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Dr. Dark Current

Sat May 02 2009, 05:05PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Is it possible to make a mains voltage/frequency wattmeter, or a wattage-to-voltage converter, without using microcontrollers or other fancy stuff? I'm probably looking for something with operational amplifiers but I'm not sure.

It should be able to measure wattage drawn from a 230V 50Hz outlet into all kinds of loads (low power factor, non-linear=SMPS etc.) with a moderate accuracy (~5%).

I'm not asking for a schematic, just a way to do it.

Thx for any ideas

tesla500

Sat May 02 2009, 05:40PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

A Wattmeter is just a meter that measures average power. Since power is voltage * current, a wattmeter could be as simple as current and voltage sensors connected to an analog multiplier, followed by an averaging circuit. The output of the averager is then a voltage proportional to real power.

Analog multipliers are relatively expensive, a microcontroller is much cheaper for the same accuracy. But it can be done, have a look at some analog multipliers here, for example.

David

Sulaiman

Sat May 02 2009, 07:05PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The easiest is to buy one!
Here in UK they're 5 to 10 pounds,
digital display of voltage, current, power, accumulated energy usage etc.etc.

Traditionally a meter that has a moving coil and a magnetic field from a coil.
The product (Real {volts x amps}) is indicated by a needle/dial as is common.

Mattski

Sat May 02 2009, 10:52PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

You can make the analog multiplier yourself using opamp circuits. A good example is found here. You use log amplifiers on your two inputs, then sum them, then antilogarithm.

This way will be more difficult than a premade multiplier chip, plus it will probably be less accurate, since accuracy will depend on all of your part values and how well the diodes in your log amps are matched. You can probably tune it to pretty good accuracy though.

I think the basic overall system setup would be a small-value series resistor to measure current, and then measure the voltage on the load. At 50Hz your system should be faster than the power, so you'll get a 50Hz varying power signal, so you'll need some way of getting RMS power out of that.

twist2b

Sat May 02 2009, 10:55PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
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Posts: 117

That is something that has always confused me -_-
How can you measure Amperage in parallel or Voltage in series... like measuring watts..... which do you do? In series or in parallel?

Mads Barnkob

Sun May 03 2009, 08:54AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

twist2b wrote ...

That is something that has always confused me -_-
How can you measure Amperage in parallel or Voltage in series... like measuring watts..... which do you do? In series or in parallel?

You calculate the power in Watt from the two measurements of Volt and Ampere.

twist2b

Sun May 03 2009, 08:25PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

MadsKaizer wrote ...

You calculate the power in Watt from the two measurements of Volt and Ampere.

Yeah, I understand that, but... I figured it out in my head the other day...

P=IE
Watts = amps*Voltage
So then to get the amps:
Voltage/Resistance

So essentially its just:
Voltage/Resistance*Voltage
Or
E^2/R
Thats pretty simple.

But you can't measure Amps without putting the meter in series... (older meters, not digital) Thats what I was saying.

Dr. Dark Current

Sun May 03 2009, 08:43PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

You must always put the meter in series, but since it has negligable resistance, it really doesnt matter.

The problem with AC circuit is that when you measure V and A and multiply, you get VA not W. You only get W when you multiply V and A in every instant and then average.

Steve Conner

Sun May 03 2009, 09:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Bob Pease of National Semiconductor has a simple wattmeter circuit:

The secret is to use transistors to multiply two signals together. The technique of doing this is called various names, such as Gilbert cell, balanced modulator, or the like.

Ordinary op-amps can't perform multiplication because they're linear devices. You can get a device similar to an op-amp that does it, called an operational transconductance amplifier (OTA)

klugesmith

Sun May 03 2009, 10:52PM

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

twist2b wrote:
That is something that has always confused me -_-
How can you measure Amperage in parallel or Voltage in series... like measuring watts..... which do you do? In series or in parallel?

I think other responders have missed the point you are looking for.
A true wattmeter can't be simply connected in series with the load,
and it can't be simply connected in parallel with the load.
It needs BOTH, thus more than two points of connection to the circuit under test.
Typically there are 4 terminals, two connected to the line and two connected to the load,
though three would suffice in single-phase applications (since current measurement in supply and return is redundant).

hope that helps.
-Rich

p.s.
I have a wattmeter which senses V through a pair of alligator clips, and senses I with a clip-on current transformer.

Electromechanical KWH meters in the USA, for split phase 120/240 volt service, commonly have four terminals and no instrument connection to neutral. They respond to the instantaneous product of (Line 1 current + Line 2 current) and (voltage between line 1 and line 2). Even if the meter is otherwise perfect, there are small errors when unbalanced loads (reactive ones in particular) cause unbalanced voltage at the meter.

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