Designing a IC 723 Power Supply with Variable Voltage and Current

The article provides tips and design considerations regarding building a well optimized IC 723 regulated power supply circuit. IC L146 is the improved version of IC 723 which can be also used in the explained design.

Within the common application, the IC 723 voltage regulator circuit offers an output of 2 … 37 volts however in numerous situations it becomes essential to be able to go have voltages as low as 0 V.

To accomplish this, an additional negative voltage becomes necessary: in our circuit this is made available from an LM337 negative regulator LM337.

It is far from enough just to attach an extra circuit upon the existing transformer in the form of positive supply: to have a negative voltage, there has to be a load on the positive supply.

This is furnished by R5/T2, which guarantees that a current runs continually when the mains AC is started up.

The circuit features an variable current limiting which is enacted by applying a voltage of 0.6 V between pin 2 (CL = current limit) and pin 3 (CS = current sense). This particular voltage acts like the total drops across R8 (relative to the output current, Io and around P3.

The last mentioned voltage may be the multiplied result of of the resistance of P3 and the current via T1.

Additional stabilization transistor T1 base is supplied by T2. Despite this two times stabilization you may still find a little ripple 10.3 percent with the current into CL.

Voltage stabilization is delivered by IC1: hum and noise are much less than 1 mV at an output of 15 V at 150 mA. The output voltage gets higher linearly as the resistance of P2 get higher.

Optimum output voltage could be predetermined using P1. The negative supply features a extended.

Time constant compared to the positive section to ensure when the AC input is turned off, it continues to be powered a little more time.

Considering this was not included, the output could briefly surge (which may harm the devices that are connected) because of the shortcoming of the 723 to drop to zero in the absence of an auxiliary voltage.

The 2N3055, when fitted on a appropriate heat sink 12 °C/W, could dissipate 30 … 40 watts. With a transformer rating of 22 V, this indicates that in more than 1 A could be dealt with.

The option of transformer is rather stringent, since strictly considering 24 V is currently somewhat excessive for the IC 723 design which will withstand pretty much 36 V. Hence, it is advisable to makes use of the IC L146, which is an enhanced model of the IC 723. This new IC is rated to take care of up to 80 V.

Remember, however, that even still the transformer secondary voltage really should not be over by a couple of volts, or else the absolute maximum rated voltages of the electrolytic capacitors and transistors will probably be exceeded.

Additional Precautions While Designing an IC 723 Power Supply

• The transformer secondary voltage must be approximately equal to the specified highest output voltage, as a minimum, that is, if this is situated over 20 V.
• Without fail make sure that the current spec of the transformer is at minimum 1.4 times the output current.
• The output voltage will be corresponding to P2Uneg/R4 volts; Ureg, must be fixed at around -5 V with P1 By fine-tuning P1 and hence Uneg a bit, the highest output voltage could be fixed accurately to 22 V.
• If the expected optimum output voltage is fairly dissimilar to this value, R4 must be modified to ensure that Uneg continues to be around -5 V.
• The maximum output current will depend on R8 and may be calculated with the formula 0.6/0.47 = 1.28 A.
• Never let the 3055 heat up to above 90 W consistently!
• Last but not least: the earth return is deliberately indicated as three concurrent lines to provide a distinct point of reference in which the voltage or current, in the final instance, is constant. Because of the inevitable voltage drops throughout the earth lines, regulation will almost always be low quality if the returns aren't isolated from each other separate.