Cheap display has weird frequency count

Jun 23, 2017 2 Replies
M

I bought an inexpensive volt/frequency led display from eBay. On a generator, I get the proper frequency display, around 60 Hz. When I tried it on a dc to ac inverter, square wave output, I get around 120 Hz. It's a two wire device, with power and signal coming off the same wire.



What could I expect to see if a scope was connected to that inverter? The thing runs everything I've plugged into it, with a bit of noise on radios and such. Drills and things work fine. So do small induction motors.


mike


It is hard enough to remember my opinions, without also remembering my reasons for them! Friedrich Nietzsche
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P

A cloud of magic smoke. ;)

I expect that it doesn't have adequate filtering, and that there's overshoot on the inverter's output.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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R

The inverter may not be putting out a very pure wave form. Frequency counters often expect a sine wave and will sometimes give a multiple of the actual signal. It could be trying to count the rising part of the wave and if a spike on the falling edge it may trigger on that. Some motors will run ok on power that is not near 60 hz. Try it with a simple analog clock and see if it times correctly or if it runs fast.

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