power analyser is inaccurate?

[eem2am]

Hello,

We were testing a PFC LCD-Television power supply.

We wanted to read the power input from the mains during active standby mode. (its around 10 to 20W , depending on TV model)

We tried using the Voltech PM100 Power analyser, but the power reading was "jumping about".

VOLTECH PM100 DATASHEET:
VOLTECH|PM100|ANALYSER, POWER, 1 PHASE | Farnell United Kingdom

Then a senior engineer came along and told us not to use the PM100 because he said it is inaccurate.

...However, there is nothing in the PM100 datasheet to suggest it would be inaccurate.

We ended up being told to use a Yokogawa PZ4000 instead........

PZ4000 POWER ANALYSER DATASHEET:
Yokogawa - Products - PZ4000 Power Analyzer



So why is the PM100 so bad?

I have heard bad reports of the PM100 in other companies....specifically its poor accuracy.....even when seen to be calibrated.

 

[chuckey]

The Farnell instrument has not got a figure for the crest ratio (?). I.e. the ratio of the maximum current (in a spike) compared to the constant repetitive wave form. i.e. it should captured every spike and RMS its value with that of the rest of the current flow. It might have a good bandwidth for a constant high frequency current, but its not the same! I seem to remember precision RMS voltmeters (40 years ago) had a crest ratio of 2000 :1. the voltage spike was not limited by the input circuitry of the voltmeter.
Frank

 

[FvM]

The standby power consumption may be actually "jumping". Did you set the measurement parameters to long averaging as suggested in the VOLTECH PM100 operation manual?

I also suggest to perform an oscilloscope current measurement, or even real power measurement, if the oscilloscope has sufficient signal processing capabilities. You have at least a change to understand why the power analyzer measurement are varying or (possibly) inaccurate.

 

[eem2am]

Thanks, I will try that long-averaging method.

I wondered about simply getting current (from a current sense resistor, we dont have many current probes, and they're always gone "walkies") on one channel of a scope, Vin (AC) on the other, and then multiplying them, and getting the time average of the multiplications which is the power.

-However, i hope i can find this in the yokagawa DL9140 scope manual.

Also, the problem is it will be in the AC line so will be alternate pos and neg, and so will average to zero.....so i have to hope the math function is easy to work out.

 

[The Electrician]

I have seen this behavior from modern digital power analyzers before. When it happened to me, it was caused by the conducted EMI from the device being measured. The later Yokogawa analyzers seem to have better immunity.

I recommend having on hand an old-fashioned electrodynamometer type wattmeter such as: **broken link removed**

as a sanity check. These old meters aren't fooled by EMI and although they are only good for about 1/2% accuracy, they provide a useful function.

 

[power-testing]

The TV standby circuit for the majority of TVs are designed with a small 2-10W flyback AC/DC converter to enable handling IR commands when the TV is "off". This standby power supply is actually quite noisy and hard to get a handle on when trying to measure "average standby power". The Voltech is likely jumping around because the power is noisy. There are three options in solving this problem if you already own a Voltech.

You can read voltech over GPIB quickly and average them.
Σ Pin/n=Average Power

Alternatively you change the measurement period in the voltech. I forgot the command.

Alternatively you can add a 1uF Box Cap across the line to filter out the noise.

1-2 uF High Voltage Ceramic Capacitor across line and neutral and the will filter out the high frequency noise.


**broken link removed**

 

[eem2am]

Hi,
I will certainly check this out.

-but today a senior SMPS engineer of 15 years experience told me that the Voltech PM100 was no use for standby measurements.

Also, Hi power-testing,
-my company used to use a 1W flyback to provide standby power..........but now they don't do this anymore......because if you use such an extra SMPS, then you have to use a relay to switch out the main PSU.
-and as you know, relays are expensive.

-also, with all the modern burst -mode able chips about, with low standby power, there is no need for a separate SMPS any more.

 

[RCinFLA]

In standby, there might be a strobing of power going on to check if the video input ports are receiving a signal to initiate a full power on. You can check to see if the TV has this function.