Voltmeter for negative voltage - Page 1
Forum section: electronics / beginners

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 04, 2018, 10:35:25 pm
content: Hey guys,I built the basic LM317/LM137 dual power supply that is shown on the LM317 schematic.Works fine and Im happy with it. Now, Im trying to use the same volt/ammeter meter (that basic mini volt/ammeter pic below).I tried using a 3PDT switch to make the commutation and switch the current probes and the voltage probe.The thing does not work to measure the negative voltage, it measures current on both sides fine but only measures positive voltage.Ive spent such a long time thinking about this that Im embarassed.Switch connections pic belowThe thin black lead is not connected to anythingThe thin red lead is connected to a 7812 fed from the positive rail of the supply

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 04, 2018, 11:17:35 pm
content: I found this and believe it is your meter.I think you need a 4PDT switch to solve this.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 08:51:43 am
content: you can do it just switch correctly the negative voltage measurement wire and voltage power line for ammeterEnvoyé de mon HUAWEI NXT-L29 en utilisant Tapatalk

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 05, 2018, 01:27:10 pm
content: I think I got,this new way o wiring it keeps the black current lead on the ground or negative depending on the switch position and also toggles the thin yellow voltage line based on which current the meter is measuring, either from + to ground or ground to the negative rail of the PS.The volt/ammeter keeps being powered by a 7812 that is fed from the positive rail of the PS. So the meter receives 12V when measuring the positive side (given that the black current lead is not tied to ground) or receives 24V when the black current lead is tied to the negative rail.Schematic attachedWhat do you guys think?will it work?

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 05, 2018, 03:51:07 pm
content: Wait, if I wire it like the post above wouldnt it be receiving 42V on the thin red wire when measuring the negative current?Because Im powering it with a 7812 that is fed by the positive rail so when measuring positive current the meter sees 12V on the thin red line.But given that the power supply is a +-30V wouldnt there be 42V between the 7812 and the negative rail?The meter can take 30V on the powering leads.Sh#t, can anyone help?

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 05, 2018, 04:27:28 pm
content: How about this. Not certain that meter allows insertion of ammeter in the high side, however.This requires 4PDT switch as well. Maybe there is a way to reduce it.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 05, 2018, 04:48:14 pm
content: Is the power supply represented by the resistors or they are actual resistor?

author: Wimberleytech
author_rank: Super Contributor
timestamp: December 05, 2018, 05:13:07 pm
content: Quote from: AngraMelo on December 05, 2018, 04:48:14 pmSorry, I intended to label this. The resistors are the LOAD. The power supply source is at the left.See updated schematic

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 05:23:14 pm
content: Ammeter gnd big black and bid red one doesn't need comutting, wire those directly.Switch only yellow and thin red accordinglyPierreEnvoyé de mon iPad en utilisant Tapatalk

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 05, 2018, 05:58:56 pm
content: Perieanuo,So you are saying that the meter reads current that is going both ways on its current leads?Where do I attach the thing black wire? Because

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 05, 2018, 06:15:42 pm
content: If the meter reads current going both ways without having to switch the thick leads, this is the only way I figure it can be connect. But I still believe I need to toggle the thin black lead. If everything goes right the meter will treat the negative rail as ground and will see the 30V coming from ground to power itI did the wiring again

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 05, 2018, 10:10:42 pm
content: I think I made a mistake, I'll draw a schematic tomorrow to clear up.Current wires needs also to change wiring from sensing common gnd to negative branchSorry, I'll revise asapPierreEnvoyé de mon iPad en utilisant Tapatalk

author: fsr
author_rank: Regular Contributor
timestamp: December 05, 2018, 11:16:59 pm
content: mmm... i don't know, you need to find some documentation, because who know how is this thing on the inside. It uses a positive supply to power the meter. The yellow wire seems to be to measure the voltage, and then the thick red/black wires to measure current (well, black probably for voltage too).First, measure both black wires to make sure that they aren't shorted inside (probably won't, but just to be sure...). Then, on the points labeled "power supply for the load", you could connect GND to + and VEE to - to measure your PSU's negative output, or connect VCC to + and GND to - to measure your PSU's positive output.(VCC means your PSU's positive output wire and VEE your PSU's negative output wire).Well, at least that's what makes sense to me, but no warranties without documentation.I think that you need to switch the signals on "power supply for the load", because even if the ammeter works in any polarity, there's only the yellow cable to measure voltage, and that means that the other cable for the voltmeter is probably the black one for the ammeter, so you cannot just leave the ammeter wired directly.And that's not all, you also need to connect your PSU's output directly when the meter is connected to the other side, so you have to switch VCC, GND and VEE on two different possitions, further complicated by the fact that the meter needs to be in series with the load to measure current. Worse: you have only one GND terminal, and it's shown that the ammeter goes in series with the load, over the black wire. If that's the only way to use it, well, that's a problem, because it's a split supply, so that the current from both sides will return on that lead (but going in different directions).That seems like several things to test, then draw how the meter would need to be connected to measure the positive and negative output, then see how a switch can provide that kind of connections. And even then, moving the switch will briefly interrupt the output.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 06, 2018, 09:53:32 am
content: Hi, you can do it like in schematic attached (eagle schematic also attached):1. you need to use switches that "break before make"2. all switches needs to commute synchronised (or find a correspondent switch, I was lazy to find one, like a old rotactor)3. avmeter works in this case for voltages between 3.5-28VdcFor point 3 solution is regulate power supply for avmeter with 7805 for example before LM317 positive branch and DC-Dc converter + 5V regulator on negative branchPoint 1 is very important.I don't like none of the approaches, I suggest you did it the right way: 2 avmeters with 2 dc-dc's and 2x5V regulators vor avmeter power supplyI have one avmetter that resemble yours and thich black and thin black are in fact one common node, thin black isn't connected in case of using with same PS for powering the meter and measure for correct measurement.If you want to open the schematic cut the .txt from sch file, open with eagle.regards,Pierre

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 06, 2018, 01:02:02 pm
content: Thank you so much for taking the time to draw everything.The wiring looks like it would work. My issue is that A 8 pole switch is very hard to find. It would be something like a rotary switch of a multimeter. The absolute best I could find is a 4 pole triple throw rotary.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 06, 2018, 02:00:03 pm
content: On other hand, a new chinese avmeter is under 5 euros.you don't have the space for a second one?switches is really ancient and bad idea on long term.PierreEnvoyé de mon iPad en utilisant Tapatalk

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 06, 2018, 02:13:40 pm
content: Everything is ready. The PS is ready on the enclosure and I decided to put the volt meter just now. Im trying to make everything fit.I understand that using 2 is by far the easiest but it just does not fit. Even if I take everything apart.

author: fsr
author_rank: Regular Contributor
timestamp: December 06, 2018, 11:44:20 pm
content: I don't think that you can use this meter even if you had two of them, because you need to connect the ammeter over the negative wire, but this is a split supply, so just imagine what will happen if you have two 1A loads, one on the positive side, and other in the negative side: the current over the GND wire will be 0A.Are you interested in a future meter project? Maybe you could "arduino" a meter. There are some small arduino boards out there, or you could use the "naked" atmega328 on your own board.

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 06:37:24 am
content: Quote from: fsr on December 06, 2018, 11:44:20 pmHave you understood my schematic?positive branch measures load crt by interposing secong gnd point, in negative branch wiring same thing happens and current resistor inside ammeter is on negative input...Envoyé de mon iPad en utilisant Tapatalk

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 06:42:14 am
content: Quote from: AngraMelo on December 06, 2018, 02:13:40 pmRemember point 2 , all switches must change state same time.otherwise you risk to burn avmeterRegards,pierreEnvoyé de mon iPad en utilisant Tapatalk

author: BravoV
author_rank: Super Contributor
timestamp: December 07, 2018, 07:48:28 am
content: Aware you're working on the switch, but just a thought, why not buy something like this ? Imo, its relatively cheap, and can do positive & negative readings and it works.Random search at Aliexpress and sorted by price->HERE... and different colors too ..

author: perieanuo
author_rank: Frequent Contributor
timestamp: December 07, 2018, 08:23:45 am
content: Quote from: BravoV on December 07, 2018, 07:48:28 amhi,it's more interesant to buy a VAmeter for a variable PS, you got crt and voltage in one location, 10 times easier to cut only one rectangle.just do the right wiring that's all.he already had 1 better device, not the same precision maybe but that's not the pointpierre

author: fsr
author_rank: Regular Contributor
timestamp: December 07, 2018, 08:30:37 pm
content: Quote from: perieanuo on December 07, 2018, 06:37:24 amYes, you put the ampmeter on the GND line when measuring the positive side of the supply. But the current on the GND line is the sum of the currents of the positive and negative sides (and the currents go in opposite directions, so they substract). KCL.1A of current on the positive side - 1A of current on the negative side, and the ampmeter on the GND wire will read 0A.I think that he needs to find some meters that can be installed on the + and - wires.