Precision voltage divider - Page 1
Forum section: electronics / beginners

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 01:28:17 pm
content: Hi guys im trying to turn 32 volts into 5v using a voltage divider. The precision of the device concerns me as because it is fed to an 16-bit analog to digital converter. Could someone provide me a circuit that will precisely step down the voltageI tried using resistors but the tolerance of them is too high....BTW I'm 15 and just got into electronics, I do not have any degree of electrical engineeringPlease dont use technical terms when describing the circuit as im a noob :/Thank You)

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 01:30:46 pm
content: You did it the right way. Using the resistors. You can get very high accuracy resistors.They get disproportionately expensive. But 1% are almost as cheap as normal ones, and 0.1% (approx), are usually affordable. Beyond that can begin to get pricey.But be careful that the input impedance (resistance if you like), of the ADC, is not loading the resistors too much.I.e. You may need an op-amp configured as a buffer, or need to use low enough resistances, so that it does not effect the accuracy too much.Do you really need HUGE accuracy ?

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 01:39:24 pm
content: Quote from: MK14 on March 27, 2016, 01:30:46 pmIm basically building a power supply with a digital voltmeter and this is why i need to step down the voltage. The voltage will only be shown to 3 decimal places. (X.XXX Volts) What value resistors and tolerance would I use. I already have 1% tolerance resistors.... Money isnt a problem but is it worth getting 0.1% and 0.01% tolerance resistors?

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 01:47:22 pm
content: Quote from: szymonm2 on March 27, 2016, 01:39:24 pmI think 0.1% are probably the ones to go for then. Power supply meters are usually somewhat accurate, but no where near, multimeter levels of accuracy, usually, except (typically) ultra expensive lab/precision PSUs.I think 0.1% should be fine..01% would usually become VERY expensive, and may need other somewhat complicated (design/construction/calibration) stuff and expenses, to achieve that level of accuracy. E.g. Pricey, precision op-amps etc.

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 01:55:52 pm
content: Correction to above:If you can find better than 0.1% (and it is somewhat cheap to yourself), then don't rule it out.For surface mount parts, these 0.05% are £1.05 + VAT.http://uk.farnell.com/vishay-draloric/tnpu0603100kazen/res-thin-film-100k-0-05-0-1w-0603/dp/1877592But I still think 0.1% is fine, for most PSU applications.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 01:58:59 pm
content: Quote from: MK14 on March 27, 2016, 01:55:52 pmOk last question. What value resistors will I use for thisThanks

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:01:30 pm
content: Quote from: szymonm2 on March 27, 2016, 01:58:59 pmDo you know what MCU ADC you will be using (presumably YES, as you said 16 bit) ?Specifically I wanted to check the input impedance. But in all likelihood, it will need an op-amp buffer (voltage follower).

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:04:16 pm
content: Quote from: MK14 on March 27, 2016, 02:01:30 pmIm using the ads1115 16 bit analog to digital converter

author: mij59
author_rank: Frequent Contributor
timestamp: March 27, 2016, 02:07:39 pm
content: The precision of the resistors is not that important, you'll still have to calibrate the system.Temperature coefficient and long term stability are more important.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:14:53 pm
content: Quote from: mij59 on March 27, 2016, 02:07:39 pmThe power supply has 3 fans and big heatsink so temperature isnt a problem ....

author: mij59
author_rank: Frequent Contributor
timestamp: March 27, 2016, 02:23:14 pm
content: Quote from: szymonm2 on March 27, 2016, 02:14:53 pmThe temperature gradient across the divider resistors should be as small as possible.

author: pelule
author_rank: Frequent Contributor
timestamp: March 27, 2016, 02:25:47 pm
content: A question coming up is, what is the qualityof the 32v you like to turn down to 5v.Is it stable over temperature and at value enough?The best precision of the devider doesn't care, if that 32v is not stable.So you may want to use a 5v reference IC instead.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:26:02 pm
content: Quote from: mij59 on March 27, 2016, 02:23:14 pmCould you please explain why that is?

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:26:18 pm
content: Quote from: szymonm2 on March 27, 2016, 02:14:53 pmWhat he seemed to mean, was that if it was going into large scale production or only one was being made by an experienced electronics engineer. There are partly cheaper ways of doing it.But for a one-off, done by someone learning about electronics (by the sound of it). Splashing out on better parts, to avoid complications, makes sense, to me at least.To the OP:As regards the resistor divider. A quick look seems to indicate, that only a few values are available at 0.1%.So something like a 10K and 1K resistor, giving a divide by 11, is what you probably need to go for.The input impedance of the ADC (you linked to above), is not too bad. Depending on specific ADC settings, as it has programmable gain control (apparently).

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:27:33 pm
content: Quote from: pelule on March 27, 2016, 02:25:47 pmI checked the 32v voltage with a school oscilloscope and it was 100% stable....

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:31:56 pm
content: Quote from: mij59 on March 27, 2016, 02:23:14 pmMy interpretation of the OPs posts, is that they are trying to give their hobbyist, run of the mill, starter, PSUs. Metering capabilities, using an ADC.Not a lab instrument, which needs 0.00000000000000000000000000000001% accuracy, over 10 years for space flight work.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:32:46 pm
content: Quote from: MK14 on March 27, 2016, 02:26:18 pmIts only a project for myself only. As I stated in my first post Im only 15 and just got into electronics. On ebay.com you can actually get 10k and 1k 0.01% or 0.1% tolerance resistors but These cost like 6$ each.

author: mikerj
author_rank: Super Contributor
timestamp: March 27, 2016, 02:34:47 pm
content: Quote from: szymonm2 on March 27, 2016, 02:26:02 pmResistors have a temperature coefficient, i.e. they change their resistance slightly as their temperature changes. You can buy very expensive resistors to minimise this effect, but in a potential divider it doesn't matter provided the two resistors change their resistances proportionally. To ensure this you need to use the same type of resistor in both halves of the potential divider, and keep them physically close together to ensure they are both at similar temperatures.

author: TheAmmoniacal
author_rank: Unknown
timestamp: March 27, 2016, 02:35:47 pm
content: No voltage signal that you can easily make is that stable, you should see a ripple on your 32V rail. Look up Dave's video on measuring PSU noise.https://www.eevblog.com/forum/blog/eevblog-594-how-to-measure-power-supply-ripple-noise/

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:36:01 pm
content: Quote from: szymonm2 on March 27, 2016, 02:32:46 pmI assumed you would be buying stuff from farnell, or similar.Some of the stuff on ebay, is probably fake/poor-quality etc, stuff from China.You SHOULD NOT need to pay more than £1 (if that), each for the resistors.It would probably be a complete waste to pay any more, because it is NOT needed for your project.

author: mij59
author_rank: Frequent Contributor
timestamp: March 27, 2016, 02:38:33 pm
content: Quote from: szymonm2 on March 27, 2016, 02:26:02 pmIf both resistors have the same temperature, a temperature drift will not affect the voltage divider ratio.

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:42:47 pm
content: Quote from: mij59 on March 27, 2016, 02:38:33 pmAre we designing a 0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001% accurate lab instrument here ?Or an electronics hobbyists, first ever PSU, which needs 1% accuracy, if that ?tl;drA beginners first PSU, does not need to worry about stuff like that. It is for high precision power supplies.See this:Quote from: szymonm2 on March 27, 2016, 01:28:17 pm

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:43:28 pm
content: Quote from: MK14 on March 27, 2016, 02:36:01 pmI was pretty sure that ebay was the best place to buy components....

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:46:15 pm
content: Quote from: szymonm2 on March 27, 2016, 02:43:28 pmWhat makes you think that ?Buying from Farnell/Digikey/RS etc, usually ensures that the components are NOT cheap fakes (poor quality/problematic etc) from China, and have dependable characteristics/datasheets.

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2016, 02:47:17 pm
content: Quote from: szymonm2 on March 27, 2016, 02:26:02 pmGood to see someone young doing something they find difficult, asking questions and learning. Youwillmake mistakes, and with 20:20 hindsight will realise that next time you would do X a slightly different way because of Y. Anecdotes like that are very important when it comes to getting a job in any field, because itdemonstrates(i.e. not merely asserts) that you do more than necessary because you enjoy it, and that you can learn. Plus it helps the interviewer find soemthing to talk about!Anyway, to your question.No component is perfect, no component is ideal. You obviously understand about tolerance, but this is about the "temperature coefficient of resistance" or tempco. A tempco of, say, 100ppm per degree C (100ppm/C) means that if the temperature changes by 10C, then the resistor value will change by 0.1% (i.e. 100*10e-6 * 10 -> 10e-3) or less. Thus a 1000ohm resistor might become a 1001ohm or 999ohm resistor.Since you are making a voltage divider, if both the resistors change by 0.1% then the divided voltage will remain the same (e.g. 10000/1000 = 9990/999 = 10010/1001). But if there is a temperature gradient then the resistors will be at different temperatures and will change by different amounts and the output voltage will change (e.g. 10010/999 != 10000/1000).To minimise that it is possible to get matched resistors that are thermally coupled, typically "array resistors". But I doubt you really need that for your purposes.BTW for future reference, there are many other "coefficients", e.g. with voltage (i.e. a nonlinearity), with time (i.e. aging), with frequency (i.e. parasitic capacitance/inductance) and so on. Don't worry about those yet, but file the information away so you can understand things in the future.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:49:02 pm
content: Quote from: MK14 on March 27, 2016, 02:42:47 pmAll I'm aiming for is 3 digit accuracy for the voltmeter inside the power supply... I dont mind paying extra 10£ for the components if I get that 3 digit accuracy.

author: TheAmmoniacal
author_rank: Unknown
timestamp: March 27, 2016, 02:49:07 pm
content: I've been buying stuff off eBay and Aliexpress for the last two years and had a good experience doing so. Buying from the larger distributors only adds unnecessary cost to hobby projects.I'd say, go with Farnell/Digikey/RS only if you feel that it's necessary. You will always get your money back on both eBay or Aliexpress if the components don't match the description, so there's not much to lose (only time).

author: daqq
author_rank: Super Contributor
timestamp: March 27, 2016, 02:49:32 pm
content: QuoteAssuming that you do not mind the occasional fake part popping up, recycled garbage sold as new and basically running the risk of getting a random part then yeah, ebay is the best placeEbay is great for some things, but I wouldn't stick anything from there into something for a customer.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 02:52:52 pm
content: Quote from: tggzzz on March 27, 2016, 02:47:17 pmGuys THANK YOU so much for all the support I have gotten. Its hard to find information about my area of interest in school. In our physics class we do really basic stuff like the ohms law and what is a circuit which is not enough for me...

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 02:56:48 pm
content: Quote from: szymonm2 on March 27, 2016, 02:52:52 pmThe following book is quite good, for learning about a lot of the more complicated stuff, in electronics:http://www.amazon.co.uk/The-Art-Electronics-Paul-Horowitz/dp/0521809266/ref=dp_ob_title_bk

author: TheAmmoniacal
author_rank: Unknown
timestamp: March 27, 2016, 02:58:10 pm
content: Another thing I want to add. If you already have a decent ohmmeter (DMM), you don't necessarily need high tolerance resistors, you just need stable resistors (low tempco etc). So if you go with an assortment of resistors, you can make your voltage divider based on the resistance measured and not the resistance marked.If the resistors are from the same manufacturer, they most likely have the same characteristics towards temperature. So by simply placing them close to each other they should ought to be "matched". Could get away with cheap resistors?

author: daqq
author_rank: Super Contributor
timestamp: March 27, 2016, 03:01:44 pm
content: There are these fellas:http://uk.farnell.com/webapp/wcs/stores/servlet/Search?catalogId=15001&langId=44&storeId=10151&categoryId=700000005448&showResults=true&aa=true&pf=111858610Prebuilt precission resistive dividers with a fixed ratio that have very low temp coefs.

author: tggzzz
author_rank: Super Contributor
timestamp: March 27, 2016, 03:21:00 pm
content: Quote from: szymonm2 on March 27, 2016, 02:52:52 pmYou're welcome. Don't neglect the basic stuff - itisimportant for many reasons, not least because it encourages a "way of thinking", a framework on which to hang more complex examples. And when I went to university I thought I knew all that simple stuff - but I didn't and got a very poor test score. Fortunately that was only two weeks into the course, so I had plenty of time to readjust my attitude.As a taster for the future, if you have any number of power supplies andanynetwork of resistors, then it can always be simplified to asinglevoltage source plus series resistor (a "Thevenin" source), or an equivalent single current source plus parallel resistor (a "Norton" source).One thing you can start doing soon is thinking about yourrequirementsrather than solutions. Based on your requirements, you can work out a specification which is as wide as possible (and therefore as cheap and reliable as possible). For example, many circuits will work within a range of voltages, e.g. TTL logic is 5V+-0.25V, LVC logic is 1.5V-5V (but slower at lower voltages). Thus a +-0.1% voltage supply for those would be overkill, e.g. a +-2% supply would be fine for TTL.

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 03:40:48 pm
content: You could put an ebay, surprisingly cheap £1.88 including postage, dual volt/amp 3 digit meter, such as this:Which does most of the stuff for you.http://www.ebay.co.uk/itm/DC-100V-10A-Voltmeter-Ammeter-Blue-Red-LED-Amp-Dual-Digital-Volt-Meter-Gauge-G-/141871975208?hash=item21083a8b28:g:5C8AAOSwKtlWj6Oo

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 04:49:07 pm
content: Quote from: MK14 on March 27, 2016, 03:40:48 pmBelieve it or not I already have this meter and is highly unnacurate... The minimum current it shows is 0.15A which is too much and plus I wanted to have the voltmeter connected to my arduino....

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 04:54:25 pm
content: Quote from: szymonm2 on March 27, 2016, 04:49:07 pmI'm probably shooting myself in the foot here...But that is an example of why I did NOT recommend EBAY, for QUALITY, 0.01% resistors.By spending more, I would imagine you can get 3 or even 4 digit meter modules like that, but which ARE reasonably accurate.But it would probably be a fair bit more (like £10, to £20), and you may need to buy from a proper supplier, rather than ebay, to get it.But if you really want to read the values into a micro, that does get a fair bit more complicated.

author: TheAmmoniacal
author_rank: Unknown
timestamp: March 27, 2016, 04:55:16 pm
content: These are the best option for those panel meters

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 05:02:05 pm
content: Quote from: TheAmmoniacal on March 27, 2016, 04:55:16 pmThis thread is going to bankrupt me!Those meters look really NICE!. (I want to buy some of them, for PSUs).Nice find. (I like his channel, and have already seen a number of his videos).

author: TheAmmoniacal
author_rank: Unknown
timestamp: March 27, 2016, 05:09:58 pm
content: I got mine from this seller:http://www.ebay.com/itm/5-Digit-0-36-DC-0-33-000V-High-Accuracy-Red-Led-Digital-Voltmeter-Panel-Meter-/121766165595They also have a trimpot on the back which I assume can be used for calibrating? (not looked into it). You can also change the resistors on the back easily if you want to try improving them.

author: szymonm2
author_rank: Contributor
timestamp: March 27, 2016, 05:25:05 pm
content: Quote from: TheAmmoniacal on March 27, 2016, 05:09:58 pmI see but this is not what i am aiming for here. Im trying to have everything connected to an arduino and 16x2 display... So in conclusion what are the resistors I buy again? And also do I need an op-amp?

author: rdl
author_rank: Super Contributor
timestamp: March 27, 2016, 05:33:14 pm
content: When I need high precision, high stability I use Vishay Dale PTF series. 10 ppm/C at 0.1 % is usually good enough for me and not terribly expensive.http://www.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyPTF5610K000BYEK

author: MK14
author_rank: Super Contributor
timestamp: March 27, 2016, 05:46:17 pm
content: Quote from: szymonm2 on March 27, 2016, 05:25:05 pmAs (I thought I said earlier), 10K and 1K, and you can just about cope WITHOUT an op-amp. But an op-amp, would give a small improvement in accuracy (probably not worth it, and would introduce its own errors as well, and complications).But if you can get other values (between 10K and 1K) at 0.1%, then go for 9K and 1K 0.1% (maybe not available, hence the 10K, 1K).The ADC seems to need about 4V full scale. But I wanted to give it a bit of headroom, anyway.

author: macboy
author_rank: Super Contributor
timestamp: March 28, 2016, 05:19:55 pm
content: Quote from: szymonm2 on March 27, 2016, 01:39:24 pmWhy didn't you say that in the first place?1% resistors will do just fine. The actual issue is the temperature coefficient (tempco). As the resistors heat up, they will increase or decrease slightly in value. If the two resistors in the divider don't "track" (change by the same %) then the divider ratio actually changes. Change in room temperature, and especially, self-heating due to power dissipated in the resistors, will cause this effect. For a costant output with a constant input, you need the ratio to be the same, even in the absolute values change. For 3 or 4 digits, you can probably ignore the effect. The point is that tempco is potentially important, but the absolute "precision" isn't. You can get relatively affordable resistors with lowish tempco; Vishay RN55 series comes to mind but there are others. The search feature at Digikey and others let you search based on tempco.You don't need precision because you can and will calibrate in software. That just means scaling the ADC value by a fixed amount to get to the DC voltage reading. So after everything is built, measure a 30 V input, grab the ADC reading, and figure out what that scaling factor is needed to get 30.00 V on the display, then build that value into the final software. For something other than a one-off project you would want some means of doing this without having to change the software, but for a one-off, you can get away with much more crude means.Since the Arduino ADC can take many thousands of readings per second, you should take many readings (at least 100, preferably more) and sum them, the apply the appropriate scaling to the sum to get your voltage reading. This improves noise and resolution by a factor of sqrt(N) (N is number of readings). You will get less flicker in the last digit of your reading this way. You don't need to take my word for it, try it yourself and see the results.

author: Cerebus
author_rank: Super Contributor
timestamp: March 29, 2016, 02:58:00 am
content: Quote from: tggzzz on March 27, 2016, 02:47:17 pmFor the benefit of our OP:Just to clarify "ppm" here stands forpartspermillion. One ppm is 0.0001%, 10 ppm is 0.001% and so on.Typical resistor tempos are 100 ppm/C or 200 ppm/C for run of the mill metal film resistors and 20 to 50 ppm for the commoner types of precision resistors that will have 0.1% resistance tolerance. Better tempco and better resistance tolerance tend to go hand in hand for the obvious reason that a 0.01% resistance tolerance is pretty useless if you have 100 ppm/C (0.01%/C) temperature coefficient as the resistor will only be within tolerance over a temperature range of 1C.You can get much better tolerance and tempco but you pay heavily for it - I have here some 3ppm/C 0.005% resistors that come in at around £20 GBP for a single resistor. Needless to say that, at those prices, you only use these kind of precision components when you absolutely need to and can't think up a way of working around the problem.

author: tggzzz
author_rank: Super Contributor
timestamp: March 29, 2016, 06:44:48 am
content: Quote from: Cerebus on March 29, 2016, 02:58:00 amYes.Buttwo0.01% 100 ppm/C (0.01%/C) resistors can be useful if the circuit's operation depends on theirratio- and thatwouldbethe case in the OP's potential divider.

author: Cerebus
author_rank: Super Contributor
timestamp: March 29, 2016, 06:41:42 pm
content: Quote from: tggzzz on March 29, 2016, 06:44:48 amWhat a few people here seem to be missing is that the OP is 15 and doesn't know enough to choose the values for a simple voltage divider for himself. So he needs a bit of educating - which is fine, I clearly remember being there myself. What doesn't help is going too far in one leap. It's probably useful to explain tempco to him at this stage. But, tempco tracking, thermal gradients, thermal emfs et al. are probably way too much to take in at one go.BTW, I challenge you to find acommercially available0.01% 100 ppm/C resistor. i.e. Not 'select on test'.

author: MK14
author_rank: Super Contributor
timestamp: March 29, 2016, 06:45:36 pm
content: Quote from: Cerebus on March 29, 2016, 06:41:42 pmEXACTLY!That is why I tried to simplify it for the OP, and just supply some resistor values, and 0.1%, since 0.1% are fairly cheap. Approx £0.23 to £0.63, depending on which ones you go for.Worrying about temperature coefficients etc etc, is for advanced level stuff, NOT beginners.EDIT:But some of the other suggestions, such as using 1% resistors, and calibrating/adjusting it using a multimeter, changing software values. Is also a perfectly good solution, for the OP. But 0.1% resistors, are so relatively cheap these days, for a one off, it does not matter that much. But they could be TOO hard for the OP to get hold of, if they never buy stuff from Farnell/Digikey/RS etc.

author: LukeW
author_rank: Frequent Contributor
timestamp: March 30, 2016, 12:00:41 pm
content: OK, turn 32 volts into 5v using a voltage divider.Here's a handy little site:http://jansson.us/resistors.htmlThis takes care of getting the best possible match between the ratios you want and the real-world series of resistor values you can easily buy.https://en.wikipedia.org/wiki/Preferred_number#E_series5/32 is 0.15625, so put that into the resistor ratio box, and select the button for voltage divider mode.Also select which E-series of resistor values you want to use.As with all software tools and calculators, it's useful and saves work, but it's also good to understand the underlying concept and maths behind what you want to do.27k + 27k + 10k will give you an exact 5V output at 32V in, using E12-series components.(The three resistors are in series, and the 5V is taken across the 10k resistor, which is the "bottom" one connected to ground.)

author: MK14
author_rank: Super Contributor
timestamp: March 30, 2016, 12:26:03 pm
content: Nice/useful online calculator, thanks! (although I'm NOT the OP, I may use the calc for my own stuff).It even does different E number series.The ADC can't handle 5V (when I had a quick look at the datasheet).I think it does 4.096V full scale, but it has lots of complicated settings, such as programmable gain control, so I'm NOT 100% sure.Also I would be temped to go for higher than 32V, in case the voltage goes up a bit.I.e. 35V or 40V full scale, would probably be better/safer. Otherwise 32.1V and more, could saturate the reading.

author: MK14
author_rank: Super Contributor
timestamp: March 30, 2016, 01:06:15 pm
content: Quote from: LukeW on March 30, 2016, 12:00:41 pmI DON'T know if you have been following and/or reading this thread ?But 10K seems to be too high a value. Because the ADC has (potentially) got a significant input impedance to it, and no op-amp buffers are being used. That could introduce a significant source of error.That's why I suggested 1K.EDIT: The ADC input impedance depends on the ADC settings, so is hard to easily work out quickly, since I'm not 100% sure, how the OP is going to set it up.But a quick estimate is that it will be around 710K. hence why I thought 1K could just about work, without an op-amp. But 10K would be really pushing it, as it would probably introduce a significant error.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: March 30, 2016, 02:35:03 pm
content: if you need precision, you can just parallel 10 resistors and then it would all average out.

author: tggzzz
author_rank: Super Contributor
timestamp: March 30, 2016, 03:43:13 pm
content: Quote from: ZeTeX on March 30, 2016, 02:35:03 pmIf and only if their values are normally distributed about the process' mean value. Components from the same batch usuallyaren'tnormally distributed about the overall process mean.Simple ways of realising that:when supplying "matched" components, it is usual to take them fromthe same batchif a batch is "cooked" (etc) for too long, then all components in that batch will be affected similarlycomponent values often aren't normally distributed, but have a two horned distribution - because the central parts are selected for sale at a higher price

author: Kleinstein
author_rank: Super Contributor
timestamp: March 30, 2016, 06:22:19 pm
content: For the divider a 10 K resistance value could be a little on the low side. At 32 V this would result in nearly 100 mW of heat for the resistor and thus a significant temperature rise. To avoid a large temperature rise one should only use a small fraction of the rated power of the resistors. So I would prefer a value in the 100 K range, even if this means adding a buffer amplifier. A constant but not so high input resistance of the ADC would only change the scale factor, so this might not be such a big problem. With a higher impedance divider one might want and extra capacitor to GND as ADCs might take pulsed input current.As the adc seems to have a relatively accurate reference, one could opt for using low tolerance resistors (e.g. 0.1 %) so one could get away without adjusting the reading to an accurate DMM. As the resolution is quite good, there is no need to really match the range very close to 32 V, so likely no need for combining three resistors. If a good voltmeter is used to adjust the scale, not accurate resistors are needed, but the stability (e.g. low temperature coefficient) is still important.If yau really need an accurate divider to 1/10 output voltage, one can use 6 equal resistors and use 3 in parallel against 3 in series for the resistor ration of 1:9. With selecting / sorting the resistors this can be rather accurate even with only 1% nominal resistors.

author: MK14
author_rank: Super Contributor
timestamp: March 30, 2016, 07:29:52 pm
content: Yes, I know, I decided it was a worthwhile compromise. What you suggested is perfectly good and WOULD potentially improve accuracy still further.Approx 90 milliwatts dissipation, on a 250 milliwatt rated part (assuming they get the 250 milliwatt parts), with 3 fans, so hopefully it is force air cooled as well. On a beginners first ever PSU project, which does not really need the accuracy they were originally claiming. Does NOT seem too bad, to me.E.g. Typical cheap multimeters, are probably more like 0.8% accurate (roughly), at 32V.I went that route so they could just buy 2 somewhat cheap (approx 50p each) resistors, and be done with it.Rather than have to take a beginner through the exercise of knowing what an op-amp is (they might already know how to do op-amp circuits, I don't know), and make an accurate op-amp circuit.Since they are a beginner, I'm more worried that they would wire it in poorly, so they would get inaccurate results, especially with current measurements. I.e. Grounding etc.