Testing voltages over long periods of time

[eujean]

Hi there,

I live in a dormitory at the university, and have noticed a weird phenomenon..

I have an ancient refrigerator, and everytime the thing "turns on" my external display goes off for a second then back on..

So I assume it takes a hefty amount of current to get the conpressor going, and the voltage drops for a moment, but how would I measure this to be sure?

Because the refrigerator turns on randomly(based on temperature) I don't know how to capture the mains voltage over a period of time.. Any suggestions?

Also this only started happening recently

[tautech]

Measurement methodology will depend entirely on the equipment at your disposal, you don't say what you have. 

Any DMM (bench or HH) with logging will do it, even correct use of a DSO with the appropriate trigger settings and using single shot mode will too.

[bitshift]

One very crude way would be to rectify the mains voltage, feed it into a small peak detector followed by a voltage divider. Then, using an arduino or similar, log the voltage to determine the lowest value.

Disclaimer: if you go through with this, post your final schematic and ask people about safety before plugging the thing in and switching it on.

For the peak detector, Dave did a nice video on them here:

[eujean]

To make things tricky, I only have 2 DMMs and a 20MHz analogue scope available in my room..

And various other tools such as arduinos, etc. I like the peak detector idea, might give it a go thanks

[Kilrah]

Because the refrigerator turns on randomly(based on temperature) I don't know how to capture the mains voltage over a period of time..

Turn the refrigerator off, let it sit for half an hour, it will have warmed up enough for you to be sure the compressor starts straight away when you turn it back on...

[danadak]

From a safety point of view -

1) Get an LED, walwart with low voltage AC output, say 12 V, off ebay. Add a series circuit,
R + LED + Diode and wire to Walwart output. Now you have a led whose brightness is propor-
tional to AC line voltage. This should cost you < $ 5.

2) Get Arduino Uno + a photocell. Use photodiode in a divider and feed to Arduino analog in.
Tape photo diode to LED. Or get an optocoupler which has LED and photo sensitive transistor
isolated output. Add a little code to do peak detection (you do not have to do an analog peak
detector, just do it in code), and start logging values, and output thru UART to PC file. Or write
a little python code and display results on PC.

3) Or use a simple Arduino DAQ solution like -

https://sites.google.com/site/measuringstuff/the-arduino

Again use a simple transformer to lower and isolate the AC line voltage.


Regards, Dana.





Regards, Dana.

[Richard Head]

Use a 230V-12V mains transformer as a current transformer wired in series with the fridge. The 12V winding must be on the mains side and the 230V side connected to a burden resistor (ESSENTIAL). This gives you isolation. Connect the scope probe across the burden resistor. Without going into the calculations a 100R resistor will give you about 5Vrms on the secondary with about 1A through the 12V side. If you set the scope to 2V/div and 10ms/div and trigger as soon as you get an input you can probably see several mains cycles due to the screen persistence, no storage scope required.

[3141592]

If your DMMs have a simple Min/Max function and a way to disable auto-off, then that would be the simplest.
Another lazy way would be just pointing a camera at the DMM... but I'd suggest going with some of the better solutions the others said, that way you'll gain experience.

If you gonna have something wired up to mains, be sure to pay attention to safety for example having a big sign over the setup warning that it's live, not leaving wires or test leads that might get pulled out by someone unsuspecting and present a danger etc...