Pyramid PS-21K voltage mod - Page 1
Forum section: electronics / projects

author: Technotronic
author_rank: Contributor
timestamp: December 29, 2023, 06:38:56 am
content: Recently fixed one of these up, it had a bad bridge diode block. I went and installedan LED voltage display readout and its operating well, delivering nearly 25A.I need a supply that will deliver 26V at a low current no more than 100mA,and also 50V at about 2A max and was curious if a voltage doubler of some sortcould be put in as a mod to double the 13.8v once for the 26V, then again for the 50V?I have studied some voltage doublers , triplers and quads online but not sure what way to go.The PS uses half the diode bridge block as a FW center tapped setup, and I wound up ditching thatfor 2 stud mount high current diodes that were in my parts bin...in the photo.I am thinking maybe a DPDT switch of some sort to bring the doublers in and out perhaps.The 2N3055 transistors are about 60V rated, so they should handle the higher voltage, and perhapsI will have to upgrade some of the caps...and maybe the op amp board would also need some changes?Or am I better off building a separate project box to just plug the output jacks into and let it handle all that?

author: coromonadalix
author_rank: Super Contributor
timestamp: December 29, 2023, 10:02:54 am
content: yes and noyou can hack it to higher voltage, but half of the current, but no adjustable currentto have adjustable current its another matter / design etc ... it will add more complexity and costsyou have to remove the xformer center tap, and change where the main ground supply is / will belucky you the psu has a floating section for the lm723but the voltage divider potentiometer need to be recalculated and a few things too, you need to get the lm723 datasheet for floating operation and values calculationsin red circle you have to check thesesbut if you have no experiences you may or will fail, switch and other parts need to have calculations, and you can not switch when the psu is powered oni hacked an ps51k to higher voltage / half current with no problem, they are all based on the same design / topologyto add digital meters i added another small psu for them since for the amps you need isolated supply too, but im on the positive rail, not by the ground as many small meter doesand the thermal switch was the 1st thing to go out lolltheses a crude / basic psu's the lm723 can make a good job if you know what you're doing, if not it will get unstable at some pointoh tweaking the xformer will get you around 25vac once filtered will be around 40vdc, you will not be able to get more, maybe around 35vdc out of the psuforget your 50v requirement, and surely the current limitation as it isbetter switch to an already made psu, voltage and current limitted, took me 3 pcb iterations to get a good morking psu, from 10ma to 20 amps, a real zero volts to 30 vdc,lm723 can't go down to zero by its design, you have to use tricksbut now it is abandonned, and i got real psus like lambda kepco kikusui for serious stuff

author: Technotronic
author_rank: Contributor
timestamp: December 29, 2023, 03:36:15 pm
content: Yes, I kept it on the bracket but used the already drilled holes and widened them for the 2 big diodes.The thermal sw really belongs on the heatsink with the power transistors somehow as they get pretty hotnaturally.Would be neat if with some sort of slide switch or 3 pos rotary switch we switch outthe full wave diodes and bring in a fw doubler off the xfmr, then double that with the 3rdpos of the sw. Somehow I guess the potentiometer running the op amp for bias must alsochange to I guess some fixed resistances to properly bias the 3055's and clamp them at26 and 50V respectively.Quote from: coromonadalix on December 29, 2023, 10:02:54 am

author: coromonadalix
author_rank: Super Contributor
timestamp: December 29, 2023, 04:26:35 pm
content: but you dont catch the fact its more than a switch additionyou need to change parts to have higher voltage output, it was calculated for the original use, not the one you intend to useand once again forget the 50 volts you will never get it