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.
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.
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
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.
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.
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.
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.
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
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 (
, 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.
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in