Symmetrical power supply elektor 12/83 - Page 1
Forum section: electronics / beginners

author: Senin
author_rank: Newbie
timestamp: September 12, 2024, 05:36:43 am
content: Hello! I’m looking to build the symmetrical power supply from the December 1983 Elektor issue, but I have a few constraints:My transformer is 2x13V instead of the 2x18V one specified in the schematic. Will this affect the supply in any way other than the reduced maximum voltage?I don’t have TO-3 LM317/337 regulators, so I'll be using the regular TO-220 ones. Will this reduce the maximum safe current obtainable from the power supply?Do you think this is a reasonable schematic? Is the current limiting implemented properly?Thank you!https://ia801702.us.archive.org/19/items/ElektorMagazine/Elektor%5Bnonlinear.ir%5D%201983-12_text.pdf

author: JohanH
author_rank: Frequent Contributor
timestamp: September 12, 2024, 07:22:10 am
content: Yes, you will be limited to about 13V. Depends somewhat on the size (power) of your transformer. A small transformer will have higher idle voltage, so test max DC output voltage with the desired load current.For 1A DC (per winding), you need a transformer that can supply about 1.5 times that, so about 40 VA with your 2x13V transformer.Modern TO-220 regulators are fine if you use brand names from reliable suppliers and adequate cooling (good sized heat sink).Can't comment the current limiting, other than most Elektor designs have worked fine. The LF356N opamp is available and should work fine.

author: Senin
author_rank: Newbie
timestamp: September 12, 2024, 07:35:04 am
content: Thank you for your answer, sir!

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2024, 09:06:32 am
content: The implementation of the current limit is a bit odd. So this is more like a current limit for some circuit protection, not a real current regulation for a really stable current. The use the raw recitified voltage as the reference for the current limit, so quite some ripple expected in current limit mode, and also quite some drift.The voltage regulation also is also not as stable as one would expect from a lab supply. One part is that the voltage over the 2 diodes D7,D8 is subtracte and this gives some tempereature dependens (expect some +4 mV/K from there). The other part is the principle heat problem with the 3 pin regulators as they get hot from current flow. Here the TO220 case can lead to more temperature rise.With only a 13 V transformer the 7812 regulator may be borderline - so one may want sufficient filter capacitance to avoid drop out.

author: Senin
author_rank: Newbie
timestamp: September 12, 2024, 09:26:00 am
content: Thank you for the explanations!I've seen those diodes in different schematics with the LM317, one of them was in the Onsemi datasheet. Is there a better way to generate that -1.2V (2 x 0.6V) negative voltage instead of using the junction drops?Can the current limiting be improved in any way without redesigning the schematic too much?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2024, 09:48:10 am
content: There are 1.2x V shunt reference chips (e.g. LM4041-1.2, TLV431,...) that could be used instead of the 2 diodes.The problem with the self heating of the LM317 is however a principle limitation of the regulator chips.The curent regulation could also use a better reference, e.g. by adding part of the regulated +-12 V supply instead. This could at least reduce the ripply and mains voltage dependence.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 12, 2024, 10:19:24 am
content: I do not understand why R14 is connected to the input side of IC7, even on the "wrong" side of current shunt R10. It would be more logical connect it to the output of IC2 (-12V). That way the current though the diodes is more stable, and thus the voltage drop.But assuming a temperature difference of 30c between startup and hot under load, The voltage over those diodes would change by about 30*2*2 = 120mV and that is noticeable, but it's not a disaster. But even a simple resistive divider would probably already be better.Instead of the two diodes, you can use a LM337 but it probably needs an extra load resistor to get to it's minimum load current.I was wondering why R14 is pulling around 20mA thought those two reference diodes. (Rectified transformer output is around -25V) Then I realized that R15 has a constant current of 1.2V / 120 Ohm = 10mA and R14 must be able to take that while still leaving some current for the diodes.If your transformer voltage gets too low, then IC1 (7812) and IC1 (79L12) will get out of regulation, but as it's only used for a power supply for the opamps, this does not seem very important.I do not like the circuit around the current limiting. The opamp circuits look very much like a differential amplifier, but they are taken out of balance by the current adjust potentiometer, which makes them dependent on the voltage ripple of the raw rectified DC... So that is why the "reference" of these diff amps (Net with R1, R2) is connected to -12V instead of GND. A higher voltage over the diff. amp makes the imbalance due to DC voltage ripple less.I guess it's all "adequate" for an adjustable current limit. It's not intended to be a precicely adjustable current source, but mostly as a coarse over current protection. If it can be set to a 20% tolerance it's probably already good enough.If you want to design a bit yourself, you can make a current mirror over the shunt resistors with two BJT's for each of the shunt resistors. Then you have a variable current, depending on the current through the shunt resistors, and you can make a voltage out of that by connecting it to GND though a resistor.

author: Senin
author_rank: Newbie
timestamp: September 12, 2024, 10:48:05 am
content: I think, for now, I will leave the current limiting as it is. I am not very experienced, and I don't think I am capable of modifying it. I will treat it as a protection feature. I will connect R14 to IC2 out and try to find a practical replacement for the two diodes. Thanks, everyone, for your help!

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2024, 11:15:00 am
content: The resistor values at the current regulation are also a bit odd. As show the shunt signal is attenuated quite a bit with the smaller resistor to the negative voltage. It should be better to have the resistor ratio more the other way around, so just enough attenuation to stay in the common mode range of the OP-amps.