Precision bench supply - MC1466L based - Page 1
Forum section: electronics / projects

author: Mephitus
author_rank: Regular Contributor
timestamp: December 15, 2015, 03:47:18 pm
content: Full design found here:http://www.sentex.ca/~mec1995/circ/ps4002/ps4002.htmlI have been looking around for a robust power supply design and ran across this one (Schematic attached) For the transformer, I was planning on rewinding the secondary of a salvaged microwave transformer. Should handle the load no problem, from what I understand.Seems to be a pretty solid design that I could build with some cheap and easily sourced parts. Preferred over the cheapo $10 chinese one I showed in a previous post. I was thinking of changing the 2N6388 and 2N6385 with a pair of 2N3055's to increase the current output. Would this work as a direct swap out? Any advice or feedback is welcome2N6388http://www.onsemi.com/pub_link/Collateral/2N6387-D.PDF2N6385http://www.svntc.com/TPDF/468.pdf2N3055http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF

author: dom0
author_rank: Super Contributor
timestamp: December 15, 2015, 04:20:24 pm
content: 0.03 % + 3 mV load and line regulation might seem good, but are very poor for aprecisionsupply.Similar for 0.01 %/K output drift, typical, not even a guaranteed value.No noise specificationsat all.I think this chip was probably designed in an age where an individual chip was very expensive, so Motorola integrated everything except the power part into a single chip to offer some cost savings in a lab PSU design. I see no reason at all to use this chip. It doesn't do anything special, and it certainly doesn't do it better than a circuit build from op amps and other discretes.For reference, here are the specs of a~40 yearold precision lab supply:line regulation: 0.0005 % + 100 µV (for voltage) and 0.005 % + 40 µA (for current)load regulation: 0.0005 % + 100 µV, 0.01 % + 500 µA (for current)output noise (measured with 3400A), 20 Hz to 20 MHz: 40 µV, 200 µA (RMS)output settling time: 50 µs to within 50 mVTempco: 0.0001 % + 15 µV/KThe reference stability and PSRR is very easy to achieve, even with crap op amps and crap references (last week I evaluated a simple dual reference supply design with Av = +1, -1, used NE5532s and a simple Zener, not even precision, it achieves more than 110 dB (e: checked my notes) of LF PSRR – although it drifted like hell and was rather sensitive to light ; swapping the random glass-packaged Zener with a LM4040 remedied both).Good load regulation is basically inherent in the design of a lab supply (integrative behaviour), but to get these specs you need to do it right. At anything more than very low currents kelvin/ sense connections are a must or the wire resistances limit performance (severely).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2015, 04:29:30 pm
content: The MC1466 is a rather antique part - hard to get one. It's usually replaced by the LM723 / µA723 , that itself is nearly as old as the 741, but still made.Don't expect high recission (low dirft) from these chips. Especially current limiting is rather crude and far from precise. The curcuit does does not look very good - I have some doubt whether it is stable witch a nasty load like a 100µF low ESR cap at the output. I have not looked at it in detail, but the way the voltage is set and a cap in the feedback to GND look really bad.Also a 2N3055 is usually good for only about 1.5 A at a 35 V supply. So you would need two of them in parallel (or series) to get so much power. To much power for a single transistor is exactly the problem with cheap chinese supplies - don't have to copy that mistake if you are not building to a absolute buttom price.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 15, 2015, 04:33:09 pm
content: Thanks for the heads up dom0. Do you have any designs of a DIY supply to recommend for a starter like myself? Trying to be reasonable with a 24v 3a spec range. I have been trying to find a reasonable used supply (sub $100), but not much luck without gambling on a "unknown/parts only" with ebay. So building one has seemed to be as cost effective, or much better for equivalent, and also providing a solid amount of experience/learning to the endeavor.

author: dom0
author_rank: Super Contributor
timestamp: December 15, 2015, 04:44:33 pm
content: Most people don't need a precision supply. A normal lab supply does just fine for them, and is more easily build (the differences aren't large in the regulator and reference circuitry, but subtle - for a normal lab supply it is sufficient to "just connect the right things together", but in a precision supply it's important how exactly you do it - and other stuff, like parts selection is much more important, while the schematic itself doesn't look too different.).So if you don't need guaranteed tempcos in the range of a dozen ppm/K or so, excellent long term stability and accurate output setting —— then there's really no problem with building any solid lab supply circuit as usual. Take a look at the older pure analog HP supplies for plenty of working examples, the service manuals include theory of operation and complete schematics. Some well-tested DIY variations on the HP design exist, e.g.this schematic (German annotations). I haven't looked for this kind of project in a long while (I just do them myself), but someone else should have a few good pointers in this regard for you - maybe even with English text on the schematic? :-)If you don't re-build a complete project (with a thermal design) you have to do it yourself. It's not difficult to get right, if you follow the recipe (add thermal resistances correctly, when using multiple pass elements don't assume perfect power distribution, and don't calculate for a ambient temperature of 20 °C and junction temperatures of Tj(max) (depending on transistor and package 125 - 200 °C. When having multiple elements, first calculate temperature rise of the heat sink, then temperature rise of each individual dice. The other way around is not commutative).

author: Mephitus
author_rank: Regular Contributor
timestamp: December 15, 2015, 05:04:44 pm
content: I was not specifically looking for a precision supply per se. Hell, for my purposes being within 1% would be more than adequate. I would love to design one, but I am still working my way through the first few chapters of the art of electronics. So as of the moment, its a little outside my abilities. But I enjoy taking a stab in the dark to try and understand basic circuit operation like this. So the advice is really helpful. I have some german speaking coworkers. I will see if they would be willing to translate it for me.Kleinstein: I have a few LM723's in my slowly growing parts bin. Since you have a deeper understanding than me, is there a design you can recommend that I could build out with a few protoboards?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2015, 06:02:12 pm
content: The LM723 is tricky to make is work at low voltages and the current limiting are really poor (uses diode drop as reference). So I would not use this old chip for a Lab supply. Its good for a fixed low noise 100 mA or 2 A (with external Transistor) supply - something more felxible than a LM317.For an own design I would go for the HP style design, using a reference chip (might use LM723 for this purpose), 2 OPs and the output transistors. I don't have a link to a really good schematics. A resonable good version is shown here:http://www.mikrocontroller.net/topic/354091Again in German, but the main part is the circuit diagram. The discussion is about slight improvements but not really that relevant.

author: dom0
author_rank: Super Contributor
timestamp: December 15, 2015, 08:05:46 pm
content: I like to use the LM4040 reference, it's not expensive, but has good tempco and drift (for it's price), especially when used in a current-regulated circuit.