Do the readings of power supplies drift?? - Page 1
Forum section: electronics / beginners

author: Rob Sims
author_rank: Regular Contributor
timestamp: August 19, 2016, 03:16:27 pm
content: As I said in a former topic, I have a mechanical engineering degree and I'm a beginner in electronics. I did have some electrical classes, but not so advanced as the electronic engineers had. My question for the advanced and experienced electronic engineers is, do the voltage and/or current readings of a power supply drift in general?? And if so, over what period of time?? Of course it all depends on brand, model etc. Do I have to be concerned about drift of readings??

author: danadak
author_rank: Super Contributor
timestamp: August 19, 2016, 06:33:30 pm
content: Yes, power supplys do drift, both short term and long term. Root causecan be reference used in supply. Or passive drift, like resistors. All thingsage, and there are thermal effects layered on top of all that. Even migra-tion effects in semiconductors, and board effects in metal junctions ofdissimilar ,metals, like solder and copper.Nominally power supplys have V specs, over a T range. Also load effectson Vout.https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&uact=8&ved=0ahUKEwj43bPZj87OAhUG8x4KHU6XBYMQFghEMAo&url=http%3A%2F%2Fwww.analog.com%2Fmedia%2Fen%2Ftechnical-documentation%2Fapplication-notes%2F294542582256114777959693992461771205AN280.pdf&usg=AFQjCNHQkJIKt-nGyJQ_kX3FNygqb5RoEQ&sig2=2e1gNUid8xtaFk3yJyQ8fwhttps://www.dropbox.com/s/ruaf9booe17jk8n/PCB%20Layout.zip?dl=0Some info on errorsRegards, Dana.

author: Rob Sims
author_rank: Regular Contributor
timestamp: August 19, 2016, 10:16:19 pm
content: But what do electronic engineers do in general when the power supply drifts?? Discard it and buy another one, calibrate it, work with the difference etc. Yes I know it all depends, but what is the best strategy when dealing with a inaccurate unit.

author: Macbeth
author_rank: Super Contributor
timestamp: August 19, 2016, 11:16:41 pm
content: I don't see any difference between a mech or elec engineer here. It all comes down to cost/accuracy/convenience. If you have a dodgy PSU you can work around it by using external meters then fine, just like you can trust the DRO or DTI on your lathe vs the marked off readings on the hand winders.Most PSU's can be readjusted, and if your accuracy is only required to be in the 100mV as most are then even the cheapest DMM is good enough.Do you have an actual problem with a PSU you can identify that someone could perhaps help you with? I'm hoping this PSU is not Fukishama here...

author: bitseeker
author_rank: Super Contributor
timestamp: August 20, 2016, 12:06:56 am
content: Quote from: Rob Sims on August 19, 2016, 10:16:19 pmIf accuracy is a requirement, companies that buy lab-grade gear have their equipment calibrated annually (or on whatever interval the manufacturer recommends). Some equipment may be regularly calibrated while others are not, the latter usually being identified with a sticker that says, "Calibration not required" or similar. There are also different levels of calibration that can be purchased, depending on product, customer, legal or other requirements.For hobby use, most of the time the calibration is not critical, especially since the actual output can be checked with a multimeter. As long as you can tell that the output voltage hasn't suddenly gone from an intended 5V to 10V, it's probably good enough. Remember that the display is mostly for reference. Then, you just have to decide to what lengths you want to go to in order to keep the multimeter in calibration, if at all.If the accuracy of the meter on the power supply is a requirement, be sure to only buy equipment that can be adjusted, either by you or a calibration lab (and which brands/models your selected lab will adjust).It doesn't make sense to buy stuff that you know won't meet your needs and will have to be thrown away unless it's one or your hobbies, in which case just throw the cash directly my way. It's more efficient.

author: crazyguy
author_rank: Regular Contributor
timestamp: August 20, 2016, 01:24:41 am
content: Quote from: Rob Sims on August 19, 2016, 10:16:19 pmto answer your question i would rather ask you a questionwhat do mechanical engineers do in general when the mechanical watch drifts ?

author: Brumby
author_rank: Unknown
timestamp: August 20, 2016, 02:35:13 am
content: While all these discussion points are valid in themselves - there is one overriding question you MUST ask, so you don't go throughunnecessarymental anguish........How accurate does itNEEDto be?

author: Rob Sims
author_rank: Regular Contributor
timestamp: August 20, 2016, 11:06:43 am
content: Ok in general, what accuracy do I need when working with transistors, op amps, microchips etc?? It all depends naturally, but what accuracy is needed in general, if let's say you want to do repairs on medical equipment etc??

author: voltsandjolts
author_rank: Unknown
timestamp: August 20, 2016, 11:46:08 am
content: All readings on everything suffer from drift, power supplies tooEnough to be a problem for your application? Probably not.

author: Rob Sims
author_rank: Regular Contributor
timestamp: August 20, 2016, 05:44:51 pm
content: Ok I got it about power supply readings drift. Enough of that.But generally what accuracy do I need when working with transistors, controllers, op amps etc?? Looking at Brumby's reply I think that's a better question. I know it all depends, but will I ever need micro volts when dealing with controllers, op amps etc.??

author: bitseeker
author_rank: Super Contributor
timestamp: August 20, 2016, 06:21:55 pm
content: This all boils down to knowing the requirements for what you're working on. Until you know exactly what you're going to be designing or fixing, and the requirements associated with it for its intended use cases, "generally" doesn't really apply. Why? Because, generally, as long as you operate within the limits of a component as given by the manufacturer's datasheet, then that's generally OK. However, that doesn't mean the device containing those components will operate as required.For example, when you mention medical devices, do you have the specifications for those devices? Are there requirements for how accurate they need to be in order to be considering to be operating correctly? Are those devices going to be in an environment where they have to be operating to specific levels of accuracy? Do those devices have regulatory requirements in addition to any technical requirements?Without knowing the product requirements, a $5 multimeter and some $3 wall-wart power supplies are accurate enough to make transistors, controllers, op amps, etc. operate. It's easy to get caught up in all the cool equipment and making all the displays match up, but unless you know that you need a certain level of accuracy or resolution, you probably don't need it (yet).You've got some good gear already. Get started on what you want to build or fix. As you learn more and/or new requirements come about, you'll adjust what's in your lab accordingly.

author: Rob Sims
author_rank: Regular Contributor
timestamp: August 20, 2016, 11:36:29 pm
content: Many thanks Bitseeker for the time to clarifying things. Happy with the points you mentioned.

author: bitseeker
author_rank: Super Contributor
timestamp: August 20, 2016, 11:55:11 pm
content: You're welcome, Rob. Looking forward to seeing threads about your projects.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2016, 02:52:12 am
content: For general purpose power supplies, 0.1 volt accuracy is enough simply because without remote sense or kelvin connections, the voltage drop through the output connections will vary that much do to current draw. At lower currents where voltage drop is not a consideration, stability will typically be better than several millivolts. If you are using a power supply to measure transistor threshold voltages, then it is best to use series resistance and measure the voltages at the device using a voltmeter.It is not too difficult to make a power supply with stability in the microvolt range or better limited only by its internal reference and any voltage dividers however it is difficult to bring this to the circuit without remote sense or kelvin connections. I once designed and built a simple replacement voltage regulator for my car and achieved better than 1 millivolt load regulation at the starter battery with a load change of about 15 amps. My best handheld meter can only measure a difference of 1 millivolt at 12 volts (40 volt range with 40,000 counts) so it was better than that but I do not know how much better. A designsimilar to thiscan achieve 1 microvolt stability from 0 to amps of load but notice that it uses kelvin connections.

author: bitseeker
author_rank: Super Contributor
timestamp: August 21, 2016, 09:21:35 pm
content: Thanks, David. Good stuff.Note that in the article linked to in David's post, the regulator referenced in sentence three should be LM109, as illustrated in the schematic, not LM108. The LM108 is the op amp.

author: Circlotron
author_rank: Super Contributor
timestamp: August 23, 2016, 11:07:43 pm
content: Quote from: Brumby on August 20, 2016, 02:35:13 amAccurate enough to stop bothering one's OCD. 😬

author: Brumby
author_rank: Unknown
timestamp: August 24, 2016, 04:48:15 am
content: Quote from: Circlotron on August 23, 2016, 11:07:43 pmLOLAvoid digital metering. (There's always room for one more decimal place.......)