Wallwart with regulators or dedicated power supply?  - Page 1
Forum section: electronics / general

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 01, 2010, 11:23:23 pm
content: I'm working on a project right now that uses some very sensitive analog circuits. So to power this project I was thinking of using a dedicated power supply like this:http://www.mouser.com/catalog/specsheets/55225L.pdfBut then I thought that since I don't need anywhere near 25W of power I should just use a wallwart with some regulators, but I'm worried that the wallwart will inject to much noise into the power rails, especially since I will then need a voltage inverter (I need a -10V to -15V) then as well. Which method do you think will generate less noise? I priced out both options and they are priced relatively the same when considering all components and connectors.

author: Feanor
author_rank: Regular Contributor
timestamp: November 02, 2010, 05:37:59 am
content: I would want to avoid dealing with the mains AC needed to power the dedicated power supply.Rectifying and regulating the low voltage AC from a "wallwart" (I had to google this we do not call them that in Australia) with out any noise problems should be quite possible. In fact you should be able to beat the specs on the dedicated power supply easily with standard voltage regulator circuits. The voltage ripple in the data sheet is in the 10's to 100'd of millivolts and the supply is a switch mode.So rectifying and regulating everything yourself would be my choice.

author: Simon
author_rank: Unknown
timestamp: November 02, 2010, 09:21:55 am
content: you could surround the inbuilt power supply with a shield, having direct access to the transformer will enable you to directly rectify a + and - rail without using inverting power supplies that will also inject noise as you will have to use switching methods

author: scrat
author_rank: Frequent Contributor
timestamp: November 02, 2010, 09:50:12 am
content: Quote from: Simon on November 02, 2010, 09:21:55 amTo obtain the two voltages + and -, the transformer has to be center tapped or you have to use a half-wave rectifier.

author: Simon
author_rank: Unknown
timestamp: November 02, 2010, 10:29:25 am
content: Quote from: scrat on November 02, 2010, 09:50:12 amCorrect

author: scrat
author_rank: Frequent Contributor
timestamp: November 02, 2010, 10:46:42 am
content: Quote from: Simon on November 02, 2010, 10:29:25 amI was considering the fact that a half-wave rectifier makes more noise (and at half the frequency) than a full-wave.The problem with wallwarts is indeed they don't come with + and - outpus, so you'd have to put there a switching converter to obtain them. A center tapped transformer would be the simplest choice.

author: Simon
author_rank: Unknown
timestamp: November 02, 2010, 10:48:24 am
content: yes the half wave would so a centre tap will be best, I would think that sensitive analogue would not really want a switching inverter due to the noise

author: Feanor
author_rank: Regular Contributor
timestamp: November 02, 2010, 11:44:18 am
content: This is a possibility for getting your plus and minus voltages from a transformer with no center tap. And no switching inverter. Only good if you are drawing very little current from your negative rail. The more current you want to draw from your negative rail the larger C1 has to be. C1 should also be rated for your transformer peak to peak voltage and your negative rail load current. So you are looking at a beefy cap, maybe a few in parallel.It works by creating another isolated AC voltage by capacitive coupling. And then rectifying this voltage in the opposite polarity. With the little full bridge packages available you could do this with 5 components. Plus the regulators you will need to smooth the ripple.Two bridges, two filter caps and one coupling cap.This is just an idea, I have no evidence to support it working other than this simulation.But it would be worth a try.

author: Simon
author_rank: Unknown
timestamp: November 02, 2010, 11:52:17 am
content: oh hang on a minute, what is to stop you doing it without the cap ? doesn't the rectifier isolate input from output allowing you to do what you want with the two outputs ? (I know I'm probably wrong)if you use two half wave rectifiers and have a decent voltage drop across the regs using linear regs you should get a good output, or make sure you over rate the smoothing caps and possibly use a slightly higher amperage transformer

author: scrat
author_rank: Frequent Contributor
timestamp: November 02, 2010, 12:02:11 pm
content: Quote from: Simon on November 02, 2010, 11:52:17 amWith the two full-wave rectifiers, if the cap isn't there, your're going to short the transformer output (not really a short, but only two diodes in series).Another solution could be a capacitive splitting: the rectifier output is splitted by two caps in series, with the center "tap" as ground. If it is well regulated, and the pos/neg power consumption isn't so unbalanced, it should work and be very simple.

author: Feanor
author_rank: Regular Contributor
timestamp: November 02, 2010, 12:24:28 pm
content: Like this? This could work I guess yes. I like this even better in fact this I have done. Don't know why I did not just put this up first. Might be that IQ of mine below room temperature or something.

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 02, 2010, 03:04:19 pm
content: Thanks for the replies but I'm a little confused at what you guys are getting at. The power supply I linked to (the model I'm looking at) is a triple output PSU. It has a +5, +12, and -12V output so no tapping of the transformer required. If I were to go with the PSU I would most definitely shield it and place it a couple inches away from any other electronics.Going back to the wallwart idea, the voltage inverter thing still seems like its gonna be an issue. I've used these things a few times before and every time I've gotten very noticeable voltage spikes that were I real pain to get rid of. If I were to go this route, I think I would have to use an LC filter.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 02, 2010, 03:32:15 pm
content: If really low noise is your goal I would think that any kind of switchers or inverters are out of the question. From my experience instrumentation power supplies are almost always linear (or batteries if ultra low noise is required) and usually rated to provide twice the current actually needed by the circuit. I don't know why you would avoid building your supply. Just start with a transformer something like this ....http://www.allelectronics.com/make-a-store/item/PCTX-816/PC-MOUNT-TRANSFORMER-16VCT-0.35A-OR-8V-0.70A/1.htmlA couple full wave bridges, some caps, maybe a choke and filter cap if you're concerned about RF.*shrug* Just my $0.02.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: November 02, 2010, 03:37:52 pm
content: Wall warts are available with triple outputs or more. If this is a one-off project, look at the local thrift stores like Goodwill. Look for supplies with multi-pin connectors. +/-12v & 5v supplies are not too uncommon. RadioShack used to sell one that was supplied from some failed computer company. It had a large 5-pin DIN plug with those voltages.Jon

author: scrat
author_rank: Frequent Contributor
timestamp: November 02, 2010, 03:49:29 pm
content: Quote from: TopherTheME on November 02, 2010, 03:04:19 pmSome of the previous posts were related to the wallwart idea. The dual voltage is achievable for example with the cap splitter, which feanor has simulated and also done.I didn't think there were common wallwarts with triple output. This makes much of the info posted above unuseful.

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 02, 2010, 06:31:09 pm
content: Quote from: JohnS_AZ on November 02, 2010, 03:32:15 pmI thought about it but I figure that something off the shelf will probably give better performance for a lower price than something I could build.QuoteWhere can I find these at? I don't see any on Mouser or Newark.I'm having a hard time finding a good linear supply that meets my needs so I might just go with a medical grade supply like this:http://us.tdk-lambda.com/lp/ftp/Specs/km.pdfIts still a switching supply but the ripple is only 50mV and I can probably easily take that out with a couple of LC filters. Although I still may have to do some shielding since I'll still have the mains coming in. Right now I'm trying to copy the PSU design off of a lock-in amplifier and oscilloscope.

author: qno
author_rank: Frequent Contributor
timestamp: November 02, 2010, 06:44:11 pm
content: Just build it yourself.A 470 uF capacitor, an LM78xx and a few decoupling caps (see datasheet LM78xx) will have an output noise of <1mV.If you only use a few milliamps the LM78xx does not have to bee cooled.If you need really really low noise put 2 in series i.e. if you need 5 volts first use a 12 Volts and then a 5 volt regulatorhttp://www.national.com/ds/LM/LM340.pdfFor low noise power supplies stay away from switchers!!!

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: November 02, 2010, 06:58:05 pm
content: Well if you have more money than time, I'd suggest that you look over these ...http://www.acopian.com/dual-screw-5-10-12-15-m.htmlEnclosed, linear, bipolar output, and 1mV RMS ripple.Personally, I'd build one.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: November 02, 2010, 08:18:38 pm
content: Here's an example from my junk box, picked up at the local Goodwill for a couple bucks.It's actually a quad output, +/- 15, 5 and 3.3

author: Feanor
author_rank: Regular Contributor
timestamp: November 02, 2010, 10:11:24 pm
content: Quote from: TopherTheME on November 02, 2010, 03:04:19 pmWith your standard two wire wallwart (No center tap)The voltage inverter would not be necessary because you can get a positive and negative rail with two diodes and two capacitors as shown earlier.Quote from: TopherTheME on November 02, 2010, 06:31:09 pmYou can beat the pants off 50mV with a standard, out of the data sheet linear regulator in fact 1mV is very realistic. Check Qno's post here.Quote from: qno on November 02, 2010, 06:44:11 pmSo you would need one positive linear regulator and one negative linear regulator. If you decided to put two in series as suggested above then you would need two positive and two negative regulators and their supporting components. That is just a handful of components for a few bucks. If you do decide to put two in series and you want +/- 12V you could use variable regulators. Start with an 18V wallwart set the first positive one to regulate to 15V and the second positive one to 12V. Then the first negative one to regulate to -15V and the second negative one to regulate to -12V.As opposed to buying a medical grade power supply from TDK-Lambda that is rated to withstand 4kV. I do not know how much that will cost but I am scared thinking about it.Just think of the money saved. I am guessing around a hundred dollars per unit.? The wallwart, and linear regulators would still be my choice.

author: TopherTheME
author_rank: Regular Contributor
timestamp: November 03, 2010, 01:21:29 am
content: Quote from: Feanor on November 02, 2010, 10:11:24 pmI can get the lambda medical power supplies for about $50, and thats with triple output. As for building one, I would still need several regulators, an inverter in one form or another, all the accompanying components, plus the extra PCB space to put it all on. After you add all that up plus the time to include it in the design, its no longer just a few bucks.But, you guys convinced me to try building it. I'll first try finding an AC-DC adapter with some kind of negative voltage output like John Chandler posted so I don't need an inverter and just use linear regulators to get the voltages I need. Every rail won't be pulling more than 200mA so I don't think heat sinks will be required, although I'll still include a fan since many components are temperature sensitive.

author: scrat
author_rank: Frequent Contributor
timestamp: November 03, 2010, 08:57:33 am
content: Building your own is a good idea, since then you can reuse the design and experience for further projects. The cost, even in the case of two regulator stages, would be lower than 50$ in components and work, if you apply datasheet schematics.Confining all of the high voltage into the wallwart (outside your board) should reduce many problems.