What does the count on a non-autoranging multimeter mean?

[joseph nicholas]

Hi, I have a really cheap Chinese multimeter.  On all the ranges like ohm, volts, etc it reads something like this: 2,20,200.  On the volts range it does the same but then at its highest end it goes to 700.  At first it though this signified the precision of the count or at the least the decimal places on the meter.  But why not just use 1, 10, 20.  Can someone point me to a site that explains this, if not may be you could explain it?

thanks, nico

BTH this scheme doesn´t prevent me using it just confusing.   

[ChunkyPastaSauce]

    EEVblog #26 - Multimeter Tutorial - Counts, Accuracy, Resolution & Calibration

[jeroen79]

On digital multimeters the ranges will be factors of 10 apart as our numbering system is base 10.
Thus it will easy to just move the decimal separator around.
The highest range being 700 instead of 2000 would probably reflect the meters rating, ie don't measure voltages over 700 with it.

On analog multimeters you sometimes find ranges with a different interval.
For example 1, 3 , 10, 30, etc.
That is a root 10 ish interval.

On analog multimeter you can just print two scales while on a digital meter you would need to divide the digital count somewhere before sending it to the display.

I do wonder about the origin of the 'counts' property.
An older Fluke multimeter actually had two ICs for the
One would generate a square wave with the frequency increasing with the voltage and the other would actually count the pulses and output the count on the base 10 display.
I'm not sure if all digital multimeters work like this but it would explain why count is called count.

[Brumby]

Hi, I have a really cheap Chinese multimeter.

I'm sure a lot of us do.

On all the ranges like ohm, volts, etc it reads something like this: 2,20,200.

You will probably have a display that goes up to 1999 - with a decimal point that shifts according to the range.  This is because full digits take up space, so they only give you 3 of those, but a '1' can be displayed using only 2 segments.  Adding a '1' doubles the range you would have otherwise.

  On the volts range it does the same but then at its highest end it goes to 700.

You aren't going to get a cheap meter capable of going up to the count limit of 2000V!  The 700V figure is a limit that the overall design and circuitry can handle.  (Supposedly.  Don't use any of the cheap meters to measure high voltages - especially high energy circuits, such as mains.  They may work - but if you get a transient, they can ... quite literally ... blow up in your face.)

  At first it though this signified the precision of the count or at the least the decimal places on the meter.  But why not just use 1, 10, 20.  Can someone point me to a site that explains this, if not may be you could explain it?

thanks, nico

BTH this scheme doesn´t prevent me using it just confusing.

Hope this helps.

Dave's video is worth a watch.

[joseph nicholas]

Ok, I got it now.  After reading the comments and re watching the video.  The meter is a 3-1/2 count meter.  It can only display 1.99 so therefore the 2x´s number base on the dial.  This tells me the capability of the display.  Then just to be safe, in the voltage mode quite illogically it jumps to 700 on the dial. I think I need an auto-ranging multi-meter.

Thanks for your comments.

nico 

[ModemHead]

I do wonder about the origin of the 'counts' property.
An older Fluke multimeter actually had two ICs for the
One would generate a square wave with the frequency increasing with the voltage and the other would actually count the pulses and output the count on the base 10 display.
I'm not sure if all digital multimeters work like this but it would explain why count is called count.

The integrating dual-slope A-to-D converter found in most handheld multimeters is actually a voltage-ratio to time converter.  The resulting time period is indeed represented on the display by "counting".

[jeroen79]

Then just to be safe, in the voltage mode quite illogically it jumps to 700 on the dial. I think I need an auto-ranging multi-meter.

It's not illogical. The displayed range tells you up to which voltage that range is usable.
In the low ranges the meter's count limits the voltage that can be measured.

In the highest range the meter's safety rating is the limit.
The display could theoretically go up to 2kV but above 700V the meter will be damaged.

This is no different on an autoranging multimeter.
Autoranging is just the meter turning the knob for you when it goes above or below a range's limits.

You'll probably see the same thing on the AC side of the rangeswitch.
The ranges will be 2*10^x except the high range.
Possibly the highest AC range will be different from the highest DC range.
If so, the ratio between the two will likely be somewhere near 1.414. Guess why?

[Brumby]

Ok, I got it now.  After reading the comments and re watching the video. 

The meter is a 3-1/2 count meter.

To be precise ... 3-1/2 digit or 2000 count.  (Don't worry too much - we knew what you meant.)

It can only display 1.99 so therefore the 2x´s number base on the dial.  This tells me the capability of the display.

Then just to be safe, in the voltage mode quite illogically it jumps to 700 on the dial.

If you look at the ranges, I think you will see 2V, 20V, 200V, 700V.  This shows there is not a 'jump' to 700V, but rather a range that is less than the logical sequence of 2V, 20V, 200V, 2000V.  This is not related to the counting function, but of the physical design limits of the circuitry.  It is quite likely the meter will measure voltages higher than 700V - but DON'T try it.  Doing so is risking damage to the meter - and to yourself.

I think I need an auto-ranging multi-meter.

I wouldn't say 'need'.  Learning how to use a manual ranging meter has it's benefits - ones I would strongly suggest are worth having.

The first is the general idea of approaching a measurement from the highest range and move down the ranges as indicated.  The second is the fact that such considerations force you to think about the voltages and/or currents you are working with.  An auto ranging meter doesn't make you do this but, to be fair, if you don't keep these figures in the forefront of your thinking when measuring circuits, you shouldn't be going anywhere near them.

Having said that, an auto ranging meter is convenient at times.  The only negative (sometimes) is the time it takes to find the appropriate range.  This delay can be annoying if you want to make several measurements quickly.  However, if you know a specific range you want to use, then there is usually a 'range select' feature which will solve that problem

Thanks for your comments.

nico

[Assafl]

Counts: There are many ways to make A/D converters. For example, you can use a resistor ladder and comparator to make a very expensive low resolution A/D (like the LM3914 series) - or some other technique.
One such technique is called a Dual Slope Converter - and has the advantage of having both precision as well as a rather low sensitivity to component drift. The downside to Dual Slope is that it is slow.
It works by comparing the voltage on a capacitor (being charged by a constant current) to the input voltage. To negate capacitor value, it then discharges the capacitor down to a reference value.
As it is charging a timer generates pulses and counts the number of pulses. Math is then used to compensate for cap value (comparing the up count & down count).
Hence the term count. The meter reads the number of pulses it counts while charging and discharging the capacitor.
(NB - to make dual slope faster there are shortcuts taken like successive approximation)

Autoranging: Does not affect count - count is the A/D conversion. Autoranging is deciding which voltage divider to use the get the most dynamic range without exceeding the max voltage the A/D can handle. In non autoranging DMM that decision is made by the user. In an autoranging DMM there is a set of resistors (a resistor network) and comparators that make the decision automatically and usually fast.

700V: That is the maximum voltage the meter can handle without burning out components (it usually has a safety margin on top of that). The reason it is recommended to not use a properly rated (CAT rating) DMM on supply side mains is because of what can happen if the rating is exceeded - at that point it can be component breakdown - but it can also be the electrical grid doing its best to get energy to keep flowing - at whatever cost to the DMM, its user and the surroundings. It can indeed explode. Hence measuring 700V on a low energy circuit might be okay - but not on the supply side.

The flip side of the 700V - is how low does it go - at the low end you have resolution (2V/2000 count) but you also have noise, accuracy of the voltage reference, and drift. That is why Dave recommends having two inexpensive meters over 1 expensive one.

[retiredcaps]

The integrating dual-slope A-to-D converter found in most handheld multimeters is actually a voltage-ratio to time converter.  The resulting time period is indeed represented on the display by "counting".

Modemhead has a write-up on DMM Basics dual slope at

http://mrmodemhead.com/blog/dmm-basics-dual-slope-adc/

[joseph nicholas]

You'll probably see the same thing on the AC side of the rangeswitch.
The ranges will be 2*10^x except the high range.
Possibly the highest AC range will be different from the highest DC range.
If so, the ratio between the two will likely be somewhere near 1.414. Guess why?

Because RMS?

[Brumby]

You'll probably see the same thing on the AC side of the rangeswitch.
The ranges will be 2*10^x except the high range.
Possibly the highest AC range will be different from the highest DC range.
If so, the ratio between the two will likely be somewhere near 1.414. Guess why?

Because RMS?

You are thinking in the right direction.

To confirm your understanding, I ask you this:
Say we have a multimeter that has a range marked 1000VDC.  Using the reasoning above, what would expect the maximum AC voltage range to be? (To the nearest 100V)

[joseph nicholas]

I feel like a contestant on Jeopardy.  Let me see.  1,000 x 1.414.   

[Brumby]

So .... your answer would be 1400V?

Incorrect.  Think more about what RMS represents.  Drawing pictures might be helpful.

(I might seem to be a little mean here - but you are very close to understanding.)

[Zero999]

It's all marketing BS as far as I'm concerned.

Technically a count is the analogue value represented by a single bit on an ADC and the number of counts is equal to 2^the number of bits. If a meter has a 16-bit ADC and an analogue range of +/-1000V, then it will have 2¹⁶ = 65536 counts and each count will be just over 30.5mV, irrespective of whether or not the display has adequate digits to display values that small on the 1000V range. Internally, any digital calculations will be performed using counts, rather than Volts or Amps to avoid rounding errors, then it will be converted to the correct units before being displayed.

[joseph nicholas]

As a practical demonstration, I already did this.  I built a power supply.  I put 127vac into a 10 amp bridge rectifier, used a 63v 10000uf capacitor and measured the voltages.  The ac voltage was less than the dc voltage on the output.  The dc voltage was something in the range of 135volts or so.  This was because the the meter was inaccurate in measuring the ac, but close enough to show the difference between the two.  My assumption was that the ac voltage had more power than it showed on my meter.

[Alex Nikitin]

As a practical demonstration, I already did this.  I built a power supply.  I put 127vac into a 10 amp bridge rectifier, used a 63v 10000uf capacitor and measured the voltages.  The ac voltage was less than the dc voltage on the output.  The dc voltage was something in the range of 135volts or so.  This was because the the meter was inaccurate in measuring the ac, but close enough to show the difference between the two.  My assumption was that the ac voltage had more power than it showed on my meter.

  you are quite lucky to walk away from that experiment.

Cheers

Alex

[Gyro]

It's all marketing BS as far as I'm concerned.

Technically a count is the analogue value represented by a single bit on an ADC and the number of counts is equal to 2^the number of bits. If a meter has a 16-bit ADC and an analogue range of +/-1000V, then it will have 2¹⁶ = 65536 counts and each count will be just over 30.5mV, irrespective of whether or not the display has adequate digits to display values that small on the 1000V range. Internally, any digital calculations will be performed using counts, rather than Volts or Amps to avoid rounding errors, then it will be converted to the correct units before being displayed.

I very much doubt if there's any calculation involved in a low cost DMM. It's a simple dual slope ADC (probably an ICL[edit:7106] or derivative). After applying the scaled input to the integrator for a fixed period, the display value is simply the number of actual hardware clock counts required to return the integration capacitor voltage to zero. The number of counts the meter has / displays is simply represents the length of the counter chain implemented in the chip..

Probably OT in terms of the OP's current questions anyway.

[Brumby]

Probably OT in terms of the OP's current questions anyway.

Not probably .... definitely.

The OP is asking fundamental questions about the use of a multimeter.  Going into the technical detail on how it derives the numbers displayed is really OTT.  It's like talking to a learner driver about how the valve timing works when they're still figuring out the gears.

[Brumby]

As a practical demonstration, I already did this.  I built a power supply.  I put 127vac into a 10 amp bridge rectifier, used a 63v 10000uf capacitor and measured the voltages.  The ac voltage was less than the dc voltage on the output.  The dc voltage was something in the range of 135volts or so.  This was because the the meter was inaccurate in measuring the ac, but close enough to show the difference between the two.  My assumption was that the ac voltage had more power than it showed on my meter.

  you are quite lucky to walk away from that experiment.

Cheers

Alex

That is quite scary.

If we take that 127VAC as a sine wave, then the peak voltage will be 180V.  When you rectify that, you might lose a couple of volts from the diodes, but you are still going to get over 175V peak.  Putting that into a capacitor only rated for 63V is ASKING for it to explode.  The fact that you only read 135VDC or so indicates there's something else happening - whether it's measuring problems or circuit anomalies it's hard to say - but even if you did have 135VDC - it's still way too high for a 63v cap.

FWIW, it's common to see capacitors being run below their rated voltage.  For example a 63V cap might run at 50V or less.  Even then, they can curl up their toes (especially the cheapies), but you never run them above their rating! - even the best quality ones.

[joseph nicholas]

Sorry, I left something out guys.  I used a transformer from an old microwave oven which was rewound.  The output from the transformer was down to about 10 volts ac.  I put this into the bridge rectifier and capacitor. 

The circuit works great a  simple and very cheap non-regulated power supply.  Next, I ran this into a cheap buck converter.  The buck converter worked in and was able to give me pretty good precision to 1 decimal place but unfortunately would only switch on with a load attached.  It drew about .25 amps using a household light bulb.  The buck convert is rated a 7amps and I am now waiting for my dummy load dyi kit to test both of them out.  Unfortunately the buck converter has no current limiting circuitry.  It will make a very basic power supply.  Not much but a start.   

[Brumby]

Sorry, I left something out guys.  I used a transformer ......

Ah ... yes ... that does make somewhat of a difference.   

[Assafl]

So how did you get to 135V DC?

Oh and one more thing - 135V AC is dangerous. But not as dangerous as a rectified 135VDC with a capacitor.

E= 1/2 CV^2 = 0.5 * 0.01 * 135^2 = 91.1256 Joules

Generally speaking - 50J - is considered enough for possible ventricular fibrillation. (if all of the energy ends up in you or you end up flying to the other side of the room depends on how contact came to happen and body resistance).

I'd be terrified if I were you...

[Zero999]

So how did you get to 135V DC?

Oh and one more thing - 135V AC is dangerous. But not as dangerous as a rectified 135VDC with a capacitor.

E= 1/2 CV^2 = 0.5 * 0.01 * 135^2 = 91.1256 Joules

No, you've got that backwards.

135VAC is much more dangerous than 135VDC and any capacitor charged to 135VDC is much less dangerous than a constant DC voltage source such as a battery.

Generally speaking - 50J - is considered enough for possible ventricular fibrillation. (if all of the energy ends up in you or you end up flying to the other side of the room depends on how contact came to happen and body resistance).

The energy is only dangerous if a high enough current can flow for a long enough period to cause ventricular fibrillation, which is much more likely with mains frequency AC than DC. 135VDC will give you a bit of a shock but nowhere near as bad as the 120VAC mains.

[Brumby]

.... I'm more interested in where the 135VDC came from.

UNLESS it is a reading error on the part of the OP - and it was 13.5VDC.  That would be about right for rectified 10VAC.