Meter, dual display VI,  3 wiring diagrams  ----  Which one to use? - Page 1
Forum section: electronics / projects

author: ez24
author_rank: Super Contributor
timestamp: December 30, 2017, 10:26:33 pm
content: Here are 3 ways to hook up a dual display VI meterI am used to seeing two methods using one or two PS, but this one came with 3 ways to wire it.Why would someone use 2 PS to use these when they come with a wiring diagram using one power supply? I ordered some of these but have not received them yet.The only thing I can think of using two is to see "0". I do not need to see "0". I want to see VI at 19v, 12v, 9v, 5v, and 3.3v (from 5 bucks using 5 meters) all from bucks furnished by a Dell laptop supply. I wonder if only 3.3 will not work? Any problems in only using the power from the bucks only, ie one supply.Sorry I cannot find the link to the source of the diagrams

author: rstofer
author_rank: Super Contributor
timestamp: December 30, 2017, 10:37:52 pm
content: They're all the same! The thin red and black wire provide the operating voltage for the meter, the thin yellow lead measures to source voltage (the source providing the current) and the thick red and black wires connect the internal shunt resistor in series with the load.The only question is: Do I want a separate power source for the meter? If yes, use diagrams 1 or 2, if not use diagram 3. If I want a separate source, do I want a wall wart? If so, use diagram 1. Otherwise, derive the meter power from some kind of SMPS driven from the source serving the load. Under all conditions, observe the voltage limitations of the meter power source. And the load voltage/current, of course.It appears to me that the thin black wire and the thick black wire are connected. I'm basing this guess on the fact that the thin black wire isn't used in diagrams 2 & 3. If true, the power source isn't truly floating.Consider the case where the Supply Power is > 28V. Maybe 48V for some kind of DC motor.

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2017, 03:36:56 am
content: Quote from: ez24 on December 30, 2017, 10:26:33 pmThink about this: whywouldn’tyou want it powered separately?As for why you would: one, so the thing can run when the measurement voltage falls outside the acceptable supply voltage range. This thing can run on 3.5-28V. If you want to be able to display anything under 3.5V or over 28V, then youmustpower it separately. For example, if you’re using this to monitor your 5V rail, and said rail falls to, say, 1.5V due to overload, would you want the meter to go blank, or would you want it to show 1.5V?Also, at least on some of these panel meters that don’t have particularly good voltage references, their calibration actually depends on the supply voltage, meaning that their accuracy is only correct at the supply voltage it was calibrated at — not good if the supply voltage is changing.Quote from: rstofer on December 30, 2017, 10:37:52 pmThat’s correct. I’ve dinked around with these and indeed, they aren’t fully floating even with a separate PSU.

author: ez24
author_rank: Super Contributor
timestamp: December 31, 2017, 04:52:59 am
content: Quote from: tooki on December 31, 2017, 03:36:56 amWell two costs twice as much and in my case would require another power source and the hardest for me is wiring it up in the confined space I have.QuoteMy voltages are going to be between 3.3 and 19v. You confirmed my gut feeling that one PS would not measure 3.3. For 3.3 I may use a 9v battery pack (Arduino battery pack). But I did not want to use the 9v PS for the 19,12, 9, and 5v supplies because of wiring.QuoteGood point but I will have two AN8008 meters mounted next to these, if I see something like this I can power off and hook up a multimeter (but I will think about this )QuoteOuchMy "other" PS is 9v (down to 6v) using rechargeable AA batteries. My goal is to get within 0.1v of the AN8008. Should I look into coming up with some sort of voltage reference for the battery pack? If so can you give me a hint into which way I should proceed? I do not need super accurate stuff, +- 0.1v is good enough.I know this is going to sound strange but I am going to use two 9v packs so I have a counter-balance (ie I need the weight to balance the weight of the bucks). So I could come up with a 12-18v voltage reference. (or even 3 packs)All my bucks, switches, terminals, meters, scopes, and FCs are going into the lid of a briefcase. When the case is opened the weight of the lid wants to make it tip over. The bottom will be my work area. In other words a lab in a case.I am using peg board to mount the parts. This is turning out to work well. I have mounted the scopes, AN8008s, bucks, switches. I have not figured out how to mount the meters yet. For the meters I am playing around with hot glue and paint sticks. I am surprised how strong a bond there is between the sticks and meter. The wood gave way when tested.If the meters for 19, 12, 9, and 5 are not accurate on one supply, I will look into voltage regulation. I should get the meters within two weeks.Thank You

author: ez24
author_rank: Super Contributor
timestamp: December 31, 2017, 05:23:13 am
content: I found this circuit. Seems it would work with 2-3 battery packs and comes in at 0.1%, which is good enough.

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2017, 12:44:21 pm
content: I assume this is for the PSU you’re going to power with the 19V laptop power supply?I mean, even this is probably wild overkill, but why not just use a 7815 linear 15V regulator to derive a clean linear supply from the 19V, just for powering the panel meters? I mean, if the 19V rail fails, everything else is going down too, so you’d only have the panel meters go out if the whole thing is off. (Just to be clear, I’m saying one power rail specifically for powering all the panel meters, not one rail per panel meter.)The lazy (and likely 100% adequate) solution is to power the panel meters off the 19V rail directly.

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2017, 12:53:10 pm
content: Quote from: ez24 on December 31, 2017, 04:52:59 amYou’ll probably be just fine. It comes down to the panel meter in question more than anything. Some of the ones I have (the kind with trim pots) are only just OK (for example, if I calibrate it to agree with my bench meter at 30V, it’s inaccurate at 1V, or vice versa). Others are spot-on, within 2 counts. (Even the very best multimeters will only ever be within a few counts of each other.) It’s hard to know ahead of time when ordering these chinese things. That said, the 4 and 5 digit models tend to be better in my (limited) experimentation.If I understand your application, you’ll just be using them to monitor fixed power rails (e.g. one panel meter for each rail). As such, you’re not expecting them to vary much, so if your meters are the kind with trimpots, you could adjust each one to agree with your AN8008 at its nominal voltage. The error on the small fluctuations you’ll see (e.g. a 5V rail sagging to 4.93V or something) will be negligible.FWIW, for my own “pimp my cheap lab power supply” project, I’ve pretty much written off the premade panel meters, because for an adjustable power supply, their refresh rate of around 3 updates per second is a bit sluggish for my taste. So I’ve been experimenting with using INA226 voltage/current sensor boards with Arduino — these things are insane, 5 digits of resolution at hundreds of readings per second, and they are pretty much perfectly accurate. (On voltage, where no calibration is needed, they agree to within one count with my 6.5 digit K2015.)

author: ez24
author_rank: Super Contributor
timestamp: December 31, 2017, 06:56:01 pm
content: Quote from: tooki on December 31, 2017, 12:44:21 pmYesQuote from: tooki on December 31, 2017, 12:44:21 pmI was thinking about this so I ordered some 7815s last nightQuote from: tooki on December 31, 2017, 12:44:21 pmThis will be the first thing to try. I need to change my profile from D.A. to L.D.A.Last night I printed out a PDF from TI on voltage regulation and will read it today. I have another post on running bucks in series, like from 12->9->5->3.3. Blueskill wrote a lot of technical terms that I need to learn. Between you and him I have a lot to read today. I have gone as far as I can while I wait for the meters. I hope they come in next week. But when I take breaks from reading I will practice gluing the meters, I have a bunch of 3 digit ones to practice with. Mounting the meters turns out to be the most difficult thing in building my lab.