Buy a cheap Power Supply or DIY/Mod my old PC PSU ? - Page 1
Forum section: electronics / beginners

author: Discotech
author_rank: Regular Contributor
timestamp: June 02, 2018, 02:28:43 pm
content: So I need a power supply for my breadboards I'm going to purchase so I can make some electronic circuits and I'm wondering whether it's better to just buy a cheap PSU like the one at Farnellhttp://uk.farnell.com/tenma/72-10480/power-supply-1ch-30v-3a-adjustable/dp/2251946?MER=bn_para_1TP_LastViewed_1Tenma 72-10480 - Bench Top Power Supply, 0-30V 3A with Single Outputor Use my spare PC PSU which is 700w and one of the high end types so probably has better components than the Tenma and obviously better reliability/safety features.I was thinking of doing something like thisI figure it would be a great project for me to wet my toes back into electronics and I was going to do it slightly different by not actually having to open the PSU as it's all modular so I can just connect the DIY side of it via the external connectors.I'm just wondering if the components used in that youtube vid are acceptable for what I'm trying to achieve mainly the step up/down converterThe PSU on the 12v rail is capable of 696w @ 58a with 90% efficiency at 50% load so it should be capable of equalling the Tenma's 30v/3a which would be 90wAlso in regards to the converters is it difficult to add my own OVP and OCP to the circuit so I don't go over whatever the rated V/A is for the converter ?

author: james_s
author_rank: Super Contributor
timestamp: June 02, 2018, 07:05:12 pm
content: I've used those little modules and they work pretty well as long as you don't try to run them at the max current they claim to be good for. Building power supplies is fun and they make a good beginner project but if you have the budget you would generally be better off buying one that's all ready to go.

author: Discotech
author_rank: Regular Contributor
timestamp: June 02, 2018, 09:46:47 pm
content: Quote from: james_s on June 02, 2018, 07:05:12 pmI have the budget but what I save by building my own PSU I could put towards a scope or other needed equipment no ? Would certainly get me closer to a DS-1054z if I can save £50 to £100 for building my own just as capable power supply

author: james_s
author_rank: Super Contributor
timestamp: June 02, 2018, 09:57:19 pm
content: It could, or you could end up spending more and cobbling together something that doesn't meet your needs. The choice there depends on how confident you are in your abilities to build something, and how much you value your time. If you are paid hourly, look at how much time you spend building and compare that to how much money you could make by spending that time at work. Of course that ignores educational value and satisfaction.Another option to consider is buying used, I have seen some deals on decent used bench power supplies on ebay.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 02, 2018, 10:00:11 pm
content: For another approach, read this thread:https://www.eevblog.com/forum/testgear/suggestions-for-road-trip-(portable)-dc-power-supply/msg1426994/#msg1426994

author: nanofrog
author_rank: Super Contributor
timestamp: June 02, 2018, 10:13:12 pm
content: Have you considered used?I mention this as you've the ability to get a decent, multi-output linear model inexpensively.For example, there's currently anIso-Tech IPS2303available on ebay.co.uk. Triple output (2* 30V@2A, 1* 5V@3A), and is capable of both CV & CC operation (CV = constant voltage, CC = constant current). FWIW, my GW Instek GPS-3020 is the same exact unit and works well. Currently, the bid is at 49GPB + 12GBP postage.Don't discount the usefulness of CC capability. You smoke fewer parts by using it.

author: Discotech
author_rank: Regular Contributor
timestamp: June 02, 2018, 11:19:20 pm
content: Quote from: nanofrog on June 02, 2018, 10:13:12 pmMy problem with used is I have no clue what to look for and what is stuff to avoid and what is a keeperI have a perfectly good PSU now, it just needs converting to give me the 30v @ 5a which seems to be a good entry level amount for a novice, for CV & CC would I not be able to incorporate those into the circuit design for the mod side of the PSU ?Near 700W on the 12V rail alone should be plenty to give me multiple outputs so I could have 30v @ 5a, along with the standard 12v, 5v and 3.3vI'm confident enough to attempt to build it, but I'm just unsure how much the components will cost me, if I can build for say £20 to £25 then that's money to go elsewhere if we're talking £50 to £80 for ready built new/second hand ones

author: nanofrog
author_rank: Super Contributor
timestamp: June 03, 2018, 02:05:54 am
content: Quote from: Discotech on June 02, 2018, 11:19:20 pmI'd suggest getting a PSU with the following:One that allows you to adjust both voltage and current (CV & CC mode operation).Multiple channels. Particularly one that has at least two of the same specs so they can be connected in either serial or parallel (double the voltage or double the current respectively).Low ripple (linear does a lot better than SMPS's in this regard).+ and - voltages sufficient for driving an opamp.And the unit I linked previously does all of this without spending a lot of money. Seller claims tested & working.A much better value than the new Tenma you linked (BTW, Tenma is a Farnell brand).

author: Discotech
author_rank: Regular Contributor
timestamp: June 03, 2018, 01:36:09 pm
content: Quote from: nanofrog on June 03, 2018, 02:05:54 amI've just been watching this vid from Dave on setting a lab upHe suggests building your own PSU over buying built or second hand and I think it's perfect for a first project but obviously buying second hand/new already built does have its merits in that you won't likely have to troubleshootI mean there's nothing stopping me from building my own for now to dip my toes in and then buying something new/used that's better right ?

author: james_s
author_rank: Super Contributor
timestamp: June 03, 2018, 05:20:22 pm
content: Well sure, if you have the time and the budget then go for the build, power supplies were some of the first projects I built and I still build them from time to time when I need something in particular. It is nice to have one commercially made power supply with all the bells & whistles though.

author: Jwillis
author_rank: Super Contributor
timestamp: June 03, 2018, 06:15:04 pm
content: Quote from: james_s on June 03, 2018, 05:20:22 pmI can understand your frustration.I've been in your shoes....No really I swiped them last night.First off .What do you want and what is your budget.You won't get fancy and cheap in the same box.You can try a cheap Chinese special switch mode power supply, but that's a roll of the dice.You might get a good one you might not.So I suggest Linear all the way.This at least guarantees a degree of isolation from mains high voltage because of the transformer.Much quieter and safer for you and your projects and much easier to fix if something does go wrong.Any of the HY3005 type linear power supplies use the same tried and true design.It doesn't matter which brand.For a single unit they run around GBP 150+SH where as a double runs around GBP 300+SH .But with a little searching you might find one cheaper either new or used.Good luck and cheers.Nice shoes by the way.

author: Discotech
author_rank: Regular Contributor
timestamp: June 03, 2018, 11:15:04 pm
content: Quote from: james_s on June 03, 2018, 05:20:22 pmTime is something I am fortunate enough to have plenty to spare, so I think I'll go the route of modifying my ex PC PSU, then if it doesn't suit my needs either attempt build another from scratch or just buy something decentI'm trying to do things cheap"ish" but even cheaply costs are starting to add up

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2018, 03:29:23 am
content: More than once I've been guilty of trying to take the cheap route and spending more than I would have to just do it right the first time. It certainly is possible to save money if you can cobble something together out of bits you already have. In fact one of my most used bench PSUs is one I built out of a 32V power brick from a thrift store, one of those Chinese switching regulator modules and a digital V/A meter stuck in a project box I salvaged from something at work. I use it often because it's compact and light weight, I initially used a 3A module but it blew up under close to full load so I replaced it with a 5A module that I derated to 3A. The constant current mode has some ugly oscillations but it does work ok.

author: rstofer
author_rank: Super Contributor
timestamp: June 04, 2018, 01:22:46 pm
content: Mostly I just use my bench PS for +5v for logic and +-15V for op amps. Having current limiting is very nice when bringing up a new project. If I were to say 'get a power supply', it would be for current limiting a new design so that it doesn't melt.I also have the Sparkfun kit for the PC PSU. Funny thing, I have never used this supply. Among other things, it has too much short circuit current. Fuses help but this PS can melt a project to the ground without getting warm.https://www.sparkfun.com/products/12867While you are at Sparkfun, search for 'power supply'. There are a lot of alternatives.Search around EEVblog, this topic comes up every week or so. Everybody wants to build a power supply with all the bells and whistles for essentially no money. A real power supply is a pretty complex piece of equipment. If I wanted to build something cheap, I would look at a project with a 7805, 7815 and 7915. These are the important voltages and 3.3V can be easily derived from 5V. Sparkfun has stuff for this. There are plenty of these 3 output power supplies on eBay.I have the Rigol DP832 and it's pretty nice.

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 04, 2018, 01:49:03 pm
content: Getting one of those Tenma supplies certainly isn't a bad idea. Having a proper and reliable voltage and current setting is very helpful.

author: Discotech
author_rank: Regular Contributor
timestamp: June 04, 2018, 03:42:25 pm
content: Quote from: rstofer on June 04, 2018, 01:22:46 pmYou are correct and it seems it's possible but you're looking at costs into the realms what a decent fully built PS will cost.Quote from: rstofer on June 04, 2018, 01:22:46 pmI just got done watching Daves power supply building videos he had (shame he never finished the design) and everything was fine until the 4th video where things just got real complicated, so I've gone from being confident to deciding it's way out of my current comfort zone.... so I think I'll just sell my PC PSU and put that money towards a proper supply that has constant V/C, now comes the fun of googling the various opposing opinions on what's best for £x

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 04, 2018, 03:49:39 pm
content: Quote from: Discotech on June 04, 2018, 03:42:25 pmIf you want one of those Tenma supplies you may want to shoot TopLoser here on the forum a message. He should have a bunch of single and dual channel units in new-like condition for sale. That should be superb bang for buck.

author: Discotech
author_rank: Regular Contributor
timestamp: June 04, 2018, 05:14:42 pm
content: Quote from: Mr. Scram on June 04, 2018, 03:49:39 pmAre they good for the money then ?I was thinking if a cheap PS costs £50 to £80 while it may do the job, the PSU is the heart of all your projects so it's probably one area you really shouldn't cheap out on, so is there anything in the £100 to £150 range that would be a no brain buy, either second hand or new ? or is £100 to £150 still considered "cheap" range ?

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2018, 06:20:03 pm
content: You'll want more than one power supply anyway. I probably have 2 or 3 on my bench at any given time, some commercially made, some I built myself, some fancy, some as simple as a wall wart.

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 04, 2018, 06:56:26 pm
content: Quote from: Discotech on June 04, 2018, 05:14:42 pmDave sells them. Is that good enough?Many folks seem to like them, myself included. Other than an issue with an earlier batch there don't seem to be horror stories about them. It's hard to beat that bang for buck, especially when you get a good deal on a second hand unit.

author: Discotech
author_rank: Regular Contributor
timestamp: June 04, 2018, 11:03:15 pm
content: Quote from: Mr. Scram on June 04, 2018, 06:56:26 pmI'd only got to watching the Korad fail video he did 6 years ago, I have lots to catch up onI've been looking at DPS5005 as well today, they seem a decent little unit

author: nanofrog
author_rank: Super Contributor
timestamp: June 04, 2018, 11:26:57 pm
content: Quote from: Discotech on June 03, 2018, 01:36:09 pmI agree it's nice to build one. But with the cost of a suitable transformer, heatsinks, bulk storage caps, and enclosure, it's cheaper to buy one if you don't already have these parts lying around.You're better off buying linear supplies from the used market IME.

author: Bassman59
author_rank: Super Contributor
timestamp: June 04, 2018, 11:42:25 pm
content: Quote from: Discotech on June 02, 2018, 02:28:43 pmA power supply with an output currentmeteris invaluable, especially when bringing up a board for the first time. You should have some idea of what the board's total current draw should be, and if the power supply meter tells you something odd (like the draw is too high) you know you have some work to do.

author: james_s
author_rank: Super Contributor
timestamp: June 05, 2018, 12:15:42 am
content: There are lots of little Chinese meters available for peanuts these days. I used one in my last build that cost only a few dollars and displays V and A on LED digits. Nice clean looking little thing, with accuracy that is plenty good for the task. They're cheap enough to be stuck in virtually any power supply you build.

author: rstofer
author_rank: Super Contributor
timestamp: June 05, 2018, 12:39:02 am
content: I would put adjustable current limiting at the top of the must have feature list. The other day I was bringing up a Z80 project and I had an address conflict (pretty easy to do). I had set the current limit to about 100 mA and as soon as it powered up, the voltage dropped right off. Probably saved several chips.Adjustable voltage is nice, maybe even important, but current limit is vital! And there needs to be a readout of the current setting (in numbers) not just some knob that you turn and hope it all works out.

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 05, 2018, 09:18:21 am
content: Quote from: Discotech on June 04, 2018, 11:03:15 pmThat's a switcher, isn't it?

author: Discotech
author_rank: Regular Contributor
timestamp: June 05, 2018, 11:01:23 am
content: Quote from: Mr. Scram on June 05, 2018, 09:18:21 amhttps://www.banggood.com/DPS5005-50V-5A-Buck-Adjustable-DC-Constant-Voltage-Power-Supply-Module-Integrated-Voltmeter-Ammeter-p-1062473.html?cur_warehouse=CNNeeds a DC supply to power it so it's really a glorified control panel rather than a fully functional PSU but if I can find a 40v 10a supply somewhere it could be usefulI have the luxury of time to decide on what to get though as most of the other things I've ordered, I've got to wait to be delivered from overseas which could take a while so I'm not rushing into anything

author: Old Printer
author_rank: Frequent Contributor
timestamp: June 05, 2018, 12:36:37 pm
content: I haunted ebay for a while and eventually wound up with a few of the small older HP 6216A units in very nice shape. It took a bit a patience, but they are built like a tank and I paid no more than $60 inc shipping for any of them. I also found some BK 1651 triple output units at good prices, under $50 with shipping. They are solid units that were also sold by Tektronix as the CPS250 so a detailed service manual and schematic were available. I just got a nice BK bench DMM for $20 shipped (non-auto ranging). The bankruptcy of the ITT schools a couple years back flooded the market with a lot of decent equipment at good prices. No way could I build anything that approaches the quality of any of these units. Just my 2 cents.