True RMS wattmeter = buy or make suggestions

[picclock]

I need one !!. Am considering a design I found in Elektor 1983 May

Any better ideas appreciated.

Best Regards

picclock

[philf]

How accurate does the meter need to be and does it need to be true RMS?

I have a plug-in meter which reads W, KWh, A etc. Mine has similar functions to:

http://www.ebay.co.uk/itm/Plug-in-Power-Consumption-Meter-Energy-Electricity-Usage-Watt-Calculator-Monitor-/162113034987?_trksid=p2141725.m3641.l6368

I don't know how accurate they are but it gives some idea of power consumption. On mine you can input the cost per KWh and it works out how much you're spending.

Phil.

[awemawson]

"Hot Wire ammeters" were the old school method of getting true rms values and they cover a wide frequency spectrum.

[picclock]

@ awemason
For Ammeters only thats true. I once used a wattmeter which summed two moving coil movement to produce results. However, in todays world the current is often taken in short pulses, and at differing points on the voltage waveform, so calculating the average power use is tricky, requiring fast calculation of the power value and averaging over the whole cycle.

The Elektor meter (May 1983) used a 4 quadrant multiplier to allow for power being taken and returned to the source and averaged it using an LM13600/ 13700 chip with some buffers and stuff.

In truth measuring power accurately these days is tricky  , hence my post. I have looked for true RMS meters but they seem very rare, and although I am reluctant to make a pcb and assemble it, it appears I may have no alternative, unless someone at madmodder comes up with a better solution  .

Best Regards

picclock
 

[PK]

If we rephrase the question: "Need electronic device, should I buy one or build one" then the answer these days is nearly always "Just buy it"

There are some exotic bits of kit that are worth building. But not many.
Of course, if you just want to make one, then fine. But it almost always costs a fair bit more to get a similar result.

PK

[picclock]

@PK
Definitely would buy if I could find one to buy, and not too many £££££'s. However I have not found anything out there as yet. There are some on ebay but its hard to know if they are true rms or just average amp times volt types.

If I get time I may just roll my own, as there have been many times in the past that a device like this would be very useful. Also could make it for DC with AC superimposed and with ranges for hi/lo voltages and currents. Price of multiplier chips is quite hi though unless you settle for the less accurate ad633 and its variants.

Best Regards

picclock

[awemawson]

Tick Tock,

You don't say what application you have in mind. Are you doing DC measurements, 50 Hz, or are you in the realms of RF, microwaves or what? It makes a huge difference to the correct approach.

 

[BillTodd]

RMS volts or current ok

No need for RMS watts  (although i cannot recall why exactly! - something to do with the rms function not giving the answer that one is actually looking for)

Bill,

(too old for this maths *%+/)

[awemawson]

The beauty of the old fashioned method (ie hot wire) is that over a wide spectrum it is irrelevant what the wave form is, whether it's steady or pulsed, and within bounds what the frequency is.

After all the heating effect IS the RMS value as I understand it.

[John Rudd]

Tick Tock,

  ......Andrew, you dont arf take some liberties.......

Btw, aint come across any Ayres Rock yet.....

[awemawson]

It was just a gentle nudge to get him to spell MY name correctly 

[BillTodd]

After all the heating effect IS the RMS value as I understand it.

Yes that's it. X  rms of an alternating current gives the same power as X dc into the same load, making the phrase "rms watts" misleading or incorrect.

[picclock]

RMS power is measured with a resistive (non reactive) load,  with in phase current/voltage. It bears no relation to the power consumed by most of todays electrical devices, which tend to take current at the peak of the waveform only, or after the waveform peak has occurred.

A hot wire ammeter is very good at measuring non sinusoidal currents. However, for power measurement, its the magnitude of the current and voltage value at a given points in the voltage waveform which will give the true power value.

For instance, lets assume a trailing edge dimmer circuit which fires 120 degrees into the sine wave. The instantaneous power at the moment of trigger will be the load current (not necessarily resistive) and the voltage 120 degrees along from the start of the sine wave. As the voltage falls the power will change tending to zero at the moment the voltage waveform reverses polarity. Additionally any phase shifted load may return power to the supply reducing the apparent wattage.

So the way I see it is that its very tricky to get accurate power readings without a fair degree of sophistication. The original electric 'Ferrari' meters (I think) used induced eddy currents and some clever magnetic stuff to get accurate power readings. Modern meters use dedicated chips to do the same job.   

Still thinking about rolling my own 

Best Regards

picclock

[awemawson]

I think that should be Ferranti meters Picclock 

I still have an original salesmans demo of the very early Ferranti meters that was made up as a desk calendar.

(I shared office space with the meter department for a period of about 10 years in my 25 odd years with Ferranti  )

[picclock]

@ Awemason

Oops !! Ferranti - Excellent catch - Clearly old age has gotten to me - Either that or I had a go faster race tuned meter.

Still a jolly clever metering system though, even though its ancient tech.

Best Regards

picclock   

[awemawson]

Tick Tock its

MAWSON for gawds sake man

[PK]

@Awemawon

It could be worse...
:-)