diy bench power supply component verification - Page 1
Forum section: electronics / projects

author: mick make
author_rank: Newbie
timestamp: December 01, 2020, 04:37:00 am
content: i recently got some parts form an old speaker system and decided that i am going to make a bench power supply with them and stated working on a schematic and wold like some feedback. i found this instructabel (https://www.instructables.com/DIY-Analog-Variable-Bench-Power-Supply-W-Precision/) and decided that i would use it as an example schematic and try to make my own but with 2 rails instead of 1. can someone just take a look over my schematic and let me know if anything looks wrong. be gentle im still learning this stuff.https://easyeda.com/adenandreana/betch-suply

author: Pawelr98
author_rank: Regular Contributor
timestamp: December 02, 2020, 03:40:47 am
content: Just use an LM723 chip, it's much better for building traditional linear power supplies than the LM317 when it comes to adding external power transistors.This thing you shown is not really regulating as it uses standard emitter follower configuration and there are two diode junctions in series.Transistor voltage drop will change and so will the extra D5 diode. So now you have a voltage that will float around depending on the current and temperature.Not good.723 has everything and is generally stable, this random schematic can be in fact unstable during the constant-current operation, as the creator of this schematic most likely never hooked up an oscilloscope to it (simulation and reality can be different).723 datasheet has everything you need to build a proper 2-37V supply with current limiting. And it's a one cheap chip, that has proven itself over those 50 years since release.Alternatively look at the L200 regulator, a bit more modern chip.The datasheet too provides some example schematics but I would just use Sziklai PNP (control)+NPN (power) for simplicity in the place of the schematic with PNP pass element.

author: mick make
author_rank: Newbie
timestamp: December 02, 2020, 03:34:42 pm
content: thanks for the recommendation im looking at the l200 as i have a heat sync that lends itself to to-220 packages that will also hold my bridge rectifiers so im wanting to use the l200. would i need 2 per rail one for voltage and one for current?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 02, 2020, 08:23:16 pm
content: The L200 has voltage regulation and a current regulation (mainly for limiting) inside. No need for a second chip.It still has the downside of voltage regulator chip with the power stage inside: with high power it gets hot and the voltage will drift a little.The L200 current regulation is more accurate than the simple 723 version, but it is still mainly for a fixed limited, not easy to use as an adjustable limit.

author: mick make
author_rank: Newbie
timestamp: December 02, 2020, 09:39:25 pm
content: i was looking at the designers guide for the L200 and they gave a basically completed schematic for a vraebel current and voltage power supply with external pass element so im going to use that.https://dokumen.tips/documents/a-designers-guide-to-the-l200-voltage-designers-guide-to-the-l200-voltage-regulator.htmlits on page 17

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 02, 2020, 09:58:33 pm
content: The current limit of the L200 can be made variable with a simple opamp.Just put the opamp in a variable amplification configuration with a potentiometer to amplifie the voltage over the shunt, and then connect the output of the opamp to the current sense input of the L200.

author: Pawelr98
author_rank: Regular Contributor
timestamp: December 03, 2020, 04:50:15 pm
content: Quote from: Kleinstein on December 02, 2020, 08:23:16 pmWith L200 it's mainly about making the pass transistor take almost all the current.Same goes for all the modern 3 pin regulators.I however prefer 723, as you can implement multiple over current protections (eg.extra protection for driver stage) and every part of the internal circuitry is accessible by external pins.Not to mention that 723 is more flexible, as you the driver is separate from pass element.This allows one thing. You can put 723 at a higher potential to overcome Vbe voltage drops.My 723 power supply has 723 at a fixed 35V positive (zener + NPN) and -2.5V negative (431 reg).Even if the input voltage to the power stage is 31V the regulation is still working fine.In most basic version a schottky diode + capacitor is enough, as the voltage at the capacitor is maintained at the peak, while the main filter cap has higher ripple.In more advanced version you can use a 50Hz charge pump, which requires no extra windings. Look up electronics-lab power supply (which is rather bad I would say) which uses negative voltage pump and just reverse it for positive voltages.With two pumps, one for positive, one for negative it's possible to get a full range, from 0V all the way up to 30-something V and a small Vdroop (Vcesat of the pass transistor).