Ultra low ripple PSU with LM350

Ultra low ripple PSU with LM350 stuffed PCB

Ultra low ripple PSU with LM350 stuffed PCB

Sometimes you need a PSU with very low ripple. With a common voltage regulator this can be achieved by some additional filtering. Here I use a LM350 with an additional RC filter before the smoothing capacitors and a capacitor in the regulation path of the LM350. After the rectifier diodes C6 and C7 are the reservoir capacitors followed by the RC filter build of R2; R3, R4, R8 and C8, C9. D5 serves as protection diode for reverse biasing the voltage regulator. R6, R7, TR1 serves as voltage divider to adjust the output voltage to15V. The 1.2V reference voltage between ADJ and out is amplified about 12times to make a 15V output. So is the noise and ripple. C11 between ADJ pin and GND reduces noise and ripple considerably. Protective diode D6 is needed to safely discharge C11 if the output is short-circuited. D7 protects against short-circuited when you daisy chain two PSU for a +/- PSU if one of the outputs is short circuited. With the above measures the ripple is down in the noise floor. The drawback of this schematic is increased power consumption. The resistors in the RC input filter get rather hot under heavy load Use 2W (minimum) or higher rated resistors and mount them with some distance to the PCB. You should keep the load under 2A. You can drive heavier loads, but watch for the temperature of the resistors. You can find more details in “Small signal audio design” second edition from Douglas Self in chapter 25 p713ff.

Ultra low ripple PSU with LM350 schematic

Ultra low ripple PSU with LM350 schematic

Ultra low ripple PSU with LM350 front view

Ultra low ripple PSU with LM350 front view

2 thoughts on “Ultra low ripple PSU with LM350

  1. This PSU have almost all the recommendations cited in the datasheet, is simple and efficient, I like it. People usually build advanced designs and forget to have a decent PSU; and that’s why I like your way, because you care about the minimum details.
    You know, I think the 350 is easily exchangeable with the 338 and if you have two secondary windings you can have negative rail, I don’t know/I never saw if this can be doable with the 350.
    You’re great! keep going, I’m avid to see more. Cheers.

    • Thank you. You can easily exchange LM350 and LM338. They only diver in the maximum current. I use the LM350 for positive and negative rails without any problems. As you said, with two secondary windings or two transformers just daisy chain two PSU and you are done.

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