Measuring RMS current and power into lamp.

[treez]

Hello, we have switch mode, 2W, xenon flashlamps which run off 20VAC, 50Hz.

Here is the input current and voltage...............
(the input features a rectifier bridge, followed by a 47uF cap)

https://i50.tinypic.com/63wppe.jpg

input current = red
input voltage = green


....It flashes once every 800ms......so the voltage and current pattern that you see above, repeats endlessly.

How do i measure the rms input current?

A meter is no good because the readout is constantly changing.

Also , how do we measure the power consumption?

..The switching frequency is around 25KHz, and you can see that some of the switching current is drawn through the supply wires....since its not all filtered by the lamps input filter.

 

[FvM]

The RMS measurement problem has been already discussed in a similar thread. https://www.edaboard.com/threads/263680/

RMS is an average quantity, you need to define the integration intervall respectively filter time constant for it.

For power consumption, a long averaging time would be suggested anyway.

If you don't have a real power meter, an oscilloscope with waveform arithmetic can possibly help, also for RMS current.

 

[treez]

Thankyou,

In the thread you kindly flagged up we, came to the conclusion that a power meter would be needed, but these are $2000+ so we wouldnt be able to. We only sell lamps and we just need to be as accurate as poss without spending that much.

I am not sure if an oscilloscope is accurate enough to be able to accurately calculate input power....at least not our TDS210 or lecroy waverunner.

The integration period is 800ms. (what you see in the window above in the image there)

The Fluke 179, despite being £250, is useless...the reading just jumps about.

it amazes me that a simple thing like measuring current , in 2012, is beyond all but expensive power meters.

 

[bubulescu]

Hello

I'm afraid that for a waveform that varies randomly (or cvasi-randomly) in time, the only solution to define its consumption would be to make averages of RMS values over large periods of time, as FvM said, such as pulse-to-pulse, but even that can't be set as a definite reference, only as a guess. For example, in the picture you provided you can set a current RMS period of 200ms, do reading (calculations) for, say, 50 values or 10sec, then average the readings to get an approximation of what you want. Of course, the longer the time-span, the better the approximation.


Vlad

 

[mtwieg]

A basic digital oscilloscope should be able to do this. Just measure simultaneous current and voltage, then use a multiplying math function and take the RMS of that. If it doesn't have the built in math functionality, then you should be able to export the waveform data to excel and do it manually.

 

[treez]

bubulescu.
...the 800ms window shown above repeate over and over....so this 800ms would be the period to average over.....but i dont know what equipment could do this...or take the long averages of several seconds that you suggest.

mtwieg:
I am assuming that you mean measuring the current with a current probe..?
....or would measuring it with a current sense resistor after the mains rectifier be ok?

.....also, for example, a TDS210 scope simply doesnt have enough storage space to take enough points over the full 800ms interval for it to be of any use in the way that you describe.........i doubt that any scope would have enough storage space to be able to store enough points over an 800ms interval in order to get an accurate power measurement...or RMS current measurment?

 

[mtwieg]

mtwieg:
I am assuming that you mean measuring the current with a current probe..?
....or would measuring it with a current sense resistor after the mains rectifier be ok?

A shunt resistor would be fine, so long as it's accurate (and you don't include that voltage drop in the lamp voltage measurement).

.....also, for example, a TDS210 scope simply doesnt have enough storage space to take enough points over the full 800ms interval for it to be of any use in the way that you describe.........i doubt that any scope would have enough storage space to be able to store enough points over an 800ms interval in order to get an accurate power measurement...or RMS current measurment?

The TDS210 probably doesn't have enough memory, but better scopes will (and to be honest, the TDS2XX stuff is pretty much the bottom of the barrel in terms of digital scopes). 1MS of memory would be plenty, and most midrange scopes will have that.

Another option: if the lamp only operates in one of two states (on or off), then you can just do measurements under each condition (requiring only a couple AC cycles), then just calculate the pulsing power requirement by knowing the duty cycle. Not a perfect method, but probably the best you can do with your current tools.

 

[FvM]

I fear in today's terms, a TDS210 won't pass any more as a "basic digital oscilloscope" due to it's restricted capabilities, e.g. poor memory depth of 2500 samples. Also no multiply or integrate in math functions. In older days, I possibly had thought of putting together an analog power and true RMS meter with sufficient averaging time constant. Today I would rather lend out a better oscilloscope from a colleage (if I didn't have it already).

Average power consumption can be possibly measured with a mechanical energy meter and a stop watch (if it doesn't start running at the low load, add a some 10W base load).

 

[bubulescu]

treez,

If you say that the 800ms period is repeating, then find a device that can do aquisition for that amount of time and simply do it several times, because, I'm afraid you need the averaging part; even if the resolution is not that great, this will help.

Good luck,
Vlad

 

[FvM]

find a device that can do aquisition for that amount of time and simply do it several times

No bad idea. If you have a true RMS meter with data readout, the method should work as follows:

Average a large number of readings after squaring the current value. Due to the nature of RMS value, the square root of average should give the correct RMS value over a respective longer time. Power measurements can be averaged directly.