AC voltometer - Page 1
Forum section: electronics / beginners

author: maurermike8
author_rank: Newbie
timestamp: January 16, 2024, 04:24:04 am
content: I am trying to measure the voltage on a hand crank ac generator and am getting readings that are jumping all over, not a big surprise. When a light bulb is added into the circuit it blinks off and on due to the way the generator works but im trying to just get an average reading. Is there a different type of multimeter that can accomplish an average reading over time?

author: inse
author_rank: Super Contributor
timestamp: January 16, 2024, 05:48:01 am
content: If you still have a analog meter, the needle movement might be slow enough to give you _some_ kind of average.If you need it more precise, use a digital scope with deep memory and let it calculate the rms/average value.By the way, when the lights blink, you have to crank faster ;-)Think of the joke with the lazy guy and the creaking wheelbarrow

author: Kleinstein
author_rank: Super Contributor
timestamp: January 16, 2024, 06:15:15 am
content: Many DMMs have a rather hard low limit in the frequency. Even an analog meter may have a problem here if the needle can follow the waveform.How low is the frequency about ?A rather fast DMM may be able to read the instant voltage and may be able to send the data to a PC for later calculation of averages, RMS, peak and other measures.A digital scope would be the ideal way, even if rather low bandwidth.

author: TizianoHV
author_rank: Regular Contributor
timestamp: January 16, 2024, 10:35:05 am
content: If you don't have an oscilloscope you can rectify the output voltage, use a big enough capacitor to smooth the ripple and use your multimeter in DC mode to measure the output voltage. Then you can add the voltage drop of the diode and calculate the approximate RMS value.

author: inse
author_rank: Super Contributor
timestamp: January 16, 2024, 12:12:24 pm
content: This will only give the peak value, conversion to RMS is not that easy if you don’t know the waveform.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 16, 2024, 01:47:11 pm
content: crank more steady then you have more steady voltage. If your meter has autorange switch to manual range.

author: guenthert
author_rank: Frequent Contributor
timestamp: January 16, 2024, 01:56:17 pm
content: Quote from: maurermike8 on January 16, 2024, 04:24:04 amLooks a bit like you're more interested in the generated (rather actually converted of course) energy. You could measure that by measuring the change in temperature of a known amount of water heated by a resistor ... but yeah, don't know of a meter which does that directly.Measuring the average voltage might be easiest with a (fast enough) DMM which can be read out by a computer (some newer meters might have such functionality built-in). Most bench-top and some hand-held DMM can be connected (older ones via GPIB only, newer ones offer various interfaces) to a computer where the data can be massaged and presented.

author: CaptDon
author_rank: Super Contributor
timestamp: January 16, 2024, 04:54:00 pm
content: Is it one of the old telephone / railroad hand crank generators with the multiple horseshoe magnets? They can light a 15 watt 120 volt incandescent bulb to about 60% brightness with rapid cranking. There is a way to get the data you want but it will be a bit complex. First, the load resistor, something of maybe 20 ohms to perhaps 50 ohms at around 10 watts, easy so far. Now, you need a way to generate data taken perhaps every millisecond or so. A digital oscilloscope can do this at slow sweep speeds. You could set the scope for .5 seconds per division which would allow the scope to capture a 5 second cranking period. You only need one channel and set the trigger just a bit above or below center so the scope triggers as soon as you start cranking. The vertical probably needs 5 or 10 volts per division, maybe more. You can export the data capture from the scope as a .csv file. This will provide the voltage that was present during each sample. Good so far? Now you need a program like Matlab or SciLab (student versions may be free?) The math program needs to do a calculation for each sample where it will calculate the current through the resistor based on voltage to obtain power (peak) at that instant. You could create a new file which contains a number related to 'power' at each instant. Do-able so far? Now the math program can perform a miriad of magic for you by printing out 'maximun instantaneous peak power', 'average power', 'peak power', 'RMS power' and even plot graphs of those values for you!!! Sounds like fun!! Enjoy mate, hope to hear of your progress!!!

author: CaptDon
author_rank: Super Contributor
timestamp: January 16, 2024, 04:59:40 pm
content: One note about some of the old telephone style hand crank generators, There is a mechanical mechanism with a set of contact points that cause the generator to act as an open circuit when you are not cranking. This stops the generator from 'bridging' the phone line. Those contacts bounce very badly while cranking and will cause ready bad data plots. If so equipped you may want to jump out those contacts. Cheers!!