are much power efficient. In the case of linear voltage regulators the difference between the input and output voltage is just wasted and for switching regulators there is almost no such wastage and that’s why the switching regulators have great power efficiency ranging up to 85% . In simple words, the switching regulator operates by taking small bits of energy from the input voltage source and then transferring it to the output with the help of a solid state switch and a control circuitry. Since the switching element is either fully open or closed at any moment, no energy is wasted across it. The control circuit controls the duty cycle of the solid state switch which in turn determines rate at which energy is transferred to the output.

The electronic circuit given here is of a simple and low cost switching regulator using the IC LM317 that can deliver up to 3A of current. The input voltage range of this circuit is between 8 to 35V DC and the output voltage can be adjusted between 1.8 to 32V DC. The output voltage can be adjusted by using the POT R4.

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