Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Proud Mary, Thu Apr 13 2017, 08:35PM

Mr Scott, making a very simple circuit called a voltage divider would enable you to measure voltages higher than the 600V setting on your multimeter.

If you need help with the voltage divider there are many here who will be able to give you good advice.

Take care around microwave oven transformers. They can kill you. :)

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
radiotech, Fri Apr 14 2017, 05:39AM

Analog meters:

Identical amp meters connected in series will all read the same value of current.

Identical resistors connected in series will all have the same voltage drop.

An analog voltmeter is an amp meter with a resistor in series.

Heed Proud Mary's advice.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Enceladus, Fri Apr 14 2017, 09:23AM


If you have a selection of large value resistors on hand this is going to probably be your best option. Just ensure you select the resistors such that you wont exceed their power rating.

Suppose you make your divider out of 10 equal 1/4 watt carbon film resistors and you're trying to measure ~4kV then the minimum resistance of your 2.5W divider can be found as V^2/P.

R=4000V^2/(10*0.25W)=6.4Meg

When the voltage to be measured is applied across all 10 resistors in series, 10% of the voltage will appear across each of the resistors. Although 680k (X10) is the nearest standard value, it would likely be best to just use ten 1M (Brown/Black/Green/Gold) resistors. That way you'll have some head room.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
radiotech, Fri Apr 14 2017, 06:44PM

Resistors have voltage ratings. Vishay has a lot of good data.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
klugesmith, Sat Apr 15 2017, 12:27AM

Enceladus: Good work teaching! Now time to learn some things. As Radiotech said, resistors come with voltage ratings that shouldn't be exceeded. For non-special ¼ watt carbon film R's, it's usually 250 V or 300 V. To complement Radiotech's reference, here's part of a datasheet about MF-series resistors from KOA Speer. Look at the next-to-last column:

At the high-ohms end of a resistor product series, the voltage rating often gets you first. When specifying 1-megohm 1/4-watt-size resistors, you have to pay attention to voltage ratings if the power dissipation will exceed 1/16 watt!

Suppose you make your divider out of 10 equal 1/4 watt carbon film resistors and you're trying to measure ~4kV then the minimum resistance of your 2.5W divider can be found as V^2/P.
R=4000V^2/(10*0.25W)=6.4Meg
When the voltage to be measured is applied across all 10 resistors in series, 10% of the voltage will appear across each of the resistors.
... just use ten 1M resistors. That way you'll have some head room.

Good example there, but it has margin issues. You started by rounding the power-limited value up to the next E12 standard value, 680 kΩ. Even if the R's were OK with 400 volts RMS apiece (566 V peak), each would then have to dissipate 0.24 watts. 0.25 watts if actual component resistance were 5% below nominal. The 10 resistors would get very hot. Even hotter if packed tightly together, or if free air circulation were compromised (e.g. inside small box, with or without ventilation holes), or if the HV divider were potted in solid material.

At 1 megohm you have a more sensible margin on the power rating, but the voltages are still too high unless you choose special resistors. I do recognize that this is a hobby forum, where it's popular to compare notes about abusing electronic components.

Scotty: You only asked about measuring 2.5 to 3 kV, e.g. using five multimeters each on 600 VAC range. ( Which would work, by the way, if the meters are approximately identical, and if you add the five voltage readings together. As a practical matter, with cheap DMM's, it could be risky for the instruments and the operator. )

So you'll probably be OK using a voltage divider with only ten garden variety resistors.
Here's an exercise for you: compute the change in the attenuation factor, due to connecting your voltmeter in parallel with the bottom resistor of voltage divider. If you want accurate 10:1 scaling, for convenience, the bottom resistor can be made a bit larger than the others. If you haven't learned enough to do this with confidence, please leave all MOT's safely bolted inside MO's.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Enceladus, Sun Apr 16 2017, 05:30PM

klugesmith,

Granted I have never tried this exact application with 1/4W resistors, but I have seen them used in high voltage applications such as connected across 1n4007 diode strings to ensure balanced voltage drops across each diode while they are blocking, as well as in small Marx generators and bleeders for TC primary cap banks. You can coddle your resistors if you like but I have never had any problems with 1/4W resistors at up to 1kV as long as I haven't exceed the power rating. Besides, they're like 5 cents. Who cares?

Also, I am well aware of how component tolerances work, but I was just trying to outline how to do it fast and dirty without it pouring smoke the second it's powered on. With 10X 1M there should be ample time to get a reading before it overheats.

It's a contradiction to label a 1M resistor as being rated for 1/4W and 250V. Ohm's law is pretty inflexible that way. It's got to be one or the other. It makes no sense to explicitly display one rating
while secretly limiting the true rating to less than half of what it's supposed to be. If a 1M 1/4W resistor is not rated to dissipate any more than 1/16W, then it's not actually a 1/4W resistor at all, is it?

I honestly didn't even know there was such thing as carbon film resistor datasheets. They have no numbers or letters or insignia except the bands. How do you reference that?

I was thinking that a capacitive divider might also work too if power dissipation is that big of an issue (although I remain skeptical that it would be.) High voltage caps are usually a common item in high voltage hobbyist junk drawers. At 60Hz it's not at all difficult to get all the reactance you'd need to limit the current to a reasonable level with just 2 caps. Maybe something like a 200pF and a 1.8nF with the output taken across the 1.8nF. (Just a rough guesstimate)

Connecting several meters in series just seems ridiculous to me but it looks like thats what he wants to do. Good luck with that Scott. Hopefully you don't end up with a pile of dead meters. If you change your mind about building a divider Scott, you can PM me if you need help.

-J

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
hen918, Sun Apr 16 2017, 07:39PM

@Enceladus
The resistors might be able to handle the voltage but if they do arc over it isn't the cost of the replacemt resistor I would be worrying about, it's the (major) damage it would do to your measuring equipment

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Proud Mary, Sun Apr 16 2017, 08:12PM


Not so fast, Lard Boy! The voltage rating of a resistor will take into account the possibility of flashover and tracking, while manufacturer's spec sheets will also show the resistor's tolerance of momentary overloads in which, for example only, high voltage short duration pulses could damage the resistor without ever exceeding the limits set by the part's rate of thermal dissipation. Such damage is not always visible, and may take the form of a permanent change in the resistor's characteristics.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
klugesmith, Mon Apr 17 2017, 09:10AM

Only if you think those ratings mean it _can_ dissipate any power <= 1/4 watt, and _can_ handle any voltage <= 250 V. But they don't! The first number means you won't overheat the part if you stay below 1/4 watt. The second means you won't overvolt the part if you stay below 250 volts. If you had a 4 ohm resistor with the same specs, you would hit the power limit at 1 volt.

For practical reasons, specs like that apply to all ohm values in a resistor "product". The top row in that table I cited covers hundreds of ohm values. All nominal 1/4-watt axial-lead metal film resistors. (but not the only 1/4-watt axial-lead metal-film resistors in that maker's catalog.)
The part numbers covered by that row share mechanical, tempco, reliability, and voltage specs, and of course a nominal power value.
Not to digress too much, but nominal power is just a starting point for thermal considerations. It indicates which curves to look at in charts of temperature rise vs. power, for different cooling scenarios, and what internal temperature the resistors can tolerate.

If you buy them by the reel, at 0.9 cents each, the reel will be labeled with the full manufacturer's part number. Likewise if you buy a handful from Mouser or Digi-key (et al), which will come in their own labeled plastic baggie.

Look at tank capacitor discussions in Tesla Coils subforum. People talk about mfr's and part numbers, not just C and V and dielectric materials. Other 4hv-ers talk about stressing components well beyond voltage ratings. How many of their creations run 24/7/365, with the maker not present to fix them when they break? Or in the case of TC's, when they stop breaking?

It's not uncommon for an industrial components distributor to have a half-million line items under Resistor. At Mouser.com I just narrowed it down to the series we're talking about - see image. See what happens if you check "carbon film" instead of "metal film".

To carry this a bit further, suppose there's a 10k 0402-size resistor somewhere in a cable modem. The bill of materials for that board won't just have it down by ohms, tolerance, size, and quantity. It'll have a modem company's part number for Resistor, 10k, 1%, 0402. Factory material planning computers have links from that p/n to one or more mfr part numbers from approved vendors.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
ScottH, Mon Apr 17 2017, 01:38PM


The capacitive divider sounds interesting. At the moment I will make a voltage divider with a 12M Ohm and a 1.2M Ohm resistor (1 watt power rating). I will wrap them with electrical tape so they wont arc over from over-volt. It is a bit rigged, but hopefully this will give me some good readings.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
klugesmith, Mon Apr 17 2017, 07:17PM

Go for it! We await your results.

Note 1. Looks like you chose R values to get an 11:1 voltage division. So we are both in the club of people who don't mind doing arithmetic to convert instrument readings to real values of the variable of interest.

Have you considered that 11:1 is the _unloaded_ division ratio, and that connecting your DMM will significantly change the ratio? The specs you pointed me to don't say whether the meter's input impedance on voltage ranges is 10 megohms or 1 megohm, and the difference is huge in this application.
You can find out by measuring 120 VAC with and without the voltage divider -- a very simple way to calibrate the divider.
Or by measuring a 9 volt battery directly, and with 1.2 meg in series with meter, and with 12 meg in series with the meter.

Voltage divider loading by the meter is also a consideration for capacitive dividers. With the C values given most recently, connecting a 1 megohm voltmeter (not uncommon at the low-cost end of DMM market) would make a high-pass filter with corner frequency of 80 Hz. You could still easily get accurate readings using the AC calibration trick given above. Waveform distortion probably isn't a big concern for you.

In either case, if you changed your low-side R or C to get division of approximately 100:1, the loading by meter would be that much less significant.

Note 2. I would skip the electrical tape. Arc-over is not the failure mode for the R's we're talking about, at these voltages. High temperatures and/or high electric fields, in the resistive layer and its junctions, lead to long-term resistance changes and increased failure rates.

Wrapping the 12M resistor with thermally-insulating tape will make it get hotter than it needs to be, if it's connected for a long time. That would call for derating the component power. Maybe not by much if the wire leads are left long, as a heat sink. Hypothetically, if that resistor were immersed in dielectric oil there'd be huge margins against arc-over and over-temperature. Then you could see whether 2,500 volts applied for a day, or for a year, makes the resistor change measurably. I bet you have no lack of MOT's!

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Enceladus, Tue Apr 18 2017, 01:35AM

Have you considered that 11:1 is the _unloaded_ division ratio, and that connecting your DMM will significantly change the ratio? The specs you pointed me to don't say whether the meter's input impedance on voltage ranges is 10 megohms or 1 megohm, and the difference is huge in this application.
You can find out by measuring 120 VAC with and without the voltage divider -- a very simple way to calibrate the divider.
Or by measuring a 9 volt battery directly, and with 1.2 meg in series with meter, and with 12 meg in series with the meter.

This is a very good point and a good suggestion. When I suggested the 10X 1M divider, I had hoped that selecting low-ish values would mitigate this source of error somewhat and side-step having to consider your DVM input Z.

Voltage divider loading by the meter is also a consideration for capacitive dividers. With the C values given most recently, connecting a 1 megohm voltmeter (not uncommon at the low-cost end of DMM market) would make a high-pass filter with corner frequency of 80 Hz. You could still easily get accurate readings using the AC calibration trick given above. Waveform distortion probably isn't a big concern for you.

In either case, if you changed your low-side R or C to get division of approximately 100:1, the loading by meter would be that much less significant.

I like the 100:1 idea too.

Also I see what you're saying about the voltage ratings now. Maybe I'll check out some of these datasheets you're talking about and/or or try a few real world tests.

Cheers

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
ScottH, Tue Apr 18 2017, 02:45PM


I simulated this in LT Spice, with the multimeter being 1M Ohms and 10M Ohms. The 4,200v source listed is equal to 3kV RMS. I came out with 128v RMS for the divider setup in the 1M config. When I changed the multimeter's resistance to 10M, the voltage increased to 243v RMS. I made the measurement between R1 and R3 (The multimeter).






Would this be accurate in the real world?

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
klugesmith, Tue Apr 18 2017, 07:43PM

>> Would this be accurate in the real world?

Yes. The exercise can be simplified, even in LTSpice, but now's not the time for details.

You don't need to quote previous posts in their entirety, especially when they are long & verbose, like some people produce from time to time.

When you get your 12 meg 1-watt resistor, I guess it would be OK to tape it before brief trials at 2500 volts AC. If it arcs internally, across the helical groove in its resistance layer, it could still fry your meter. Please be sure to tell us if that happens.

There are several kinds of overvoltage protection shunts that could be placed in parallel with the meter. I don't know how long any can carry the short-circuit output current of a MOT. Gas tube surge arrestors, and at least some of the solid-state devices, switch to a low resistance "crowbar" state when triggered by overvoltage.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
ScottH, Thu Apr 20 2017, 01:56AM

I just tried it. I first tested the voltage divider with 120v, and I got a 35:1 reduction. I tested a single MOT with no caps, and got a 94v output. The meter wouldn't give me a reading with resonant caps on. I tested the meter after to see if it was shorted out, but it still works fine. Thank you for the tips

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
klugesmith, Thu Apr 20 2017, 07:33PM

Here are some traditional high-voltage resistors on Ebay, around 10 megohms, for not much money. In my experience, the seller called bloodblister is a good guy.

You could get one and stop worrying about overvolting your ordinary 1-watt resistor.
Or, as everyone says, use a string of ordinary resistors.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Proud Mary, Fri Apr 21 2017, 06:43PM

Has it been established that the input impedance Zin of Mr Scott's instrument is 10 MΩ on the range in question?

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
radiotech, Sat Apr 22 2017, 05:31AM

Mr. Scott could set his meter to the highest AC voltage range and make sure auto range is off.

(1)Then connect to a voltage close to maximum. (i.e. 500 V AC)

(2) Then connect s variable resistor in series until the reading is half the first reading. (i.e. 250)

(3) Leaving the now found resistor in place, go back to step 1.

Now the source of 500 V AC reads 125. then 62,5, etc.

When 500 V AC is read as 62.5, the meters maximum range has been multiplied by 8.

The loading will be the resistance found in step 1 , which is the |Z| of the meter itself, plus all the resistors strung in series.

(4) Stop process when maximum new voltage exceeds what the highest voltage range needed becomes.

At this point,the overall |Z| in of the test gear will be the sum of the resistor value found in step 2, plus the sum of all resistors used to achieve the desired ranging.

This method does need knowledge of the meters |Z| before hand.


How bleeding confusing is this .

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Proud Mary, Sat Apr 22 2017, 10:32AM

I think Zin = 10MΩ has been a general international standard for digital multimeters for some years now, but I don't know it for a fact, and would want to see it in the meter's spec sheet to be certain.

Or, of course, you can measure Zin for yourself. If you Google for 'how to measure input impedance of voltmeter' and similar forms of words you will find enough answers to fill a wheelbarrow.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Sulaiman, Sat Apr 22 2017, 01:23PM

I have been using these 'sticks' for some years now with zero failures




46 x (2M2, 0W5, VR37) resistors in series = 100 MOhm, 23 W nominal, per stick
(46 x 2.2 = 101.2 MOhm but due to tolerances in the batch that I bought actual resistance = 100.4 +/- 0.2 MOhm each stick)
(I had originally planned for 45 x 2.2 = 99 MOhm)
I rate these sticks at 10 kV, 10 W, each, (50 or 100 uA is just nice for a moving coil meter), safely, reliably.
(240 V per 0.1" hole-to-hole @ 10 kV)
Add in series for 20, 30, 40 kV etc.
add in parallel and/or series to load eht power supplies.
I have used 33 kV peak 'flyback' spikes across each stick.. no problem
no encapsulation/insulation, just VR37 resistors on 0.1" fiberglass matrix board, smooth/blobby soldering.

The other two sticks are 1.5KE350CP TVS diodes in series, also about 10 W each stick.
one stick just clips my 5 kV, 23 mA OBIT, the other does not.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
ScottH, Sat Apr 22 2017, 01:53PM


I measured my meter's resistance with a resistor and a battery. Its 1M ohm. I used a cheaper meter for this experiment.

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Sulaiman, Sat Apr 22 2017, 02:22PM

the good news is ;
oscilloscope probes such as x1,x10, x100, x1000 can be used directly into your meter as oscilloscope inputs are nominally 1 MOhm.
convert your meter leads to a BNC connector.
e.g. or or etc.
for a 10 MOhm input meter, add 1M1 across the meter leads, (or 1M + 220 K pre-set resistor for callibration)

Re: Will Connecting Several Multimeters in Series Increase the Voltage Rating?
Proud Mary, Sat Apr 22 2017, 03:41PM


This sounds sensible to me, Mr Scott, as it means you've 600 μA going in there, and 360mW getting wasted on your 600V range, but you ask folk like Sulaiman and Klugsmith who have spent a lot more time thinking about this kind of thing than I ever have.

The best part about sites like this is being able to help people on their way.