Double Regulator Supply - Page 1
Forum section: electronics / beginners

author: Radiohead
author_rank: Contributor
timestamp: December 16, 2015, 03:04:30 pm
content: Hello All,I’m a digital EE and graduated ~1 year ago and my goals is to designing a power supply. I dislike to work directly with a transformer or with mains voltages, since I’ve never designed something without a proper bench power supply or batteries powering my projects. I currently lack the necessary number of (working) power supplies for a symmetrical bias supply to prototype a proper bench power supply. Therefore I’d like to build a double adjustable supply with the LM317's cousin, the LM1086. In addition to the double supplies, with this project I'll get some experience of the mechanical construction and assembly as well.I am aware that the regulator isn’t current limited and I’m not bothered adding current limitation or other tricks to supply more current. If you do have a simple design which is current limited and not to I might be interested, but I’d rather design a proper PSU later on with all the extra’s (current limiting, V&I displays, etc.). This project is just a learning exercise and something to do for the next few days/weeks in between holidays (and when family comes over). Eventually this supply might be salvaged for the transformer and enclosure when I have a proper current limited bench supply design. I could build a symmetrical regulator with an LM337 or other negative regulator, but having two independent floating supplies is far more flexible. when I need an symmetrical supply I’ll create one by connecting both regulators in series.I would like to ask If you have any comments or know any improvements for this design. Especially the mains part is where I’d like to know if there are additional safety devices I need to place or other precautions in addition to placement and isolation.Just a few questions:In Prasimix’s PSU design he uses an Varistor, Gas Discharge Tube and an ESD diode on the mains input. I guess this is a bit paranoid for a low power supply like this?I’ve chosen to use a IEC connector with an integrated line filter, as this is the easiest solution and re-usable compared to a non-integrated solution. Is it necessary or a total waste and should I use a non-integrated solution?I’ve seen a lot of products and power supplies which have an additional sleeve over mains wiring, but what is the name of this isolation sleeve/application?Note on the schematic: Not all parts have been specified yet, as I don’t want to use Farnell or Digi-key for this small project, and it takes quite a while to find everything.Regards,Radiohead

author: macboy
author_rank: Super Contributor
timestamp: December 16, 2015, 03:59:49 pm
content: Note that you have just shy of a constant 4 mA flowing through the voltage setting resistors (1.25 V always across R3) so your constant current load needs only to be 6 mA not 10 mA. You can get variations of LM317 which require only 5 mA minimum; I am not sure of LT1086.The LT1086 (and LM317) do not have current regulation, but are current limited. The exact value depends on the temperature, the Vin-Vout difference, etc. but is usually max 2 A, dropping quickly as Vin-Vout increases; this is a function of the "safe area protection" which limits power dissipation. Read the datasheet, and take note of the comments in the "stability" section. With your 24 V input, you may get < 100 mA when outputting a low voltage.Other than that, it should be a very basic but functional power supply. consider getting a 3 or 4 digit LED panel meter to monitor output voltage, these can be had for a couple dollars and a couple weeks' wait from ebay etc.side note: If you can deal with a little extra noise on the output, you can buy a cheap DC-DC buck converter power supply (LM2596 based) on ebay and wire it up to track a few volts above the output voltage. This provides a pre-regulator to keep the Vin-Vout small so that you can get full current at any output voltage, and get higher efficiency too. Food for thought.

author: Ian.M
author_rank: Super Contributor
timestamp: December 16, 2015, 04:08:36 pm
content: Make it modular and pick a case with an easily obtainable and replaceable front panel (e.g. a simple flat aluminium sheet). That way the only evidence left after you upgrade/repurpose it is any redundant mounting holes in the bottom of the case.Don't be suckered into getting fancy with panel meters and such. Make it too nice and you'll have less incentive to work on the proper bench supply project.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: December 16, 2015, 06:00:00 pm
content: Quote from: macboy on December 16, 2015, 03:59:49 pmDesign for the 10 mA part, you want to be flexible and not stuck with the special version of the part.Now if you have a 10 mA minimum load spec, then the current sink cannot typically consume 6 mA and the voltage divider 4 mA and presto... Due to tolerance stackups, probably half the time, you will not meet the minimum load specification.So instead, let the load consume lets say 10 mA, and then do the calculation if you feel like it...Also, the schematic does not show a 10 mA current sink. A diode drop across a 49.9 Ohm current sense resistor is more like 14 mA.