Neat back-EMF experiment

[BrianG]

Ok, I was bored. In between projects so I decided to hurt myself.

I made a quickie oscillator circuit out of a 555 timer IC with a frequency of ~2Hz and a duty cycle of ~80%. Easy enough. So, I decided to hook the output of that to a small relay that is hooked to a 7H inductor (yeah, that's 7000mH). Then I hooked my Fluke meter to the inductor output and powered up the circuit with 6v. However, when I try to get a back-EMF reading of the inductor, it "reboots" my multimeter, which has a max voltage of 1000v. Hmm, I tried a voltage divider, but something's not right.

Sooo, I wisely () decided to hold the probe ends so I can use my "manual measurement" method. All I can say is OUCH! My hand is still kinda numb typing this.

Anyone know how to compute inductive kickback using the scenario above. Been a long time since I played around with the formulas and a quick net search didn't turn up with the info I want. And I'm too lazy to search more.

[jhautz]

Ummmm.... Here is what I heard.

Blah, Blah, Blah..... Ouch.


You crack me up with your experiments man. Sorry I'm no help, just a smart @$$.

[What's_nitro?]

I think you inadvertently discovered cold fusion.

Nah but the components must be very close to perfect resonance to have that degree of potential difference in the back EMF signal. It makes sense- huge inductor, low frequency drive... Throw a cap in there and have some real fun!

[littlegiant]

Interesting experiment there. By the way will you be including minis like the mini-lst in your rc speed calculator?

[BrianG]

Originally Posted by What's_nitro?

...Nah but the components must be very close to perfect resonance to have that degree of potential difference in the back EMF signal. It makes sense- huge inductor, low frequency drive... Throw a cap in there and have some real fun!

No resonance involved really. Just a matter of a large value inductor. And a cap would make it less: When the inductor is charged, so it the cap. But when power is removed, the magnetic field collapses inducing an opposite voltage spike. This would have to discharge the cap (since the voltage is reversed) and then charge it back up, and would just lessen the spike effect.

Originally Posted by littlegiant

Interesting experiment there. By the way will you be including minis like the mini-lst in your rc speed calculator?

Provide the tooth counts of the gears involved and I'll add them. It's due for an update soon anyway, so the more I can stuff in there, the better.