Linear Regulated Dual Polarity Power Supply by LM317 and LM337

This entry was posted on Friday, August 3rd, 2007 at 11:32 am and is filed under LM317, Power supply, Variable Regulator. You can leave a response, or trackback from your own site.

The circuit is fairly standard of a linear supply, and the function of each part can be described as follows:

TX1 is a center-tapped transformer which converts the line voltage from 120 or 240VAC to a lower value which can be managed by the filter circuitry. See note 1 for more details.
Diodes D1-D4 rectify the AC from the transformer output into a pulsing DC waveform.
Capacitors C1a and C2a are large electrolytics which perform bulk filtering duties of the pulsing DC waveform into a raw DC voltage. C1b and C2b are small film capacitors bypassing the electrolytics to improve transient response and filter high frequency line noise.
U1 and U2 are the complementary adjustable voltage regulators LM317 (+) & LM337 (-). The output voltage may be programmed easily with two external resistors.
Resistors R1-R4 set the regulated output voltage. The LM317 & 337 regulators develop 1.25V across R1 and R3, respectively. This reference voltage causes a constant current to flow through the voltage-setting resistors R2 and R4 which generates an output voltage approximately equal to the following relationship:
apacitors C3 and C4 improve the regulators’ ripple rejection from 65dB to 80dB (typical) by preventing ripple voltage from being amplified at the output of the regulator.
Capacitors C5a and C6a are electrolytics that stiffen the output voltage and reduce output impedance. Capacitors C5b and C6b are small film bypass caps (similar to those at the regulator input) to filter any high frequency noise present at the output. The size of the four bypass capacitors is not particularly critical–the most important quality being they have reasonably low impedance at high frequencies, which is true of virtually all modern polyester, polypropylene, polystyrene, or mylar film capacitors.
Finally, D5 and D6 protect the regulator from damage in the event the input is shorted. Lacking these, the output capacitance can dump its charge instantaneously through low impedance paths internal to the regulators. The diodes bypass the regulator and allow the charge to safely dissipate.

1A 48VCT (24-0-24) transformer is about the practical upper limit of commonly available models. This limit is set by the maximum input voltage of the regulators, and also by the bulk filter capacitors which are rated at 35VDC. Actually, these are floating regulators so they can withstand considerably higher input voltages as long as the input-output differential does not exceed 40V. Select a transformer which will output a slightly higher peak AC voltage than the DC voltage you expect to obtain from the power supply. Recall that peak AC voltage is 1.414 times higher than the RMS value specified by the transformer.

Read More Source :

http://orca.st.usm.edu/~jmneal/dual_psu/index.htm

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Linear Regulated Dual Polarity Power Supply by LM317 and LM337

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