DIY Lab gear suggestions - Page 1
Forum section: electronics / projects

author: ivan747
author_rank: Super Contributor
timestamp: June 05, 2011, 01:39:17 am
content: Hello there!My intention is to make a post containing lab gear that can be built easily. Links to schematics and tutorials are recommended. You can add more if you want. Here is some gear I have come across:A quick and cheap power supply from an ATX supply. No regulation or current limiting.http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-SupplyIdea: adapt a bench power supply to add some regulators and current limiting.A grounding wrist strap (useful when shipping fees are higher than the strap itself):http://www.instructables.com/id/How-To-Make-A-Grounding-Wrist-Band/Request: a grounding post connection tutorial.Quick regulated breadboard supply:http://www.instructables.com/id/Make-A-Breadboard-Power-Supply/Current range adapter for multimeters, Dave's uCurrenthttp://alternatezone.com/electronics/ucurrent/uCurrentArticle.pdf[\li].Any other suggestions?

author: cyberfish
author_rank: Regular Contributor
timestamp: June 05, 2011, 02:59:36 am
content: I think current limitation is very important for a lab power supply, especially for powering up custom-designed circuit for the first time.I would be very scared to power up my circuits for the first time on an unlimited (or with a limit way too high) power supply. Many things can be permanently damaged by <1 sec surge of high current. And without a current limiter, you won't even notice there is a short until it starts smoking.

author: Bored@Work
author_rank: Super Contributor
timestamp: June 05, 2011, 08:21:23 am
content: Quote from: ivaneduardo747 link=topic=3650.msg48630#msg48630A quick and cheap power supply from an ATX supply.[/quote

author: ivan747
author_rank: Super Contributor
timestamp: June 05, 2011, 12:05:27 pm
content: Quote from: BoredAtWork on June 05, 2011, 08:21:23 am

author: ndictu
author_rank: Regular Contributor
timestamp: June 05, 2011, 12:57:44 pm
content: I came across this one recently:http://tuxgraphics.org/electronics/201005/bench-power-supply-v3.shtmlThis one has voltage/current limits. Is it any good and worth building?

author: Lightages
author_rank: Unknown
timestamp: June 05, 2011, 03:16:08 pm
content: Quote from: ndictu on June 05, 2011, 12:57:44 pmIt doesn't look bad, just don't expect to build a fully working power supply in a case for 24 Euro. You need to buy a transformer, rectifier, capacitor, case, fuse holder and fuse, power cord, display, and controller. If you add up all the parts needed to complete the project you end up closer to $90 USD, and then you need to build it. For $96 USD you can buy this:http://www.goodluckbuy.com/zhaoxin-ps-302d-30v-2a-linear-dc-adjustable-power-supply.htmlSo "is it worth building?" depends on what parts you already have on hand and if you think your time to find the parts and build and test the kit is worth only $5 to $10.

author: ivan747
author_rank: Super Contributor
timestamp: June 05, 2011, 03:53:41 pm
content: I would actually build that supply if I happen to have most of the expensive things. The only thing I don't like is that it uses an AVR. I don't have a programmer for them and they are not sold in my country.What about function generators? And don't forget the classic programmer (if a serial port is still found around).

author: ndictu
author_rank: Regular Contributor
timestamp: June 05, 2011, 04:36:43 pm
content: Quote from: Lightages on June 05, 2011, 03:16:08 pmYeah, I know my single unit can't match price of mass production, I'm thinking more about learning stuff by making my own, but at least producing a good, usable one while doing so.

author: Unknown
author_rank: Unknown
timestamp: June 05, 2011, 04:54:00 pm
content: I think the tuxgraphics design is stupid because of the use of an MCU in the feedback loop. This severely compromises transient response and noise rejection. Last time I checked, the control algorithm was also extremely naive (one DAC unit per ADC sample period, i.e. 2048 ADC sample periods, which is something like 100us). I believe it hard hard time tracking even 60Hz ripple, let alone faster transients. Even an anciest uA743 has a few orders of magnitude more bandwidth than this arrangement. A fully analog setup (eg. the ELV design I've linked to multiple times) may be slightly more expensive and complex (depending on if you count software or not), but will perform much better. You can always add digital control and monitoring if you consider this important, but unless you're going for some serious DSP, the correct place for an MCU is outside the feedback loop, just providing reference levels.I don't see the point of using an ATX supply unless you want lots of current. By the time you've added adjustable voltage, adjustable current limit, filtering and load resistors to meet the minimum load requirements, you might as well build a lab supply from scratch. And whatever you do with the ATX supply, you still have a grounded supply that you can't put in series with another supply.

author: Lightages
author_rank: Unknown
timestamp: June 05, 2011, 05:37:45 pm
content: [quote author=ndictu link=topic=3650.msg48698#msg48698 date=1307291803Yeah, I know my single unit can't match price of mass production, I'm thinking more about learning stuff by making my own, but at least producing a good, usable one while doing so.In that case, it would be better to not just build a kit. A power supply is a good basic learning experience, design to finish. Don't buy a kit, look at some other schematics of other designs and see what you can come up with yourself. Design from application notes from the manufacturers, breadboard a couple of designs, and build your final design. Make your first one a simple single channel low current design with good specs. That way you won't be dealing with lots of smoke when things go wrong and when you have it finsihed you can use it for many things.I agree with the previous post that a CPU is not the best way to control the output quality and it complicates the design when you are just learning.

author: ndictu
author_rank: Regular Contributor
timestamp: June 05, 2011, 05:50:01 pm
content: Quote from: Lightages on June 05, 2011, 05:37:45 pmThat is pretty much what I want, I just wanted to know how good that design is, you know, to start somewhere. I can then change it, improve some things, remove unnecessary stuff and so on.Good point about the micro feedback, I should probably make something out of discrete analog parts.

author: Unknown
author_rank: Unknown
timestamp: June 05, 2011, 06:04:22 pm
content: This threadabout DIY power supplies may be relevant. The HP AN-90B I linked to may be a good read. The ELV schematic is a proven, well-tested design, which is not too complex. The only thing I might omit is the over temperature protection. The disadvantage is that the text describing the circuit is in German. If anyone has a design of similar quality with good documentation in English, please post.

author: Lightages
author_rank: Unknown
timestamp: June 05, 2011, 06:54:45 pm
content: A good place to start out for any project is to look at this website:http://www.epanorama.netand for the specific topic of adjustable power supplies this link:http://www.epanorama.net/links/psu_linear.html#adjustablelinearpsuI built quite a nice power supply a few years ago and designed it from information from epanorama and from application notes from the chip manufacturers. The one problem I faced is I used tantalum capacitors in places that I wanted good characteristics, but alas when tantalums fail they usually short out and in this case it took out quite a few other parts.Good luck with your builds!

author: ivan747
author_rank: Super Contributor
timestamp: June 06, 2011, 07:07:02 pm
content: I finally found what I was looking for:http://tgbuilds.wordpress.com/projects/diy-bench-supply/It's an ATX supply modified with adjustable voltage and current limiting with a display. Cool. Now I only need some function generator and I will save around 150 bucks.

author: Unknown
author_rank: Unknown
timestamp: June 06, 2011, 07:29:33 pm
content: How does the adjustable current limiting work? Can't find any reference to it in a glance. Does it go down to 0V? To me it looks like basically the same as a standard LM317 circuit (which is not bad, but certainly not equivalent to a commercial lab supply) with some useless switches added.

author: phil_jp1
author_rank: Regular Contributor
timestamp: June 06, 2011, 07:37:53 pm
content: I think, that building regulated power supply from computer PSU it's not so good idea, and here's why:1. Ripple noise could be pretty high - bad for powering OP amps and analogue circuitry. (needs heavy filtering)2. When trying to get voltages higher than standard - it could became unstable or will not give you the current you need. (really depends on what supply you will get)3. It's NOT isolated from ground. - sometimes could be very handy to connect different psu or other equipment in series.But computer PSU's really good for powering some heavy consuming stuff and the biggest plus - they're very cheap.I think it would be better to build one not regulated power supply from computer PSU and then build ground isolated linear power supply with low ripple noise and adjustable voltage/current. - anyways one power supply is never enough and you will end up with two or three of themAnd if you built an electronic load you always can use it with any power supply to limit the current!In one week from now I will do in-depth video tutorial on how to convert computer PSU to lab bench power supply.If you need that info - you can checkhttp://jumperone.com/for this video in one week.Or subscribe to my youtube channel:http://youtube.com/user/JumperOneTVWeek or two after that I will do tutorial on how to design and make your own adjustable linear power supply on lm317. (but for now without current limit)P.S. Bread board power supply - well... if you have ordinary adjustable PSU - you will never need one.- Phil

author: ivan747
author_rank: Super Contributor
timestamp: June 06, 2011, 07:55:43 pm
content: Quote from: alm on June 06, 2011, 07:29:33 pmI don't think I'm the right guy to explain concepts on forums but I will try:A current limiter is an op-amp feedback loop. It uses a current sense resistor on the output to measure the voltage on it and, according to Ohms law I=V/R. Configuring the op-amp's gain you can compensate a smaller resistor. The op-amp feeds a transistor and senses the current flowing through it. The op-amp will try to adjust the transistor to make the current flowing through the circuit to be the voltage being fed to the non-inverting side. The inverting side is connected to the sense resistor.An LM317 has an op-amp inside of it and the Adjust pin is used for feedback.

author: ivan747
author_rank: Super Contributor
timestamp: June 06, 2011, 08:01:14 pm
content: Hmm... I think I started one of those threads where everyone agrees and do a post about it. Man, those are explosive. This thread was about cool lab gear that could be built by anyone in the hobby and turned into a discussion about DIY power supplies and ATX supplies.By the way, idiot question: can an ATX supply be isolated from the mains by cutting the path between the Earth and the circuit ground and tying Earth to the chasis?

author: phil_jp1
author_rank: Regular Contributor
timestamp: June 06, 2011, 08:17:41 pm
content: Quote from: ivaneduardo747 on June 06, 2011, 08:01:14 pmYes it can, BUT not entirely. You will end up with few small value filtering caps going from the mains (Neutral and Hot) to the ground of power supply (not earth, but black ground wires)

author: ivan747
author_rank: Super Contributor
timestamp: June 06, 2011, 08:36:34 pm
content: Quote from: phil_jp1 on June 06, 2011, 08:17:41 pmWhy do they do this?

author: phil_jp1
author_rank: Regular Contributor
timestamp: June 06, 2011, 08:52:37 pm
content: Quote from: ivaneduardo747To protect output circuitry from switching noise. (there's capacitive connection between transformer's windings, and those caps making a return path for current to lower PSU output ripple & noise)

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 06, 2011, 09:12:42 pm
content: Quote from: ivaneduardo747 on June 06, 2011, 08:01:14 pmyes. i did this stupid thing that powering my project right now. switchable earth/isolated connection, but i seems to default it to grounded most of the time, not floated.

author: Unknown
author_rank: Unknown
timestamp: June 06, 2011, 11:45:36 pm
content: Quote from: ivaneduardo747 on June 06, 2011, 07:55:43 pmWhere is the sense resistor in that schematic? Where is the LM317? The only thing I see is an LDO regulator with a resistive divider connected to what's presumably the adjust pin, plus some panel meters and switches to make it into a lousy DMM.Note that cutting the ground connection will create a safety hazard. The equipment was not designed to be double isolated, otherwise there wouldn't be a ground connection. This means that a single fault can connect mains to the secondary side or the chassis. This isn't an issue as long as it is grounded, since it will just blow a fuse. If the ground connection is broken, you have a potentially lethal situation on your hands. Cutting ground connection will also destroy the EMI suppression.

author: ivan747
author_rank: Super Contributor
timestamp: June 07, 2011, 12:04:48 am
content: Quote from: alm on June 06, 2011, 11:45:36 pmI just got confused. Could you post a link to the schematic of the current limiter you're talking about? Thanks!The current sense resistor is a very low value resistor (less than 1 ohm) that is in series with the output.

author: Unknown
author_rank: Unknown
timestamp: June 07, 2011, 06:11:01 pm
content: Quote from: ivaneduardo747 on June 07, 2011, 12:04:48 amI was talking about the schematic you posted just one post above mine:Quote from: ivaneduardo747 on June 06, 2011, 07:07:02 pm

author: ivan747
author_rank: Super Contributor
timestamp: June 07, 2011, 11:32:34 pm
content: Quote from: alm on June 07, 2011, 06:11:01 pmI think it has no limiter. The creator says there will be one on the next version.