I'd welcome feedback on my bench PSU block diagram - Page 1
Forum section: electronics / projects

author: seb1982
author_rank: Regular Contributor
timestamp: March 13, 2013, 01:22:01 pm
content: Hi everyoneI'm starting to design a bench power supply as my first major electronics project for *mumbles* years. I've sketched out a block diagram of the kind of functionality I want (link below), but I'd appreciate any feedback you could give me - thoughts, improvements, ideas etc. Anything that might make my design functionality better, really!I know that it's not massively more complicated to make a switch-mode supply, but I'd really rather stick to linear until I really know what I'm doing with switch-mode stuff. I can always design another power supply later (can never have too many!)I'm not at all experienced in powering projects from the mains, though, so to be honest I'm not even sure if what I've sketched out can be done safely with transformers, so it may be a case of back to the drawing board before I've even started! I'd much rather be safe than sorry, though!I'm not anticipating using all the outputs at the same time to any degree of their capabilities, hence the somewhat conservative fusing.Anyway, any thoughts, ideas or features I haven't included would be most appreciated and helpful.Link to Google docs .pdf:https://docs.google.com/file/d/0B6fjL3QED4PlTHdBUXdRNXJkRTg/edit?usp=sharingThanks ever so much.

author: 2nOrderEDO
author_rank: Regular Contributor
timestamp: March 13, 2013, 03:51:15 pm
content: Hi seb1982!Your block diagram seems quite good, here I have some tips for you:-About the quickblow fuses: If you are going to use massive capacitors for filtering, I recommend you to replace them for a NTC or a polyswitch. Other way they will blow up the first time you power up your power supply.-Such a design is going to need a single HUGE transformer, very expensive one I tought or some small - medium transformers (I think you have choosen last one).-I heartily recommend you to read "The art of electronics", Chapter 6. There you would find much more than what I can write here, and you will learn so much. You can found an "pdf evaluation version" of this book on google ;')If (Questions == TRUE) {Ask();}

author: mariush
author_rank: Super Contributor
timestamp: March 13, 2013, 04:11:01 pm
content: Ditto for the big capacitors causing a big surge when you power the power supply.30v 4A is a big on the high side... and will cause a lot of power being dissipated in the pass transistors.I'd suggest going with 2 transformers, each with 2 secondaries of 15v 4A - then use relays to chain them as needed so that you won't have so much power dissipated on the heatsink.ex:http://www.digikey.com/product-detail/en/VPS36-3600/237-1286-ND/666172http://www.digikey.com/product-detail/en/VPS36-4800/237-1287-ND/66617317$ for 2x18v 130VAA toroidal is 80$ for 2x30v 225VA :http://www.digikey.com/product-detail/en/182F30/182F30-ND/454580So you can get 3 x 2x18v 130va for 50$Otherwise, imagine you want 3.3v DC current limited at 4A - you're pushing 30v AC ( about 40v DC at 4a = 160 watts?) through the pass transistors to get 13 watts?I'd think of consolidating the rest on a third transformer that may just as well be an identical 2x15v 4A - maybe you save some money buying in volume, ordering 3 identical transformers instead of ordering a toroid and two separate transformers.One secondary of that third transformer can get you fixed 15v, 12v, 5v , 3.3, 2.5v and could also give 12v or so to the voltage reference - with 15v ac secondary you can use some low drop regulator to get 15v DCThe other secondary could be used for fan, and to get +/- 5v DC for the meters (for example if you use something like ICL7106) and maybe to also have -15v , -12v ...With the smaller secondaries you may not even need a fan, provided you use a nice heatsink or maybe you just recycle a small microatx computer case or something similar (old ibm/compaq desktop springs to mind, or some of those dell optiplex cases (slim) ... just connect the heatsink on the metal case for better heat dissipation

author: Ghydda
author_rank: Regular Contributor
timestamp: March 14, 2013, 07:51:58 pm
content: Hi and congrats on you talking the plunge into DIY electronics againNo obvious mistakes other than the above mentioned quick blow fuses.With regards to the 30 volt / 3 amp outputs, sure you'll burn a great deal of power in the pass transistors when supplying low voltage and high current to a load from those outputs. And so what?If that really never happens for more than a few minutes at a time then you could go about using an underrated heatsink. As you've said yourself you will rarely utilize the full potential of the supplies.That being said, I would never ever run a power supply without a thermal protection circuit. Regardless of the size of the heatsink - imho - there should always be a thermal protection circuitry.If you choose not the use the tried and tested method of switching between two input voltages to reduce power losses, then you must pay extra careful attention to the safe operating area of your pass transistor. Very few power transistors can handle 3 amp collector current and a drop voltage of 40-50 volt. Most designs dissipating more than 50-60 watts use two or more pass transistors in parallel to stay within that SOA.Cheers