Panel meter connection issue - Page 1
Forum section: electronics / projects

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 07, 2015, 04:17:43 pm
content: I'm having a bit of a problem with a circuit I've been trying to get working and I'm hoping someone here might be able to help me out.The first pic is the circuit I've built which seems to work both in simulation and the real world.The second pic is the connections for themeterI got to show the load current.The third pic is the way I laid out the thing on a pad per hole board. I use a 0.1 grid which gives me the holes at the intersections of the grid lines which helps me see what I'm trying to do.The way I have things hooked up is red at source and yellow at ground which puts the 0.05 ohm sense resistor into the circuit from the meter. As far as I'm aware, I should be reading the current from the DUT but what I'm seeing is the actual input voltage.I'm using a 6V 1A wall wart to use as a DUT for testing purposes. I've only got the red and yellow wires hooked up which should mean that I should only be getting a reading at the current portion of the meter but I'm seeing the reading at the voltage side and I should be only seeing a mV reading but I'm seeing the full input voltage.I'm now fully confused as to where I went wrong with this and figured I'd see if I could get a few more eyes on the thing to point out where I went wrong with this.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2015, 04:47:23 pm
content: it isn't cleat what you are trying to do at all. Why are yellow and black tied together? Shouldn't you be using the meters internal sense resistor as the sense current resistor. Anyone would be confused by this.

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 07, 2015, 05:01:05 pm
content: The black wire isn't even connected at this point. The meter has a common positive configuration I have them connected at a common ground so I could remove either the black or the yellow wire to see if the meter was outputting to the correct meter with the correct values which isn't happening at all. Regardless if I have the yellow at ground or the black at ground, the voltmeter is the one that shows a reading, the current meter never outputs anything. I don't see how this is confusing at all.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 07, 2015, 05:10:47 pm
content: If the black and yellow wires are connected together, then no current can flow through the meter.Thus, the meter indicates zero current. Now what was the question again?Things may be clear to you, but until you get your board layout completely labeled, and revise your schematic to show the meters, etc, it is clear as mud to the rest of us.Making things harder on us simply limits the amount of useful help you can get here.It would help to simply tell us what this is supposed to be?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 07, 2015, 05:46:47 pm
content: From a noob observation it looks like when the 10 IRF530's turn on, their maximum on state resistance (according to the Vishay datasheet) is 0.16 ohms/ea. I believe your current sense resistor is too big in comparison. Can you put a load resistor (100ohms?) in-line with the DUT?

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 07, 2015, 05:53:01 pm
content: I don't have the yellow and black connected at the same time, just the yellow and red which puts rsense into the circuit as it's part of the meter itself. I'm trying to get the meter to read the current but all I get is the voltage on the voltmeter side which should only be displaying 0000 as far as I know if I only have those two hooked up which is what's confusing me. I guess it's just a matter of a crappy panel meter that either isn't built right or simulation differs from reality so much in this instance that I won't be able to use these meters for what I want to do in this instance.100 ohm resistor didn't change anything, I just tried it.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2015, 06:20:06 pm
content: If it makes you feel better, blame the meter. I've used lots of these meters and they work fine. How about drawing out the real schematic. It will be obvious then. Really, what in the hell is this supposed to do.These meters generally have the option to be self powered or by an ISOLATED supply. Sometimes there is a jumper that must be removed. Generally the small black wire must not be connected to anything.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 07, 2015, 06:41:43 pm
content: The meter measures the current flowing INTO the yellow wire on its way back to ground (through the large black wire)If the large black wire is disconnected, then no current will flow and the meter is accurately reporting zero current.

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 07, 2015, 06:42:37 pm
content: I've posted the schematic I worked from in LTSpice but I'll put up the one from Eagle as well. My thought process was that if I hook the red wire to source and the black wire to ground I'd have the 0.05 ohm resistor in the circuit as Rsense in the LTSpice schematic and I would be able to read the current flowing from the DUT on the meter. What I'm reading is zero on the current meter and the voltage on the voltage meter which seems wrong to me. I got the circuit from the net and wanted to try it out because it was dead simple or so it seemed to me. Should I perhaps just put Rsense in series with the DUT?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 07, 2015, 06:44:28 pm
content: None of your schematics actually show that combo voltage/current panel meter.It would appear that you don't understand how that meter works, or else you have not properly represented it in your schematic diagram.

author: Seekonk
author_rank: Super Contributor
timestamp: January 07, 2015, 06:56:42 pm
content: The yellow and the black should replace the sense resistor, yellow to the lowest point. That is if they measure the voltage on the load side. May need to reverse. Red to highest source voltage. You likely will need a lower reference voltage. Little red wire can go to your regulators voltage. See if it powers first before you connect this little red wire as it may already be internally connected. Don't connect the little black wire This is what you want, not what you designed. Not responsible for smoke.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 07, 2015, 07:16:12 pm
content: Quote from: Seekonk on January 07, 2015, 06:56:42 pmThe sense resistor is BUILT INTO THE METER between the large yellow wire, and the large black wire.The large yellow wire is NOT the "lowest point" depending on what frame of reference you are talking about.The large black wire is HARD GROUND. The large yellow wire is "VIRTUAL GROUND" and is connected to the high side of the current sense resistor.The large yellow wire is intended as the ground return for the load. The current through the load back to ground passes through the sense resistor inside the meter.The meter measures the voltage across the current sense resistor and calculates the current to display.If the main current path through the load is not returning to ground INTO the yellow wire and out through the large black wire, then the meter won't work properly.

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 07, 2015, 07:35:31 pm
content: In the current configuration of the schematic, these meters won't work is my thinking. I tried the way I have this laid out in the attached board image but things get really wonky that way. Rsense from source to ground, DUT from drain to ground. This is why I came here asking because in the attached pic is how I originally had things and I get no voltage and very little current. If I hook my multimeter to the DUT while I have it hooked to the load, I get no voltage reading at all, it's as if the power is shunting directly to ground. This is why I'm extremely confused and why I came here asking what I may be doing wrong.Quick edit to add how it's hooked up on the schematic.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 07, 2015, 08:37:20 pm
content: What is "Rsense" for? The sense resistor is built into the meter (between the yellow and black leads)Putting an external resistor across there is guaranteed to cause the meter to not operate properly.Do you not understand now the meter works?You appear to be trying to make an electronic load(?)It seems quite possible that meter will not work in that configuration because that is not what it was designed for.

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 07, 2015, 08:52:43 pm
content: Rsense in the schematic is a representation of the resistor in the meter. I guess I'm not quite explaining myself correctly. I had thought that I could measure the voltage across DUT with red and black and the current across the internal sense resistor in the meter with yellow to black. If I hook things up this way, as in the schematic I just posted with the colors on it, I do not get a voltage measurement at all and very little current which doesn't adjust. It doesn't help that the meters only have that crappy connection diagram and not a better representation of how things are wired internally on the meter. If the internal sense resistor is connected across yellow and black then how would connecting it like I've shown in the latest schematic not work? Black is ground, red is positive for voltage and yellow is split between ground with the sense resistor to measure the voltage drop across it. How does connecting it like I originally did short things out to the point that I can't get a reading even with my multimeter while it's hooked up?

author: Seekonk
author_rank: Super Contributor
timestamp: January 08, 2015, 12:13:09 am
content: Your ability to make pretty looking schematics far exceeds your understanding of electronics. It would seem the best thing for you to do is make a schematic of how you hooked things up. That way you would understand what you drew and then we could correct it in a logical way. What you have provided leves out the most important details. This is fairly simple and we shouldn't have to guess what you are doing. Perhaps sleeping on it and looking with fresh eyes is a first step. The laws of physics haven't changed. Electronics always does what it is supposed to do.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 01:13:59 am
content: Did you actually measure the current sense resistor in the meter?Do you know what voltage to expect per amp on the yellow node?There are several reviews on YouTube of those Chinese volt/amp panel meters.This one reviews several of them:According to what I see there, the current shunt looks like a heavy piece of wire, so I would expect very low resistance (and very low voltage across it).

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 08, 2015, 01:51:48 am
content: Quote from: Seekonk on January 08, 2015, 12:13:09 amHere's the simulation I did that seems to support what I want to do but my lack of understanding is getting in the way of. This is exactly how I hooked things up, including the meter. I'm sure that you can work that out from what I'm presenting you here but just in case not, red to V2 +, black to V2 - (this is DUT), yellow to Qx source. Now either I am seriously lacking in my basic understanding of how things were to be hooked up (this is how I originally did things) or the meter itself isn't up to par with being used this way. Since I don't have a schematic of the meter, all I could do was guess based on the information I had at the time, and yes, I searched out many of these and all, ALL of them just give you this picture or a variation of it on how to hook it up. I'm thinking I should have just gotten separate meters instead of an all in one. I'm also starting to think that I should stop asking questions here because it never seems to work out that I get answers to questions but end up having to work things out myself but then again, maybe that's a good thing.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 02:00:14 am
content: How about a simple "overview" of how everything is supposed to work? Dunno what "V2" means here? Dunno what "Qx" means, either?Either you have some fuzzy thinking about how things should work here, or else you aren't communicating your understanding to us very clearly.That is why we keep asking more questions.I continue to be baffled why you seem to be unable to even tell us what this thing is supposed to be?Is it supposed to be a load device of some kind? I don't think we are even yet all on the same page here.What is that supposed to be in your zip file? I didn't recognize either of the file types?

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 08, 2015, 02:38:44 am
content: Quote from: Richard Crowley on January 08, 2015, 02:00:14 amYes, it's a load. V2 is the simulated voltage of the device under test. Qx refers to Q1 - Q10, the mosfets wired in parallel. The files in the zip are the LTSpice simulation and the model for the TL071 op amp (has to be in the same folder as the .asc file). The load isn't really my issue though, it works. I'm having trouble understanding why I can't get the meter to read things the right way. I'm just going to wire up a separate rsense circuit and use two multimeters to read things out, this is too much of a pain in the ass.

author: rdl
author_rank: Super Contributor
timestamp: January 08, 2015, 02:59:17 am
content: In the second picture you posted (meter-conn.jpg), just replace the "power supply" with your load and replace the "load" with your DUT. As far as I can tell that should work.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 03:07:27 am
content: Your diagram says "50mV/A". How did you arrive at that number?Is that actually a measured value? How are you testing this?Have you actually tried testing the meter independently in an isolated test circuit?Is the power supply voltage to the meter within the specified range?Does the meter actually measure voltage properly between the large red and the large black wires?Does the meter actually measure current properly between the large yellow and the large black wires?

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 08, 2015, 04:23:14 am
content: Quote from: rdl on January 08, 2015, 02:59:17 amI can give that a shot but it seems rather counter-intuitive to me.Quote from: Richard Crowley on January 08, 2015, 03:07:27 amThe original schematic used that value and I recalculated my values to use that number which proves out in the simulation.Quote from: Richard Crowley on January 08, 2015, 03:07:27 amI tested with a 6V 1A wall wart using the sense resistor in the meter and multimeters to check the value of pin 3 on the op amp, the voltage across the sense resistor, and the actual current drawn while under test.Quote from: Richard Crowley on January 08, 2015, 03:07:27 amNot yet but I'll put something together tomorrow I suppose.Quote from: Richard Crowley on January 08, 2015, 03:07:27 amYes. Meter power can be from 4-30V and the reading on this meter goes from 0-100V and up to 10A.Quote from: Richard Crowley on January 08, 2015, 03:07:27 amYes it does and it seems to be reasonably accurate as well.Quote from: Richard Crowley on January 08, 2015, 03:07:27 amNo, and that's why I asked here as I'm either doing something dreadfully noobish or this meter just can't be used in this fashion. I can get a nice reading across the sense resistor with my multimeter but I can't get it to output to the display.

author: rdl
author_rank: Super Contributor
timestamp: January 08, 2015, 04:41:08 am
content: Quote from: liquibyte on January 08, 2015, 04:23:14 amThe panel meter measures the current by reading the voltage drop across its internal sense resistor. That resistor is connected between the yellow and black wires and that resistor must be in series with your device under test so that whatever current is flowing out of your load and into the DUT also flows through the sense resistor.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 04:48:15 am
content: Quote from: rdl on January 08, 2015, 04:41:08 amIn the latest schematic diagram, it appears to be connected properly.If the meter is not displaying the current properly, then it may simply be broken. Especially if the voltage across the shunt looks OK.Considering the ultra-low cost of these mass-produced commodity gadgets, that wouldn't be beyond the realm of possibility.

author: rdl
author_rank: Super Contributor
timestamp: January 08, 2015, 04:54:33 am
content: Actually, I wasn't thinking clearly. The DUT is probably some kind of power supply. That being the case, then the V+ out from the DUT would go to the electronic load, then out from the load to the meter's yellow wire. The black wire from the meter would then go to back to the DUT negative.edit: Actually, I think it should work either way. It's all basically just one big current loop with "dut", electronic load and panel meter sense resistor all in series..

author: liquibyte
author_rank: Frequent Contributor
timestamp: January 08, 2015, 05:12:50 am
content: I'll do up a test circuit tomorrow for the meter and report back. I have two of these and unless they're both broken or from a bad batch then I've obviously done something wrong here because they behave exactly alike. If I hook them up like the latest schematic I get 0V and a fluctuating 300mA or so and it doesn't change while varying the pot. If I take the sense resistor out of the equation, I get a proper voltage reading. I can't get the current part of the meter to work right at all. I'm fairly confused about the whole thing at this point, I thought it would work right off the bat.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 08, 2015, 05:35:28 am
content: Have you actually measured the voltage and current with a separate DMM just to prove the values you are assuming should be there?I would probe across the questionable panel meter with a DMM to confirm that you should see some voltage.I would connect an external DMM (in current mode) between the load transistors and the yellow meter lead to confirm the current.

author: Seekonk
author_rank: Super Contributor
timestamp: January 08, 2015, 08:35:55 am
content: Most of these meters use the low side for the current sense to reduce part count. Part count can be reduced even further when current is measured in only one direction. If you don't get a current reading, reverse the yellow and black wires.As said before the current sense resistor is just a shorting wire. In your schematic you used a very low resistor also. Consider using the Yellow Black wires in series with that existing resistor. That will insure there is enough feedback voltage. That will also change the voltage measurement of the meter. See which side of the resistor gives the best reading.It will all seem better in the morning.