Category Archives: Power Supplies

High Current Regulated Supply

The high current regulator below uses an additional winding or a separate transformer to supply power for the LM317 regulator so that the pass transistors can operate closer to saturation and improve efficiency. For good efficiency the voltage at the collectors of the two parallel 2N3055 pass transistors should be close to the output voltage. The LM317 requires a couple extra volts on the input side, plus the emitter/base drop of the 3055s, plus whatever is lost across the (0.1 ohm) equalizing resistors (1volt at 10 amps), so a separate transformer and rectifier/filter circuit is used that is a few volts higher than the output voltage. The LM317 will provide over 1 amp of current to drive the bases of the pass transistors and assumming a gain of 10 the combination should deliver 15 amps or more. The LM317 always operates with a voltage difference of 1.2 between the output terminal and adjustment terminal and requires a minimum load of 10mA, so a 75 ohm resistor was chosen which will draw (1.2/75 = 16mA). This same current flows through the emitter resistor of the 2N3904 which produces about a 1 volt drop across the 62 ohm resistor and 1.7 volts at the base. The output voltage is set with the voltage divider (1K/560) so that 1.7 volts is applied to the 3904 base when the output is 5 volts. For 13 volt operation, the 1K resistor could be adjusted to around 3.6K. The regulator has no output short circuit protection so the output probably should be fused.

Copyright 2005 Bill Bowden

DC to DC Converter

The circuit above is a DC to DC converter using a standard 12 VAC center tapped power transformer wired as a blocking oscillator. The circuit is not very efficient but will produce a high voltage usable for low power applications. The input battery voltage is raised by a factor of 10 across the transformer and further raised by a voltage tripler consisting of three capacitors and diodes connected to the high voltage side of the transformer. The circuit draws about 40 milliamps and should operate for about 200 hours on a couple of ‘D’ alkaline batteries. Higher voltages can be obtained by reducing the 4.7K bias resistor. More information on blocking oscillators can be found here: Blocking Oscillators

Copyright 2006 Bill Bowden

Bench Power Supply

This unit delivers 0 to 20 volts at up to 4 amps in 0.1 volt increments. The entire device runs on a PIC16F870 (about $3 in small quantities). This is basically a switching power supply with the voltage regulation done in software. The PIC used here has analog inputs (used to measure voltage and current) and hardware PWM (pulse width modulation) output used to control the power.

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Only two controls are used on the front panel – an ‘on/off” push button and a rotary encoder. The on/off button is a ‘soft’ control; the unit actually stays powered up all of the time. In the ‘off” mode, the display is blanked except for the far right decimal which acts as a standby indicator, and the voltage output is set to zero. Also, while in this mode, the rotary encoder is not active so that the previously selected voltage will be maintained when the unit is powered back on.

The displays are standard red 7-segment LED’s. They are multiplexed in software to simplify the circuit design. A set of 6 small pnp transistors activates each one in turn at about a 100 hz scan rate.

Because of the nature of switching supplies, it is actually possible to get more amperes out of the unit than the transformer is rated for. I got over 4 amps out at 5 volts. As you increase the voltage, less current is available.

The software listing is included here as is the Schematic. Like other projects shown on this site, you are welcome to use whatever information you want but this is not a step-by-step guide to making your own. Some of the parts used here were scrounged from the junk box and the values are unknown – they just seem to work.

[img:57b252c034]http://www.circuitdb.com/download.php?fileID=143[/img:57b252c034]

Copyright 2006 [url=http://members.cox.net/berniekm/]Luhan Monat[/url]

2 Watt Switching Power Supply

In this small switching power supply, a Schmitt trigger oscillator is used to drive a switching transistor that supplies current to a small inductor. Energy is stored in the inductor while the transistor is on, and released into the load circuit when the transistor switches off. The output voltage is dependent on the load resistance and is limited by a zener diode that stops the oscillator when the voltage reaches about 14 volts. Higher or lower voltages can be obtained by adjusting the voltage divider that feeds the zener diode. The efficiency is about 80% using a high Q inductor.

Copyright 2006 Bill Bowden

2 Cell Lithium Ion Charger

This circuit was build to charge a couple series Lithium cells (3.6 volts each, 1 Amp Hour capacity) installed in a portable transistor radio.

The charger operates by supplying a short current pulse through a series resistor and then monitoring the battery voltage to determine if another pulse is required. The current can be adjusted by changing the series resistor or adjusting the input voltage. When the battery is low, the current pulses are spaced close together so that a somewhat constant current is present. As the batteries reach full charge, the pulses are spaced farther apart and the full charge condition is indicated by the LED blinking at a slower rate.

A TL431, band gap voltage reference (2.5 volts) is used on pin 6 of the comparator so that the comparator output will switch low, triggering the 555 timer when the voltage at pin 7 is less than 2.5 volts. The 555 output turns on the 2 transistors and the batteries charge for about 30 milliseconds. When the charge pulse ends, the battery voltage is measured and divided down by the combination 20K, 8.2K and 620 ohm resistors so that when the battery voltage reaches 8.2 volts, the input at pin 7 of the comparator will rise slightly above 2.5 volts and the circuit will stop charging.

The circuit could be used to charge other types of batteries such as Ni-Cad, NiMh or lead acid, but the shut-off voltage will need to be adjusted by changing the 8.2K and 620 ohm resistors so that the input to the comparator remains at 2.5 volts when the terminal battery voltage is reached.

For example, to charge a 6 volt lead acid battery to a limit of 7 volts, the current through the 20K resistor will be (7-2.5)/ 20K = 225 microamps. This means the combination of the other 2 resistors (8.2K and 620) must be R=E/I = 2.5/ 225 uA = 11,111 ohms. But this is not a standard value, so you could use a 10K in series with a 1.1K, or some other values that total 11.11K

Be careful not to overcharge the batteries. I would recommend using a large capacitor in place of the battery to test the circuit and verify it shuts off at the correct voltage.

Copyright 2006: Bill Bowden

Variable Voltage and Current Power Supply

Another method of using opamps to regulate a power supply is shown below. The power transformer requires an additional winding to supply the op-amps with a bipolar voltage (+/- 8 volts), and the negative voltage is also used to generate a reference voltage below ground so that the output voltage can be adjusted all the way down to 0. Current limiting is accomplished by sensing the voltage drop across a small resistor placed in series with the negative supply line. As the current increases, the voltage at the wiper of the 500 ohm pot rises until it becomes equal or slightly more positive than the voltage at the (+) input of the opamp. The opamp output then moves negative and reduces the voltage at the base of the 2N3053 transistor which in turn reduces the current to the 2N3055 pass transistor so that the current stays at a constant level even if the supply is shorted. Current limiting range is about 0 – 3 amps with components shown. The TIP32 and 2N3055 pass transistors should be mounted on suitable heat sinks and the 0.2 ohm current sensing resistor should be rated at 2 watts or more. The heat produced by the pass transistor will be the product of the difference in voltage between the input and output, and the load current. So, for example if the input voltage (at the collector of the pass transistor) is 25 and the output is adjusted for 6 volts and the load is drawing 1 amp, the heat dissipated by the pass transistor would be (25-6) * 1 = 19 watts. In the circuit below, the switch could be set to the 18 volt position to reduce the heat generated to about 12 watts.

Copyright 2006: Bill Bowden