Buck Boost as a bench power supply questions - Page 1
Forum section: electronics / beginners

author: lagman
author_rank: Contributor
timestamp: July 25, 2014, 01:00:10 pm
content: Hello,I'm an EE student, and I would like to build a decent power supply for my projects.Right now I mainly play with power LEDs, batteries and power converters. For this I need a safe PSU that can supply at least 3A between 2V and 30V.I already have a PC power supply on my bench that powers my hobby charger. So I have a good 12V source.Knowing that, I would like to build/assemble a small buck-boost converter that has a voltage adjustment knob as well as a current limit knob. I also need LCD screens with at least 3 digits and with a decent accuracy.I looked at Dave's PSU project. But I'm quite lost...I looked into linear regulator designs, but I don't really need to have a perfectly clean output, so a buck-boost topology would better suit my needs as it's more efficient.I looked on ebay and found this:http://www.ebay.fr/itm/DC-Boost-Buck-Converter-5A-Constant-Current-Volt-Regulator-Dual-Display-/161166152412?pt=AU_B_I_Electrical_Test_Equipment&hash=item25864052dc.It seems to suit all my needs, but I would have to remove the blue pots and place some decent knobs and put the whole thing in a case. Is that doable?I'm lost right now, I don't know what to do. So any advice is welcome.I also realize that these questions have been asked many times before, so I'll be happy if somebody can point me to other threads that could answer some of my questions.Thanks in advance!

author: mij59
author_rank: Frequent Contributor
timestamp: July 25, 2014, 01:31:39 pm
content: Hi,You may have a look at this review.

author: polishdude20
author_rank: Contributor
timestamp: July 25, 2014, 05:54:30 pm
content: Quote from: mij59 on July 25, 2014, 01:31:39 pmI've got this little unit and it work pretty well. The buttons make it much easier to control than pots and it works great! But it's only a buck converter which might not be a problem.

author: mij59
author_rank: Frequent Contributor
timestamp: July 25, 2014, 06:36:16 pm
content: Quote from: polishdude20 on July 25, 2014, 05:54:30 pmThe unit you've chosen is also a buck converter, the text is misleading.Please check other sellers on eBay for the same unit.

author: polishdude20
author_rank: Contributor
timestamp: July 25, 2014, 07:47:04 pm
content: Quote from: mij59 on July 25, 2014, 06:36:16 pmWhat are you saying? I know it's only a buck converter but it may not be such a big issue for him.

author: lagman
author_rank: Contributor
timestamp: July 25, 2014, 09:53:06 pm
content: Thanks for the link!I was skeptical about the buttons but if you say it's better than knobs I believe you.As I'll use a 12V input, a buck regulator is quite limiting.On a side note I would like to be able to log the voltage and current on an SD card.I was thinking if using an Arduino to do that using a sense resistor and the analog inputs. Is there any issue doing that?

author: Phaedrus
author_rank: Frequent Contributor
timestamp: July 25, 2014, 10:01:24 pm
content: Keep in mind that switchmode regulators will have significant ripple and high frequency noise. I would not expect great signal integrity if you go north of 100KHz in your projects unless you filter the hell out of the PSU output and take pains to thoroughly decouple everything.But if you're only doing low frequency you'll probably be fine.

author: lagman
author_rank: Contributor
timestamp: July 25, 2014, 10:05:16 pm
content: I don't even own an oscilloscope yet. My projects are very basic for the moment. It'll always be time later to build a linear PSU.Right now I need something powerful (3A), relatively cheap and small. That's why I thought a buck boost would be best. Is that ok?

author: Rick Law
author_rank: Super Contributor
timestamp: July 26, 2014, 12:37:41 am
content: Quote from: Phaedrus on July 25, 2014, 10:01:24 pmI have the same concern, otherwise I think it works rather nice (from the video, not personal experience).I would have prefer a 2004 lcd, see the volt/current presets plus actual volt/current/power on the same screen.

author: mij59
author_rank: Frequent Contributor
timestamp: July 26, 2014, 04:22:19 am
content: Quote from: polishdude20 on July 25, 2014, 07:47:04 pmIt will be a problem if the 12V power supply is used to power the converter.

author: nsayer
author_rank: Regular Contributor
timestamp: July 26, 2014, 05:54:12 am
content: Holy crap! What a find! That thing isnicefor $15!I've ordered one and am going to the local surplus place tomorrow and buy a 24 VDC unregulated supply for it.

author: SeanB
author_rank: Super Contributor
timestamp: July 26, 2014, 06:11:14 am
content: Ordered as well, and have some 24VDC supplies to use with it anyway.

author: Neverther
author_rank: Regular Contributor
timestamp: July 26, 2014, 06:12:51 am
content: I have built mine from LM2577/2596 combo (2596 has current limiting setup) to old VFD housing. The original display on the housing has opening that is exactly wide asthesemeters (3-12mV difference against my better 40 000 count meter)Voltage is just connected to terminals while current meter is single range version of the above with salvaged 20A shunt from fried multimeter connected to LM4562 (best I had in the junkbox at the time) amp.Some filters from ATX supply and 10 turn pots with 14 turn locking knobs I picked from trash and its good enough.2577/2596 are bolted to thick trashpicked heatsink and they cool the chips and traces at the bottom so I've had no problem pulling constant 5A at lower voltages, spikes areound 7A. Max voltage is adjusted to 28V and lowest is the 1.25v reference for 2596 (but you can go lower if you play with current limiting/separate amp, not just divider like I have, but you might werck stability with insufficent filtering, delays/overshoot, etc). I think my feed for it is 16V or 19V laptopbrick...

author: Strada916
author_rank: Frequent Contributor
timestamp: July 26, 2014, 06:44:56 am
content: Siliconchip magazine April, May & June 2014 buck boost hybrid 100W. 0 to 40v and 0 to 5A fully variable. Lab power supply. Requires 12 to 24v @ 10A

author: Rick Law
author_rank: Super Contributor
timestamp: July 27, 2014, 01:23:27 am
content: Quote from: nsayer on July 26, 2014, 05:54:12 amJeezzzeee... temptations, temptations...Quote from: SeanB on July 26, 2014, 06:11:14 amHoly cow! More temptations...But I am using a 2577 based power supply board already...The fictitious figure on my left shoulder said:"so why are you worrying about noise... just go for it."The fictitious figure on my right shoulder said:"Rick, you already have more than one lm2577 based CC/CV boards, why do you need another... just stay away from it; just say no."Mean time, my eBay thumb is itching and scratching it is not helping...

author: Rick Law
author_rank: Super Contributor
timestamp: September 19, 2014, 08:02:54 pm
content: Hate to revive old thread, but this is the best place really.Since a number of you said you purchased one, do you mind sharing some experience on noise level of this power supply?When drawing say 6.5V@<20mA, at 300mA, how's the noise like? at 500mA? 750mA? 1A?I use an LM2596 based board as a power supply. At <20mA, the output is relatively noise free with a near-square wave noise of about +-20mV at 100-ish KHz. By 300mA, it goes to about +-100mV to +-150mV.If one of you owners can post a oscilloscope picture of the outputs, I will much appreciate it.ThanksRick

author: lagman
author_rank: Contributor
timestamp: September 19, 2014, 08:23:50 pm
content: In the end I didn't buy anything so I can't help you... Sorry.I just switched school and the new one is much better and has a lot of equipment. For this reason I don't really need to buy a PSU for the moment as I'll do all my projects at school. Nice!

author: Rick Law
author_rank: Super Contributor
timestamp: September 19, 2014, 09:44:59 pm
content: Quote from: lagman on September 19, 2014, 08:23:50 pmHolding my breath, a couple others said they did brought it... We shall see.Thanks!

author: eas
author_rank: Frequent Contributor
timestamp: September 22, 2014, 08:14:46 am
content: Quote from: Rick Law on September 19, 2014, 09:44:59 pmI've got one I was going to hook it up to my scope for you and capture some traces this weekend, but, well a project started working, and I ran with it. I'll post something Monday if someone else doesn't beat me to it.

author: Rick Law
author_rank: Super Contributor
timestamp: September 22, 2014, 10:52:15 pm
content: Quote from: eas on September 22, 2014, 08:14:46 amThanks! I would appreciate the info a lot when you post it!

author: Lightpath
author_rank: Newbie
timestamp: October 01, 2014, 01:31:10 pm
content: I am interested in this as well- I have several decent quality 12v supplies, and I was thinking of buying a bench top power supply for my projects/tinkering. I'd rather just use the 12v supplies and buck converters, but I am not sure of the downside to that.Anyone have any thoughts on this sort of thing? Is it better to just go out and buy a decent variable output supply? I am mostly just learning electronics at this point, and doing some arduino projects.

author: nowlan
author_rank: Frequent Contributor
timestamp: October 01, 2014, 02:34:52 pm
content: I think a real lab supply will offer you protections such as constant current/eFuse.Once upon a time, I used to use an adjustable power pack, which had 5,9,12v etc.You slide the switch to setting you wanted. Most likely LM317 inside.The ebay boards might be okay, if your using the same voltage all the time.Adjusting it will be a pain without a nice dial or display.

author: Rick Law
author_rank: Super Contributor
timestamp: October 01, 2014, 06:27:35 pm
content: Quote from: Lightpath on October 01, 2014, 01:31:10 pmYeah, it is of course better to "go buy a decent variable output supply" but far less fun. If you just want something that works, go buy one that works. As a hobbyist, one of the thing I found educational is to improvise. To improvise means I have to learn what something needs, and learns how to supply that something adequately. The other issue is $. One is always making a trade off about quality vs $. Less I spend on item A, more I have for item B, C, and D. You can spend a million dollar on a power supply but someone can spend two million and make a better one. So the issue really is "is it good enough for what you use it for."I used a boost+buck cc cv board as power supply for a while now. Attached is a photo of the one I use.Not the one that started this thread, but your question seem to be about typical cc/cv type boards as power supply instead of a specific cc/cv board, so that is what I am addressing.When I purchased them > 1 year ago, they were in the range of $11-12 USD in single quanty. (I have three of this kind). Now they can be had for under $6. Note that like many of those cheap boards, their quality varies.I have found it adequate for over a year, but now looking to something better, read on.The downsides are:1. ripple/noise (+-100mV typical and worst around +-150mV, depending on supply source as well. Noise level is slightly lower with 12V SLA battery as power source). Typically, it is an up-to-100mV saw-tooth wave riding on DC.2. wear out trimpot (if wiper loose contact, voltage is uncontrolled and may hit max!)3. cc value drifts after set but settles, cv is much firmer (+-2% spec looks accurate).4. This particular one is very low power I would not use it for over 5W, but some higher power ones are out thereThe upsides are:1. it is boost+buck, which means you can pretty much choose any power brick (or battery) with adequate power.2. Voltage regulation is very stable (within the ripple/noise) unless load varies significantly.Plus or minus, you choose:You can pretty much tame lot of the noise with a huge capacitor, but it defeats the CC say if you shorted something, your huge capacitor will continue to supply component-killing power. So, when needed, I add a 1000uF or even 4800uF to trim the noise down.Yes, you can "digitally control" them. I had fun doing it. I had removed the trimpots and use an Arduino to control them successfully: 0.1 ohm low-side shunt, ADS1115 as ADC for voltage and current sense, analogy circuits driven by digital produced comparative voltages (12 bit dac for CV and arduino's pwm as dac for CC) so once set it response with analog electronic speed instead of a slow program loop. But difficult in packaging it well and cost make it not worth while to convert for day-to-day use. The ADS1115 Adafruit breakout board alone is more expensive then the power supply discussed in this thread. Even with digital conversion, it would still be the same board with all the draw-backs but one: CC could be programmed to actually push the load to CC verses trimpot setup which really is a current-limit with CV.I found this cc/cv cheap board to be adequate for a year now using mainly for poking around and build a few projects using this to power the initial prototypes for the projects. But the noise issue bother me a lot. I am never sure in my test circuits if my oscillation (or noise) started because of my power supply or my circuit. I am trying to experiment things now that I would like something with cleaner power.By the way, I also have a couple of 5A ones for heavy duty. Their noise could be up to 300-500mV's. Not sure if it is the different chip or just the nature of bigger load, bigger swing. Percentageseemto be about the same if I plot noise vs power but never actually did calculate it as a percentage.Hope this helps answer your question.Rick