Electronics For You
LM317T and LM337T are well known and low cost adjustable voltage regulators with current limiting up to around 1.5A and power dissipation up to around 20W.
It is useful to have bipolar power supply with LM317T and LM337T producing symmetrical output power supply.
In that case LM317T will produce the positive output voltage and LM337T will produce the negative output voltage.
The issue is how to adjust simultaneously both output voltages.
The usual solution is to build tracking voltage regulator with operational amplifier (OA) which track the positive or the negative output of the power supply.
One of the issues in that case is that the power supplies and the other parameters of the OA can be a limiting factor for the required power supply.
In order to avoid the limitations of most of the OAs we will use stereo potentiometer to adjust simultaneously both output power supplies.
The good quality stereo potentiometers have differences between the two potentiometers in the couple less than +-5%.
If that difference is too large we may use additional potentiometers to adjust the output voltages to be exactly the same or with the required difference.
Here we will use that solution to produce two symmetrical power supplies.
As additional advantage of the proposed solution we have both output voltages adjusted from almost ground level.
Description of the circuit
Figure 1 presents the unregulated section of the power supply.
The AC mains is applied to the connector CON1.
The double switch SW1A+SW1B, the fuse F1, the resistor R1 and the capacitor C1 have protective functions and preferably should not be omitted.
The transformer is selected according to the needed maximal output voltage and current.
In that case it should be 2x18V to 2x20Vrms for more than 0.5Arms in order to produce output regulated voltage up to +-15V.
The bridge rectifier BR1 should be for at least 1.5A/50V.
It can be built from diodes 1N4001 or better, e.g. 1N540X are preferred.
The main filtering capacitors C6 and C7 should for at least 2200uF/35V and preferably 4700uF/35V.
The positive unregulated voltage V1 and the negative unregulated voltage V2 are applied to the regulators shown on Figure 2.
Figure 2 gives the regulated section of the power supply with LM317T and LM337T and with stereo potentiometer P1A+P1B for simultaneous adjustment of the output voltages.
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