Introduction Regulators Other Options Supply Project

Fixed Regulators

Regulators have been around for some time. No more fussing around with complicated circuits as you can see from the following simple circuit layouts. For the TO220 case you must use a suitable heatsink as quite a bit of heat must be dissipated. On the other hand, the smaller TO92 case is limited to 100mA. The T092 is very handy for onboard applications where voltage regulation is needed to prevent voltage depletion in smaller circuits. In this case an anticipated battery-voltage drop would affect the proper operation of oscillators, references, and other critical requirements.The (TO92) small regulators can also used for and is an ideal choice for an adjustable voltage reference with a ten-turn trim potentiometer.

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The following drawing shows how the whole circuit should be assembled. For a dual polarity supply the negative regulator connection to ground (common) is shown with a broken line. Please note the positive end of the capacitor in the negative regulator is tied to ground as positive with respect to the negative output.

If the regulator is to be connected close to the rectifier the .1uf capacitor at the input of the regulator may be omitted. An important point worth mentioning is that the regulator needs a 1.5V headroom to regulate at a specific voltage. For example if we need 12V regulated then the input supply must be at the very least 13.5V. 14V or more would be recommended for good regulation.

Adjustable Regulators

Adjustable regulators are physically the same in appearance as fixed regulators. Precautions in their use and application are identical to the fixed regulator.

Make note of the pin connection differences as shown in the following diagram.

The circuit is not a perfectly adjustable regulator in that the minimum voltage is rectricted to 1.2V and adjustable to a maximum of 37V at its rated load with proper heatsink. The values shown for the resistors are those recommended for proper operation. Resistors 240R should be 1/2 watt resistors.

Dual tracking regulator

A dual power supply is shown above using two separate 5K potentiometers or a dual 5K potentiometer can be used for a dual tracking power supply. The advantage of using the potentiometers for adjustment is that maximum voltage allowed by the regulators can be used but while using dual potentiometer for a tracking supply , precision may not be accurate. Good tracking voltage can only be obtained if the dual potentiometer is well matched and such a unit can be very expensive.

The circuit below will give you accuracy and precision tracking within a few milivolts but at the sacrifice of voltage limitation as dictated by the op-amp maximum permissible supply voltage which should be no more than plus and minus 15 volts input voltage for a regulated output voltage of 1.250 volts to approximately 13.4 volts at a maximum current of 1.5 amps .

To obtain such precision matched resistors should be used and the op-amp must have a very low offset current , the TL071 a FET type meets this requirement and works very well . You must also insure that the regulators used are of the same manufactured brand.

The Circuit

The positive side of the circuit is adjusted with RV1 in the conventional way. From the junction of RV1 and R1 to the LM317 the positive reference voltage is applied to pin 2 input of the op-amp through the 10K resistor and the inverted output reference from pin 6 is applied to the control pin of the LM337. Pin 3 is connected to ground reference zero . If the op-amp used does not have a zero offset current it will be reflected in difference of the inverted reference voltage applied to the negative regulator as compared to the positive reference applied to the positive regulator and the error will be reflected at the negative regulated output.

Other op-amps can be used such as OP-08, TL084, LF356 or similar. Op-amps like the 741 , 301, can also be used but the offset has to be manually adjusted with the use of a 10K miniature potentiometer connected between pins 1 and 5 in the case of the 741 and the negative ouput reference matched to the positive one by adjusting the 10K pot.

Note that the op-amp supply source is taken from the +/- input voltage, this is necessary to obtain the negative voltage reference. The 10uf capacitors are used to increase ripple filtering under heavy load . The 2.2uf capacitors used at the output must be solid tantalum types for best result. Resistors R1,R4 should be 1/2 watt resistors all others can be 1/4 watt .

The total operating current of the circuit is less than 30mA . With proper care of assembly with heatsinks and choice of components, tracking and regulation should be within + or - .5%.

On the next page we'll look at other options.

Introduction Regulators Other Options Supply Project