power measure

[cgrahl@blablabla_nospam.com]

Hi!

I need to measure the power of some devices. I'm thinking to made a
digital ampermeter, since I already know the voltage (220 V), I only
need to measure the current to get the power (according to the formula
P = I x V), right?
Does anybody have a schematic or a link to a digital ampermeter?
Or have another idea to measure the power of a device?
This must be digitally made, since this value will be used in a PIC
controller.

Thanks in advance.

Carlos Augusto Grahl

 

[Ken Taylor]

Hi!

I need to measure the power of some devices. I'm thinking to made a
digital ampermeter, since I already know the voltage (220 V), I only
need to measure the current to get the power (according to the formula
P = I x V), right?
Does anybody have a schematic or a link to a digital ampermeter?
Or have another idea to measure the power of a device?
This must be digitally made, since this value will be used in a PIC
controller.

Thanks in advance.

Carlos Augusto Grahl

You are assuming the voltage is 220V, because that's what the supply
authority says (I'm assuming this is mains 200VAC) - this may not be the
case at any particular time.

It is probably going to be easier and safer to get a digital multimeter with
a serial (RS232/485) link to your uP. There are meters which will read power
of your load by measuring V and I directly. The more expensive ones even
tell you the power factor.

Ken
..

 

[Don Stauffer]

Hi!

I need to measure the power of some devices. I'm thinking to made a
digital ampermeter, since I already know the voltage (220 V), I only
need to measure the current to get the power (according to the formula
P = I x V), right?
Does anybody have a schematic or a link to a digital ampermeter?
Or have another idea to measure the power of a device?
This must be digitally made, since this value will be used in a PIC
controller.

Thanks in advance.

Carlos Augusto Grahl

Why do you need a digital one? Analog ones work fine, and they are
cheap. You need a multimeter. The problem is that you did not say what
power consumption range you need. If less than 10 amps worth, you can
buy a meter much cheaper than you can make one for.

 

[Vidar Løkken]

Why do you need a digital one? Analog ones work fine, and they are
cheap. You need a multimeter. The problem is that you did not say
what power consumption range you need. If less than 10 amps worth, you
can buy a meter much cheaper than you can make one for.

RTFP. He needs it to input to a PIC circuit. Make that with a analogue
one.
But maybe just opening a cheap DMM, and figuring out a suitable place to
get the value is the cheapest way, if, as said you wan't to measure

10A. You could also use a shunt resistance and a A/D converter. This

will be a bit of work to build. But best chance is really to buy a
el-cheapo dmm, and figure out where to get a binary value out of it.

 

[quietguy]

Maybe things have changed, but as I recall most (of the cheaper) analogue
multimeters don't have an AC current range

David

 

[Ken Taylor]

They can do, but they are picky about wave-shape.

Ken

message

 

[CWatters]

Hi!

I need to measure the power of some devices. I'm thinking to made a
digital ampermeter, since I already know the voltage (220 V), I only
need to measure the current to get the power (according to the formula
P = I x V), right?
Does anybody have a schematic or a link to a digital ampermeter?
Or have another idea to measure the power of a device?
This must be digitally made, since this value will be used in a PIC
controller.

Article on Digital meter with a Serial Port
http://techsupt.winbatch.com/TS/T000002007F9.html

 

[Jasen Betts]

Hi!

I need to measure the power of some devices. I'm thinking to made a
digital ampermeter, since I already know the voltage (220 V), I only
need to measure the current to get the power (according to the formula
P = I x V), right?

only if it's 220V DC if it's AC power can flow the opposite direction during
some of the cycle meaning that VxA is more than the watts used.

Does anybody have a schematic or a link to a digital ampermeter?
Or have another idea to measure the power of a device?
This must be digitally made, since this value will be used in a PIC
controller.

you have to start analogue and convert it wit soome sort of anallogue to
digital converter, (I think they make PICs with inbuilt ADCs.)


if you're measuring AC you'll have to measure voltage too and multiply the
readings (volts * amps) average that and then take the square root. for
this you're going to want a microcintroller that knows how to multiply.
(since frequent ADC readings give better accuracy, and software multiply is
typically 5-20 times slower than hardware multiply.

not being familiar with the PIC line I can't reccomend a particular device.

 

[Jasen Betts]

---
Wrong. You need to _measure_ the voltage and not assume anything.

And wrong again. Power, in watts, is:

P = EI(cos phi),

that's assuming that the current is a sine wave...

 

[me]

that's assuming that the current is a sine wave...

actually any wave. E is RMS value...

 

[Jasen Betts]

---
If it's AC, the voltage across the load alternates as does the
charge flowing through it. Power though, continues to be dissipated
in only one direction. For example, an incandescent lamp doesn't
light up for one half-cycle and then dim the next.

I was thinking of an reactive load... I'm surprised you didn't catch that
with a reactive load for some period of the cycle the current oposes the
voltage, during that period the instantaneous wattage of the device is
negative. - energy is flowing from the device into the supply.

No. You need to measure the voltage and current and, if necessary,
convert the values read to RMS, then measure the phase angle
between the voltage and current, get the cosine of that angle and,
finally, multiply the voltage, the current, and the cosine of the
phase angle together to get power.

how do you compute phase diference when there's no gaurantee that the
current looks anything like a sine wave it seems to me that that, it would
be harder to compute than calculating the average of a number of
voltage.current products.

Well, then, how about recommending one you _are_ familiar with?

Atmel ATmega48 - 20 MHZ risc microcontroller with plenty of 10-bit A-D
channels.

Bye.
Jasen

 

[DaveM]

I was thinking of an reactive load... I'm surprised you didn't catch that
with a reactive load for some period of the cycle the current oposes the
voltage, during that period the instantaneous wattage of the device is
negative. - energy is flowing from the device into the supply.


how do you compute phase diference when there's no gaurantee that the
current looks anything like a sine wave it seems to me that that, it would
be harder to compute than calculating the average of a number of
voltage.current products.


Atmel ATmega48 - 20 MHZ risc microcontroller with plenty of 10-bit A-D
channels.

Bye.
Jasen

I would recommend that the OP investigate four quadrant analog multiplier
chips, such as the Analog Devices AD633 and AD532, and the OnSemi MC1495,
etc. There have been several power meters designed and available on the
web using these and similar devices.
Start by looking at the design at
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1160
You should also look at the datasheet for the MC1495. Lots of circuit
ideas in it. The MC1495 is obsolete, but the circuit design ideas can be
used with other devices as well... Or, if this is a one-off design for you,
the MC1495 is still available from some surplus outlets ( www.jameco.com ,
www.cietron.com , etc.)
The basic multiplier circuit has been designed for you. All that's left for
you to do is to control the amplitudes of the voltage and current waveforms
into the multiplier circuit.

Cheers!!!!
--
Dave M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!