can someone verify this power supply ? - Page 1
Forum section: electronics / beginners

author: BeetleJuice
author_rank: Contributor
timestamp: January 28, 2021, 09:11:11 pm
content: hi guys, found (maybe here) this power supply, it uses 2 LM723 and has a low amperage and a high amperage design.can someone more experienced provide an opinion if these designs are worth building? or maybe some tuning required? or maybe another similar supply suggestion?here are the links and picshttp://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htmthank you for your time!

author: Grandchuck
author_rank: Super Contributor
timestamp: January 29, 2021, 06:24:47 pm
content: Looks OK in general, but I cannot verify the design. The following link is for a supply design that has been homebrewed many times:https://www.amazon.com/Regulated-Continuously-Adjustable-Protection-Education/dp/B07Q1N5HNX/ref=sr_1_25?dchild=1&keywords=lab+power+supply+kit&qid=1611944076&sr=8-25The kit is available from several sources. I have built two of these and they work well. You will also need a heatsink, possibly a fan, a power transformer, panel meter (cheap digital volt/amp units are readily available), cabinet, line fuse, line cord, line switch, jacks and so on. Are you sure you want to build it? Nothing wrong with that but a decent used one might be a better idea.

author: ledtester
author_rank: Super Contributor
timestamp: January 29, 2021, 07:13:32 pm
content: Quote from: Grandchuck on January 29, 2021, 06:24:47 pmThere's has been a lot of discussion about that design on these forums. Some links that come to mind:https://www.eevblog.com/forum/beginners/bangood-psu-enhancements/http://www.paulvdiyblogs.net/2015/05/tuning-030v-dc-with-03a-psu-diy-kit.html?m=1https://www.eevblog.com/forum/beginners/diy-0-30v-0-3a-again/

author: BeetleJuice
author_rank: Contributor
timestamp: January 30, 2021, 02:31:13 pm
content: been there done that ! i have the old greek version of that psu and it still doesn't work...back in 1997 it worked .did all the upgrades and it doesnt work.i just wanted to know your opinion about the psu i found. more likely i will go for the 1982 elektor schematic.i am still searching for a better board like in eda or eagle to avoid printing the board in house.if you have anything new about the 1982 project you will help me a lot.thank you!Quote from: ledtester on January 29, 2021, 07:13:32 pm

author: BeetleJuice
author_rank: Contributor
timestamp: January 30, 2021, 02:34:40 pm
content: i try to revive as many parts from the 0-30 0-3a complete power supply i have built from 1997 and make a new good working psu.i have all the parts, i will need a good, trusty pcb and i guess i will order from jlcpcb . i try to avoid printing it in house.thanks for the reply!Quote from: Grandchuck on January 29, 2021, 06:24:47 pm

author: Grandchuck
author_rank: Super Contributor
timestamp: January 30, 2021, 06:00:46 pm
content: I have attached my version of this supply. There are several versions out there and also several modifications have been made and published over the years. One reason that it has been criticized is that 3 amperes is a tad too enthusiastic. It can reach that but should be modified for constant use. Also, some have used a poor choice for the transformer. Good luck with yours!

author: Grandchuck
author_rank: Super Contributor
timestamp: January 30, 2021, 06:06:52 pm
content: Here is some info for the circuit:There are three important voltages in this circuit that can be used when troubleshooting. The first is +38 volts shown at the top. This is determined by multiplying the ac input voltage to the bridge rectifier by 1.414: 28 x 1.414 = 39.6. Subtracting 1.4 volts for two diode drops in the bridge gives about 38 volts which is expected with light loads.Another important voltage is +11.2 volts shown at the output of OA2. The Zener is a 5.6 volt device so it is reasonable to assume that the voltage across the amplifier output and its inverting input is 5.6 volts. Knowing that both operational amplifier inputs are normally at the same voltage leads to the conclusion that the voltage across the 10 k ohm resistor to ground is also 5.6 volts. Thus, the voltage at the output of OA2 is expected to be 2 x 5.6 volts = 11.2 volts. Last, the second Zener diode at the bottom of the schematic regulates the negative supply terminals for OA1 and OA3 to -5.6 volts. These three voltages are important for understanding how this supply works and they should be verified early in the troubleshooting process.How does the current limiting work? Let’s make another calculation by assuming the current potentiometer is set to its midpoint. The voltage from the wiper to ground will be:Vwiper = 11.2 V x (5 kΩ)/(110 kΩ) = 0.509 VThe 33Ω resistor has been ignored. Notice that this voltage is applied to the non-inverting input of OA1 along with the drop across the 0.47 Ω resistor. If the load current happens to be 1.083 amps, then the drops are equal and OA1 is on the verge of crossing into negative saturation. OA1 serves as a comparator in this circuit. When the power supply is not in current limiting, the output of OA1 is at positive saturation and it has no effect on OA3 which serves as the error amplifier for the power supply. When the drop across the 0.47 Ω resistor is more than the voltage across the wiper and the bottom of the current set potentiometer, OA1 switches to negative saturation and OA3 turns off and so does the pass transistor. Also, the overcurrent LED comes on at this time.Last, looking at OA3 shows it to be a non-inverting amplifier with a gain of 3. This gain is set by the 56 kΩ and 27 kΩ resistors. With a gain of 3, the range of output voltage for the supply is 0 to 3 x 11.2 volts, or 0 to 33.6 volts, ignoring the drop across the output transistors. This supply also has a 10-turn 1 kΩ variable resistor that serves for fine adjustment of the output voltage. Generally speaking, this is a 0 to 30 volt dc supply with a current capability of about 2 amps. If you analyze the comparator trip point when maximum current is flowing through the 0.47 Ω resistor, it is about 2.17 amps. This same circuit can be used for higher current by decreasing the value of the 100 kΩ resistor in series with the current limit potentiometer. Other components will have to be modified as well.

author: BeetleJuice
author_rank: Contributor
timestamp: February 01, 2021, 08:17:34 pm
content: i checked today my 0-30 0-3 psuwe got the following:psu works fine with no load , minimum volts can be set to 0.02 VDC and max volts with my 28VAC trafo can go up to 31 (didnt check max volts). i used the trimmers to set the range from 0.02-26volt to be on the safe side.first problem thats gonna BLOW YOUR MIND:if you increase the voltage slowly , at around 7 volts you got a JUMP to 8volts on the analog voltmeter. at oscilloscope you see a strange wavy pattern . after around 9 volts the wave are more like a flat line.second problemlower than 0,5VDC the led of the current regulation stays onthird problemif you bridge the output you can get max amps 0,3ADC and the psu makes a buzzing noise , sometimes the analog amp meter needlle jumps to 3A and then returns to 0,3A and that happens again and again. R7 goes smoking hot.if you use a 10ohm load , and with a max current of 2,5amps you wont notice any problem at all, the psu looks like its fine , apart from the tiny jump from 7 to 8 volts (hard to notice on a 0-30vdc analog meter)keep in mind i made the version R7 of the mod of the original greek psu from 1997.the mods i did were the latest from this site:https://www.electronics-lab.com/community/index.php?/topic/40835-0-30v-0-3a-latest-data/any ideas where to look what to check?i can upload video too.Quote from: Grandchuck on January 30, 2021, 06:06:52 pm

author: BeetleJuice
author_rank: Contributor
timestamp: February 02, 2021, 09:39:05 pm
content: found most errors, you won't believe it:the original pcb from 1997 had differences so the mod could not work if you did not cut 2 traces on the pcb...right now everything works as it shouldwill inform you tomorrow!thank youQuote from: Grandchuck on January 30, 2021, 06:06:52 pm

author: Grandchuck
author_rank: Super Contributor
timestamp: February 03, 2021, 12:58:11 pm
content: Glad you found the problem ... Good Work!

author: Terry Bites
author_rank: Super Contributor
timestamp: February 03, 2021, 01:50:43 pm
content: No such thing as amperage!Do you mean Current?

author: rsjsouza
author_rank: Super Contributor
timestamp: February 03, 2021, 04:01:47 pm
content: Quote from: Terry Bites on February 03, 2021, 01:50:43 pmI disagree:https://www.merriam-webster.com/dictionary/amperage