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Will Connecting Several Multimeters in Series Increase the Voltage Rating?

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ScottH

Thu Apr 13 2017, 06:24PM

Registered Member #61373 Joined: Sat Dec 17 2016, 01:45PM
Location: San Antonio, TX
Posts: 87

I know this sounds silly, but if you hooked up one black lead from one meter to a red lead of another meter, using 5 multimeters set to the AC voltage setting, can I find a way to measure the output of a high powered 2-3Kv AC source (MOT setup) without overvolting my meters?

Is it possible to also measure the current output with the MOT output connected to the string of meters?

Since the voltage setting uses high impedance, and each meter is rated to read 600v, would the voltage be divided among the 5 meters evenly and the impedance of the 5 meters add up?

Proud Mary

Thu Apr 13 2017, 08:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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. :)

radiotech

Fri Apr 14 2017, 05:39AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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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.

Enceladus

Fri Apr 14 2017, 09:23AM

Registered Member #61428 Joined: Sat Jan 14 2017, 12:39PM
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Posts: 50

Proud Mary wrote ...

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 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.

radiotech

Fri Apr 14 2017, 06:44PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Resistors have voltage ratings. Vishay has a lot of good data.

klugesmith

Sat Apr 15 2017, 12:27AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Enceladus wrote ...
[making a voltage divider] ... best option. Just ensure you select the resistors such that you wont exceed their power rating.

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 Â¼ 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 kÎ©. 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.

Enceladus

Sun Apr 16 2017, 05:30PM

Registered Member #61428 Joined: Sat Jan 14 2017, 12:39PM
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Posts: 50

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

hen918

Sun Apr 16 2017, 07:39PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

@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

Proud Mary

Sun Apr 16 2017, 08:12PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Enceladus wrote ...

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.

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.

klugesmith

Mon Apr 17 2017, 09:10AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Enceladus wrote ...
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.

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.

Enceladus wrote ...
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?

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.

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