The mad DIY challenge: Would anyone actually build a PSU like this...? - Page 1
Forum section: electronics / general

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 01, 2013, 05:41:35 pm
content: OK, seems I have just wasted a few perfectly good hours for no good reason...A while back Mr. free_electron posted a tear down of an HP/Agilent E3631A triple rail PSU. In passing he suggested that one could do worse than cloning the analog regulator from the E36xx series, if you wanted to build your own DIY PSU.I also suspect people will have noticed PSUs being a hot subject among many forum contributors, beginners in particular. Additionally I felt I could use another PSU, so I decided to take free_electron's idea and run with it (hope he doesn't mind too much...).If I came up with a suitably useful clone, then I might want to just publish it for others to build. This might be an alternative to the numerous, 'lesser' designs floating around on the 'net. I also intended to use the opportunity to show how I would do thermal design, heat sink selection and similar key subjects, and ask for peer review of the 'cloning design decisions' made.To those, who may not know the E36xx design, then the analog regulator is built from fairly common components. Nothing outrageous. Just LM317T / LM337Ts, a voltage reference, opamps and a bunch of transistors, diodes plus passive components.The catch is that you really need N+1 mains transformers to build a clone, where N is the number of output rails. So you'd need 3 mains transformers for a dual rail PSU, which was my initial goalpost. HP's designers worked around this sticky point by specifying a custom transformer with umpteen secondaries, but that wouldn't be an option for a DIY clone, of course.On the other hand, then the hard ceiling when it came to cost, would be the price for a new Agilent E3620A: Agilent's 'low cost' dual rail analog +/-25V @ 1A PSU with multi-turn pot adjustments, 7 segment LED readout and not a whole lot more. In Denmark this PSU will cost you [the equivalent of] about 680 USD excluding VAT/sales tax.So after doing a few basic design maneuvers, I started to price out the components, none of which were hard to find. To put it bluntly, then this project doesn't look like it will get off the ground. Just the majority of the electronic components, bought in small quantities like DIYers usually do, will set you back 171 USD. Nor does it seem likely to me that potential builders would already have the majority of the components, stashed in their junk box or somewhere similar.Mind you, that is *just* the cost of the major components. That means: No PCB, heat sink, connectors, readout (meters or digital) or chassis, nor any of the details I haven't even thought of yet. My design was for a dual rail +/-15V @ 1.5A PSU, which is slightly different from the E3620A.Given this, are there anybody out there, who think they might actually want to consider building such a PSU, if a tested design was available? Given the likely cost and time investment needed, I personally have a hard time justifying spending more time on this idea. A shame, really...

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 01, 2013, 06:07:20 pm
content: I think most myself included don't start a project like this based on building it for a cheaper price but as a learning experience.Sent from my Milestone X using Tapatalk 2

author: mariush
author_rank: Super Contributor
timestamp: August 01, 2013, 06:11:27 pm
content: I don't know how you got to 170$Transformers are not that expensive, especially if you get some standardized ones. I've bought 18v rms 75va (2 separate 9v rms windings) for about 20$ from Farnell .. that's enough for about 15-16v 2.5-3A .. so two transformers plus a couple of relays to switch taps between 9 and 18v would only cost you about 50$Then you have a couple of extra small transformers (separate power for opamps, relays, mcu or separate multimeter chips) for about 10$ each ... opamps, voltage reference, passives, microcontroller another 5-10$ , seven segment displays another 5-10$ if you don't just go for a 16x2 or 16x4 display that's cheaper... ten turn pots or rotary encoders another 10-15$It can be done at under 100$, much lower if you buy in 25-50-100 quantities.Have a look at the Tenma psu I bought a while ago and think how much can everything cost ... it's not that much:https://www.eevblog.com/forum/product-reviews-photos-and-discussion/tenma-72-8695-2-x-32v-3a-5v-2a-mini-review-teardown/?topicseen

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 01, 2013, 06:15:46 pm
content: Quote from: ahnuts72 on August 01, 2013, 06:07:20 pmYep, that is also my foundation for DIY in the home lab. But this time the intent was to get something useful in the end, and I'd feel a bit silly if I end up paying more than the price of a brand new Agilent PSU. However, if a sufficiently large number of people are interested in the project, then that might very well push me over the edge, and try to build it anyway.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 01, 2013, 06:23:32 pm
content: Quote from: mariush on August 01, 2013, 06:11:27 pmI had to use the Danish retail prices from Mouser, not sure if better options exist over here. The general price level is lower in the US, I believe?My current design calls for a pair of 2x9V / 50 VA toroids from Triad, plus a single 2x18 V / 12 VA E-core, also from Triad. The High/Low range switch for the transformers is electronic, accomplished via an optical plus an ordinary triac. This detail doesn't cost a whole lot. It is the number of such details, when combined with the careful, conservative design on part of HP's engineers, which makes the price climb. There are a lot of components in even a basic design. So far I haven't dared to even guess at how large the PCB would be.

author: ahnuts72
author_rank: Regular Contributor
timestamp: August 01, 2013, 06:24:53 pm
content: I also count the knowledge learned as part of the experience so to me it works out in the end.Sent from my Milestone X using Tapatalk 2