I am referring below schematic where 42V is generated from LM78L05 regulator IC. But I am not able to get how LM78L05 can generate 42V? Which concept is used to get such high voltage from 5V regulator IC.

P48TA and P48TB are points taken from step down transformer.

Power Supply Transformer Supply


2 Answers 2


I think you are missing a large input filter capacitor. This thing is two blocks.

The first block involving Q3 is a capacitance multiplier that reduces the ripple (mains frequency times two) coming from the missing filter capacitor.

The second is a 5V regulator sitting on top of a 36V zener diode that regulates the output voltage to 36V+5V = 42V. The zener is mostly biased by the GND current from the regulator with a bit of bootstrapped current through the 5.1K resistor. D16 is to protect the regulator in case the input is shorted.

The regulator has its "GND" reference at +36V so it is regulating the output to 5V relative to GND which gives a total of 42V out.

You can test this with a 78x05 by wiring a diode, LED or zener diode from the GND terminal to circuit ground. Say you use a 3.1V (@ a few mA) white LED. The LED will glow with fairly constant brightness and the regulator output will be 8.1V, assuming the input is more than about 10V and less than the maximum voltage for the regulator plus 3.1V.

  • \$\begingroup\$ Q1? ( Do you mean Q3 ?) Further to large input capacitor, is there scope for improvement in terms of noise, stability. Also 36+5=41V. Sorry for highlighting, but its small error. Thanks. \$\endgroup\$ Apr 9, 2016 at 5:32
  • \$\begingroup\$ Yes, fixed. The cap multiplier won't work without an input capacitor so it's a waste of time. Of course there is room for improvement, but first you have to define what the goals are. Noise and stability might be terrible or wonderful depending on the application. The 78L05 has bad ripple rejection so you might be able to simplify this by just using an LM317 variant. \$\endgroup\$ Apr 9, 2016 at 5:37
  • \$\begingroup\$ It's biomedical instrument ( based on hematology principle ). And these voltages are given to OP-AMPS (+/-12V) after adding one more regulator to +42V Vout. Added one more image, please check. \$\endgroup\$ Apr 9, 2016 at 5:43
  • 3
    \$\begingroup\$ For biomedical hire a consultant. You are going to pass tons of compliance tests, no chance you can do it alone for the first time. Anyway before that you have to explain what exactly is done with 42V. You have to define maximum ripple, bandwidth, etc. \$\endgroup\$
    – user76844
    Apr 9, 2016 at 5:50

They add 36V of the zener diode to the regulated 5V. The regulator doesn't even "know" that it is not connected to GND, but to 36V instead.


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