EE required for DVM to have 1/100th result ? - Page 1
Forum section: electronics / beginners

author: Audiofanman
author_rank: Contributor
timestamp: January 11, 2025, 02:27:54 pm
content: Im not an EE, please take it easy on me ..Single question marks are rhetorical (unless my thought process is way off), doubles are in fact open questions...Back story,I have a 500VDC PS that Id like to monitor by checking voltage occasionally and safely while in use.For obvious reasons, bringing 500V out of the chassis is out of the question.I thought that if I could determine the value of a dropping resistor reasonably accurately,I should be able to create a 100v to 1v output, and place the resistor in series with my PS and my DVM.5V to a test tip for is much safer...What I thought was a start;I need to know what my DVM needs amperage wise to display a 1V reading (I would guess reasonably accurately).Is this a measurable spec ? Is it generally available by the manufacturer and if so is it reasonably accurate ?If not available, how would one determine this ??I hooked up two DVMs, one reading voltage and another reading resistance to each other,One DVM read 0.489 VDC which is the output of the 2nd DVM reading a resistance of 9.136MΩUsing Ohms law I get 5.35E-9 or 53.5 nAUnfortunately, this does not give me a 1v reading, and I would LOVE to think that (1/.489) x 53.5nA= 1.094E-8 or 109.4na would give me 1Vbut cant know for sure without testing....Im thinking I need to now add another resistor of any value (but know) in series and determine how that value affected both theVDC reading AND amperage, then with at least those 2 data points be able to determine a ratio that works?(I say as if I can actually do the math, in reality Im very challenged with math and would be many hours of pondering and many more trying, so help here would be fantastic!)Again, Im no EE, but I feel like this might get me half way there, the series resistance of my meter in circuit would be, as the DVM's roles are inverse..The First DVM's resistance (The one reading voltage) has an unknown resistance when measured (Other meter reads OL)..This is where I start feeling like I hit a brick wall.My mind starts screaming.. Im making this too difficult and REFUSES to chooch any further ..Oooh look at the pretty squirrel running in snow.. LMAO..Anyone care to chime in, may be point to the well veiled path I should be traveling on ??

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 11, 2025, 02:58:38 pm
content: It's better to use a voltage divider than just a series resistor. But if I were you I simply use a DVM designed for 500V. Using a resistor in series doesn't really make it safer.

author: NE666
author_rank: Frequent Contributor
timestamp: January 11, 2025, 03:02:45 pm
content: Please don't take this the wrong way, I say what I say next I say because I think it's in your best interests. And yes, I'm no EE either, we all had to start somewhere and we all had to learn, so please don't think either that I'm here just to condescend.The frank part; from what you've written above, it's clear that you don't understand enough about what you are proposing for this to be a safe exercise for you. Honestly, 500v DC isn't trivial and one slip will result in either significant damage to equipment, or your death. It really would be best for you and your equipment, if you were to ask someone qualified to build a solution for you.But, as I said, we all have to learn somewhere. So to answer your question; NO - you do not want to be placing a resistor (of any kind) in series between the output and your voltmeter. What you require is a resistive voltage divider - go take a look a wikipediahttps://en.wikipedia.org/wiki/Voltage_divider. The resistors used here go between the output and ground, and you measure the voltage between ground and their intersection.With such a divider, you then have 3 selections to make w.r.t. to your resistors. a) the ratio of resistance, which sets your division ratio, b) their absolute series resistance, which limits the current flowing through them using Ohm's law, and c) based on b., their power rating to handle that current. You'll also need to make sure that they are rated for at least 750 to 1000v operation.You don't need to concern yourself with the current drawn by your multimeter, its input stage will have a very high input impedance on the voltage range you propose and will draw negligible current.However, you need to consider the safety aspects of all of this - just using a voltage divider and bringing a nominally lower voltage outside of the PSU chassis is nothing like a guarantee that a fault condition couldn't still expose either you or your equipment to the full PSU output voltage. The only really safe way would be to handle all of this inside the chassis, bringing out a low voltage representation of the PSU output via an optically-isolated measuring circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2025, 04:28:39 pm
content: Modern DMMs have a very high input resistance. Usually some 10 M (linke the measured 9.1 M). A series resistor to get an 1:100 attenuation would need to be at 990 M ohms. Such resistors are available, but no longer that stable and easy to handle. A voltage divider would indeed be more practical. This would be somethink like 10 M and 101 K. The input resistance from the DMM would be in parallel to the 101 K and lower the combined resistance by a bout 1%. So it can make sense to include it in the design / calculation, but there is no need for a super accurate value.Normally the manuals give the value for the input resistance, or it is written on the meter. The first test was OK though 9.1 Mohm is relatively low - could be an issue with leaky cables. There is no need to measure with 1 V, the resistance chould be the same at 0.1 V and 10 V. If at all the voltage range at the meter may make a difference.That the 2nd test with reversed role does not give a reading around 10 M could be from from having a meter with very high impedance (e.g. > 10 000 MOhm). This sometimes found in low voltage ranges and with better bench DMMs. In this case there is no more need to include the input resistance - it would be high enough for normal use.A divider is a little safer than just a series resistor, as a series resistor could still charge a capacitor to a dangerrous level. Otherwise the resistance would still limit the current enough to be safe. One should make sure the resistor (10 M part) are safe to use for high voltage and have some reserve. Often this may mean using a few not too small (e.g. 1/2 W size) resistors in series (e.g. 3 x 3.3 M or 5 x 2.2 M). Also make sure not to allow for an acidential short.

author: Audiofanman
author_rank: Contributor
timestamp: January 11, 2025, 04:53:00 pm
content: @kleinstein,You must have been reading my mind..You touched on most every point of the reply I almost posted (You beat me)My now edited reply to all...Thank you for the reply's so far!While Im new to EE, I am not new to dealing with the voltages..I have a basic but well equipped bench, dim bulb, variac, isolation TT, etc. and am 100% the one armed bandit when working on it..I appreciate responses so far...For normal use, bringing 500V outside the chassis is just not going to happen. full stop.It looks like I didnt write enough in my OP, left out some needed info, my bad...chose not to design a voltage divider as there would always be current flowing through the divider even when the meter wasn't connected. Sure I could add a switch, but complexity now increases... Using probe tips as a switch was far as I hoped to go...At least as far as my situation is concerned, Im puzzled with what additional fault condition could present itself... (ANSWERED BY Kleinstein) Thank you...When my DVM is on the voltage setting, I assumed its actually the current through an internal resistor that's being measured... If my DVM does NOT work that way, well I'm the dummy lolol..Now that I think about it, that may have been a bold assumption and one I should not have made ..Please some one get me off the hook or hang me hahah... LMK...Anyway, I need to digest the reply by Kleinstein (and others) Thank you !!

author: TimFox
author_rank: Super Contributor
timestamp: January 11, 2025, 05:04:27 pm
content: Old-style passive “VOMs”, such as the Simpson 260, apply the voltage through a high-value resistor to a sensitive microammeter. The 260 uses a resistor equal to 20k\$\Omega\$ x (full-scale volts), using a 50 uA meter.DVMs, however, usually start with a lowish full-scale (maybe 200 mV) circuit, preceding it with a switched voltage divider, that presents a constant resistance at the terminals.Bench DVMs often present a very large input resistance on the lower scales, using a divider above a few volts.

author: BennoG
author_rank: Frequent Contributor
timestamp: January 11, 2025, 08:55:53 pm
content: Can you tell us what the capability's are of the 500V psu. Mainly the max current delivery.for example 0.1A or 100A or 10kA. This gives us an idea in what direction we need to think to actually start measuring this voltage.for example at 0.01A the proposed measurement methods will have influence on the output voltage.on the other end 10kA you need special safety measurements because a failing resister can take out a whole room or even building.Benno

author: Audiofanman
author_rank: Contributor
timestamp: January 11, 2025, 09:49:48 pm
content: BennoG,The PT spec is 800/850 Vac CT :(400-0-50-400) Vac/150mAGenerally up to 120ma of that current would normally be used (in an output tube)The 400 - 0 - 400 taps, rectified and smoothed out are producing 520v right nowTubes being used now are using only drawing 55mA,I believe the PS is mostly unloaded so voltages are 25-30v above where they should be...Normally 25-30v over isnt a big deal. It is when the design voltage was supposed to be already a maximumId like to see what happens to the 520v when transformer is loaded down a little more, and generally have an "easy access" test point for the future so real world conditions could be easily be monitored.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 11, 2025, 10:05:52 pm
content: To keep everything safely within your chassis, here's what I might do:Get yourself an inexpen$ive LCD panel meter, like this one:https://www.jameco.com/z/PM428-Jameco-BenchPro-3-5-Digit-LCD-Digital-Panel-Meter_108388.htmlMeasures 0-200mV full scale.Build yourself a voltage divider as described above. Since you need to use powers of 10 so the meter displays accurately, and therefore your full-scale voltage will be 2000 V, you need a 1:10,000 voltage divider.If you use a 1MΩ resistor for the top resistor of the divider, the total resistance of the divider will be over a megohm, which shouldn't unduly load the supply. (Total draw @ 500V will be 0.5mA, about 1/2 watt.)If this is too much, then use a 10MΩ resistor.You'll also need a ~9VDC supply for the meter.If you can cut a (rectangular) hole in the front of the unit, all the better.

author: TimFox
author_rank: Super Contributor
timestamp: January 11, 2025, 10:09:30 pm
content: Note that most of these digital panel meters require that the 9 V power supply be "independent" of the input common (low) terminal.(There is an internal connection somewhere between the positive and negative terminals of the power supply to the input common, but you can't mess with it.)Of course, the easiest solution is a 9 V battery, since the panel meter draws very little current.I have used an isolated DC-DC converter from a grounded supply: low power ones are cheap.These panel meters are readily available with 3-1/2 or 4-1/2 digit readouts at reasonable prices.The input resistance is very high, making a voltage divider easy to construct.The circuit board is normally configured for 200 mV full scale, but there are holes for a voltage divider and the decimal point can be selected at other holes.(I wouldn't put 500 V onto that board, but a good 10 M\$\Omega\$ resistor, as suggested above, can be mounted on appropriate standoffs.

author: floobydust
author_rank: Super Contributor
timestamp: January 12, 2025, 05:05:16 am
content: I was thinking along these lines for a circuit. You get about roughly +/-15% adjustment range so a multimeter with 10MEG or 9.1MEG etc. doesn't matter much.The two input resistors are 1/2W because you need a working voltage rating of say 350V each, it's for the HV they see not the heat.

author: NE666
author_rank: Frequent Contributor
timestamp: January 12, 2025, 10:54:58 am
content: Quote from: Audiofanman on January 11, 2025, 02:27:54 pmI'm beginning to wonder whether that would not in fact be both the easiest and safest thing for you to do.How about fitting a pair of *quality* shrouded banana sockets (of the appropriate voltage rating) to the chassis, and simply connecting your DMM to those (with appropriate, shrouded test leads) whenever you wish to make a measurement? When not in use, keep them filled with blanking plugs.https://cpc.farnell.com/staubli/66-9131-22/4mm-socket-shrouded-red/dp/CN24577https://cpc.farnell.com/tenma/72-9350/test-leads/dp/IN06177

author: Audiofanman
author_rank: Contributor
timestamp: January 12, 2025, 03:51:52 pm
content: floobydust, wow thank you!My unwillingness to use the voltage divider is 100% unfounded!I just modeled your circuit in ltspice and using 10k for the 20k pot gives me 5.06v.. Pretty much spot onAdditionally, when not in use, I see the current draw is only 112uA, Heck I have a bleeder resistor across my 1st cap drawing 10 times that much...I like it, thank you..What I like about this is I could duplicate this on a reasonably small pcb a bunch of times and actually have multiple test points...Time to pick some components and start a PCB layout !

author: BennoG
author_rank: Frequent Contributor
timestamp: January 12, 2025, 04:11:28 pm
content: Make sure you have sufficient rated resistors for the 2.2M Ohmhttps://www.tme.eu/en/details/vr37000002204fa100/tht-resistors/vishay/https://www.digikey.nl/en/products/detail/vishay-beyschlag-draloric-bc-components/VR37000002204FA100/7350816the other parts you can use normal or SMD for but don't use them for the 2.2MBenno

author: floobydust
author_rank: Super Contributor
timestamp: January 12, 2025, 07:19:19 pm
content: It's important to keep electronics "touch safe". You never know when a pet, kid or a person touch them. So I would keep any test jack below ~50V for that reason.At an amplifier shootout party, one guy drank a bit too much and stumbled by his tube amp, kicked and broke some 866 merc. vapor rectifier tubes (FWB) and it was quite the boom. He was not hurt but I realized that old gear, made the old way is too dangerous for nowadays.I did not Spice sim or spend much too time on those circuit values. You can also look at screen voltages as well.As BennoG mentioned important to use decent HV-rated dropping resistors.VishayVR37are the best I think.Also, the spacings at the hot (500v) end of the resistor needs to be several mm away from anything to prevent arcing.For HF scope analysis you would need a shunt capacitor across the HV resistors, if you look for MHz RF there.

author: Audiofanman
author_rank: Contributor
timestamp: January 12, 2025, 09:57:08 pm
content: Great minds!!!Thank you, HVR37 (3.5kv rated) already in cart ;-)I have never done SMD, need a bunch of goodies before I get going with that,A good PCB holder, Magnification of some sort etc..Presently learning how to import mouser component models, footprints etc into KiCad..Like filename extension salad.. CAF...

author: EEVblog
author_rank: Unknown
timestamp: January 12, 2025, 11:40:32 pm
content: Quote from: Audiofanman on January 11, 2025, 04:53:00 pmSo you want to bring a "safe" voltage test point out to a front panel without normaling loading down the existing supply?In that case, yes a (properly voltage rated) 100M+ series resistor should work nicely.EDIT: I see you found a solution already.

author: Audiofanman
author_rank: Contributor
timestamp: January 23, 2025, 01:30:28 am
content: Thought Id share what you folks helped with !Not 100% but close...I didn't add the 2.2MegΩ resistors to the board, I will point to point 2 in series before the leads to the board..Once its tested, If anyone wants any of the kicad files, Id be happy to share!

author: EEVblog
author_rank: Unknown
timestamp: January 23, 2025, 04:03:38 am
content: Quote from: Audiofanman on January 23, 2025, 01:30:28 amNeat solution, well done.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2025, 07:25:05 am
content: There are some odd traces, that look like some shield or similar. These may cause a rather small creep distance, so a small distance on the PCB. This could be an issue with safety. From the Photo it looks like the larger size resistors are on the other side compared to the rendering.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2025, 09:31:21 am
content: Is the power supply referenced to earth ground or does it float?

author: Audiofanman
author_rank: Contributor
timestamp: January 23, 2025, 10:47:58 am
content: The resistors are as designed, all on the top. Im new to kicad, its only the second pcb board I ever had fabbed up so the render may show some of my lack of knowledge with the design layers...The power supply is referenced to earth ground, nothing floating..And yes, the traces are smaller than I wanted, my 1st board for another project came out exactly how I wanted it and I modeled the same trace width.. I think I may have missed something on the checkout process where options are offered for the pcb board manufactur...While it's low voltage on the board and ideally I'd have bigger traces to minimize voltage drop, it is a high enough impedance circuit and I'm hoping the traces won't affect the readings too much. If they do, I guess I send it out again...On pause testing until I figure out the best way to supply a reasonably clean and adjustable 100-500vdc to the (2) 2.2meg ohm series resistors before the board. I'd like to fully test before implanting into my monoblocks..

author: floobydust
author_rank: Super Contributor
timestamp: January 23, 2025, 07:48:49 pm
content: The board looks fine for starting outThere is always endless perfection here on the forums but at the end of the day you want something that works, to sit down and listen to music.I think it's possible to calibrate it with less than 500V - if you input say 30V calibrate it for 300mV reading. It depends on the test equipment you have.So the monoblock is off and unplugged, caps discharged, you inject say 30V there on B+ with a lab supply. I've done this before, the tubes are an open-circuit when cold so you can do strange things. Try calibrate it like that.Use great caution with a multimeter to the +500V rail. Not a cheapo, test leads in good shape, a glove on the hand etc. Or just alligator clips, no probing around. It depends on your skill level. I've had multimeters that couldn't take the 500V without drama.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2025, 08:04:20 pm
content: My concern is not about the thickness of the tracks, but the distance between tracks. How much distance is required depends on possible dirt / dust that can build up.The pictures are not very clear as the critical part is just below the resistors. It does not look good (no reason to reduce the distance), but it could still be acceptable. A picture of the layout (without the parts) would show the details.

author: floobydust
author_rank: Super Contributor
timestamp: January 23, 2025, 08:05:56 pm
content: This is a 5V board. The HV dropping resistors are not on it, they are mounted elsewhere and feed into the terminal block. My understanding of OP's build.

author: Audiofanman
author_rank: Contributor
timestamp: January 23, 2025, 09:00:41 pm
content: Correctumundo, at most this board will see 6 volts and just a few mA.. that said small voltage drops from imperfect connections could mean nothing or something.. I'd like to know which side of the scale is most likely..Turns out I bought pair of 24v adjustable boost converters that have 300v out.. I think that should work well enough..

author: floobydust
author_rank: Super Contributor
timestamp: January 24, 2025, 07:43:52 pm
content: You increased the power to 6V?Let us know how it works out, I'm wondering what the bias stuff is about and these triodes or yours. Or maybe tetrodes lol.