Help with detecting current/voltage with a coil and a spinning magnet

[physicurious]

TL;DR

Why does my little generator hack not work?

Playing around with electricity and magnetism. Well, trying to anyways. Wound some wire up and down the length of a 4" screw several times. Thought when I spun the magnet close to the coil I would get a detectable amount of current on my voltmeter. Anyone know why I'm not reading anything? I know the current and voltage generated would be AC, at least I think since the magnet is flipping north/south, but thought I would at least see the needle wiggle a bit when having it set to DC milliamps. Or get a small amount of DC current just by moving one side of the magnet towards the coil quickly. I tried setting the voltmeter to AC voltage on the lowest setting but not getting anything either. Anyone have any ideas why? Also I'm a little confused why the voltmeter seems to measure AC voltage in decibels. If anyone can give me some insight into that one too that would be appreciated! Thanks all!

 

[Baluncore]

Welcome to PF.

Check your connections by switching to a resistance range.
Do you get low ohms?
If not connected, check meter leads alone for ohms.
Then select lowest DC voltage range.
Rotate the magnet across the end of the coil, not next to it.

 

[Rive]

Anyone know why I'm not reading anything?

Could you please provide some details about your magnet? Where is it from? Do you (by any chance) have a type, description, anything?
These flat, ring-shaped magnetic things can be tricky...

 

[DaveE]

TL;DR Summary: Why does my little generator hack not work?

Also I'm a little confused why the voltmeter seems to measure AC voltage in decibels. If anyone can give me some insight into that one too that would be appreciated! Thanks all!

Your analog multimeter should have several different scales. Somehow they will be labeled for which functions they are appropriate for. The dB scale is only useful for AC quantities, and, frankly not that useful anyway IMO. There will also be a scale in Volts (Current, whatever the function that's selected), just like the DC voltage scale. On some meters it will be the same scale markings, others may be slightly different.

Here's an example where the AC scales are red, the DC scales are black. Except for maybe the name of the company, every mark on the face of that meter matters.

Why someone would use it with this sort of meter is a bit of a mystery to me. It's for audio applications where you really want to know about power levels. It's a bit archaic, nearly useless, IMO. But I'm sure someone here will say they use it all the time, LOL.

 

[Baluncore]

By using the lowest DC voltage range, you are getting closest to the sensitive meter and will see the needle swing either side of zero, or vibrate, as you rotate the magnet.

The AC ranges on a multimeter now employ a couple of silicon diodes that need half a volt before the pointer moves. Once upon a time, germanium point contact diodes were used. They were more sensitive, but leaked when they got hot.

 

[Tom.G]

Take the magnet off of that metal rod and wave the magnet past the end of your coil. The closer the magnet is to the coil and the faster it is moving, the higher the voltage induced in the coil.

You could also suspend the magnet from a string and spin it.

Also try different orientations of the magnet in relation to the coil;
A) Some of that shape magnet are magnetized with the different poles on different sides
B) some are magnetized with one pole on the inner diameter and the other pole on the outer diameter
C) some are magnetized with alternating poles around the flat surface (like the spokes on a bicycle wheel, one spoke may be North and the next one South)

As @Rive said in post #3 above, "These flat, ring-shaped magnetic things can be tricky..."

A bar or horseshoe magnet will probably give better results. (even a 'refrigerator magnet' would be better)

Check out this post from about a year ago, you will notice that the peak voltage is only 10mV (0.01V), so your meter probably can't read that low. Also, that magnet in the video was rotating pretty fast, around 240 rpm (1/4 second per revolution).
https://www.physicsforums.com/posts/6503403/

Have Fun!
Tom

ps. Please let us know what results you get!

 

[Rive]

Why someone would use it with this sort of meter is a bit of a mystery to me. It's for audio applications where you really want to know about power levels. It's a bit archaic, nearly useless, IMO. But I'm sure someone here will say they use it all the time, LOL.

I do NOT use them (), but way back when a decent DMM was way too pricy and hard to get these were supposed to give you a decent indication for pulses (which could go unnoticed on a DMM).

Or so I was told. 20+ years old story, and that's at this end of the world.

That time DMM I had made AC measurement with a single diode (!), as peak rectifier Was fun to measure any non-sinus with it

 

[berkeman]

Thought when I spun the magnet close to the coil I would get a detectable amount of current on my voltmeter.

There are a few obvious issues. You need more of that WD-40 spray lubricant on your drill handle. You need to turn up your speaker so it's easier to hear the magnetic field. You need to wiggle a real magnet (like a bar magnet or horseshoe magnet) by hand at one end of your pickup coil instead of spinning whatever that thing is on your drill.

(Just kidding on the first two)

 

[physicurious]

I got a wiggle!!!! LOL It's not much but it's definitely there! Turns out I was moving the magnet on the wrong side of the coil. Thanks for the tips @Tom.G and @Baluncore ! And all the extra info too. I didn't know a ring shaped magnet could be any other way than just having each face be north/south.

Thanks everyone for all your responses! You're all awesome! Half of me was expecting people to just write something like 'just google it.' or 'go ask chatgpt.'

@DaveE Yup! That looks pretty damned close to my multi-meter. I was able to figure out how the scaling on it works when I plugged it into an 120v AC outlet. Not sure why I didn't think of that before. Make's sense though for audio. The decibels were throwing me, it has a little table with voltages and decibels so I thought you had to read decibels and convert to volts using the table somehow.

@berkeman Message received! More WD-40!!

So I feel like I got a pretty good memory of spinning something really really basic in elementary school, and I'm pretty sure it didn't even have a coil. Feel like it was more or less just a loop of wire that spun near a magnet. Am I imagining things because I remember it lighting an LED or something. The amount of amps I'm getting though, even with coiling the wire, I cant see that being enough of anything to light an LED.

Oh! few of you asked about the magnet. I dunno its just a home depot magnet.