switching induction loads

[chrisatpsu]

i see in posts about being careful about switching induction loads.

can someone explain what's happening, as compared to a resistive load?

and keep it simple  : )

[trekster]

When the magnetic lines of force collapse on an inductor, a voltage is generated.  Sometime many times greater then what was first applied.

Ron

 

[chrisatpsu]

so, if you stop supply electricity to a motor, then the forward motion of the motor as it's winding down is now creating electricity?

[tbone321]

If it was a permanent magnet motor then yes, it will act like a generator until it stops but that it not the issue.  The real problem is due to the magnetic field collapsing at the motor windings. When that happens a voltage is generated in the winding (ciol) and it builds until discharged.  Just think of the ignition coil in your car.  Now granted, that coil was designed with that in mind but just about any coil can do it, just not as quickly.  To get around it just put a light bulb in parallel with the motor and it will give the windings a discharge path ad stop any spikes.

[trekster]

When I was in Jr High (I am 54 now) a few guys would make shockers with 1 single 9 volt battery.  It was kind of a contest. We made several and I was the winner.  I found an old relay from a gas furnace.  I mounted it on a board and wired it up like the diagram below.  This would energize the relay but as soon as it moved it would break the contacts and then close making contact and starting over.  It had a high pitch BUZZ.  Off the coil I had heavy lamp cord (SPT2).  At the ends of the wire I took aluminum rods about the size of a fat pencil and connected the wires.  With a little old 9 volt battery that if touched with your hands you could not even feel it, this thing zapped you so bad you could not turn loose of the rods.  Your arms and hands would tighten up and one guy kicked me to turn it off.  He was the tough guy that said "anything you can make with a 9 volt battery I can take"   He changed his statement when I finally let off the button.  You could do stuff like this back then.  Today you would be hauled off to jail probably.  Or worse.

I am telling this story for an example of what magnetic lines of force and a coil can do.  The 9 volts felt like 150 if I had to guess.

The good old days 

Ron

[gforman]

Ah the good old day's  My trick was a old model T ignitor (coil)  with a couple of batteries strapped to it.  it worked on the same principle as the relay setup and then ran the output through a transformer to step up the voltage.
 

[chrisatpsu]

so when you energize a magnetic field, you want your circuit to be protected from the possibly crazy amounts of voltage that can happen once that field fails?  am i getting this right?

[steve hughes]

yep, you're right...now days,
1. someone would call the police department and complain
2. I would have to go take a report
3. I would probably have to contact county or district atty depending on how far parents wanted to push
4. Someone would probably go to jail for a practical joke (parents want to teach the other child a lesson, even tho theirs might have done something pretty close last week)
5. I would contact the parents of the child that did the joke and instead of them asking me if they could take care of the problem, (i.e. can we get a switch and take care of this in front of you so someone won't call CPS, and then he/she would be grounded for AWHILE....would get a statement like, "well he is just expressing himself?!?!?! and I can't believe that he even needs to get a ticket?!?!?!"

AND YOU WONDER WHAT IS WRONG WITH THE "NEXT" GENERATION???

[D56VillageNut]

so when you energize a magnetic field, you want your circuit to be protected from the possibly crazy amounts of voltage that can happen once that field fails?  am i getting this right?

Chris,

You're on the right track.  When using a relay (inductive load) the "protection" is to place a suitable diode in parallel with the coil of the relay.  I've got some small relays I'm contemplating using for some things.  They have just such a diode built into the DIP package and that was one of the key things I looked for when shopping for them.  Here's a schematic of the particular relay I've got and you can see the diode connected across the coil.

Alan T

[chrisatpsu]

some of the things that the "300 in 1" electronic projects kits dont tell you  : )

last november, i built a parallel port mechanical relay controller using the TTL signals from the parallel port passing through transistors (typical 2222a types) on the base.
i had the +5v coming from the molex hdd cable and passing through the relays before being connected to the collector side of the transistors.
I then had the emitter connected to ground
the relays i used were simple radio shack 5v trigger, 250v AC @ 1AMP relays


so basically, i should have had a diode across the the 5v contacts? (i don't believe they have it built in)

[jstjohnz]

Normally you want to put a reversed-biased diode across a relay coil.  The coil is an inductor, and the voltage across an inductor is L*di/dt. in other words it's proportional to the rate of change of the current through the inductor.  If you instantly switch the relay off, the rate of change of the current approaches - infinity.  Thus you generate a very high negative voltage across the coil.  The diode suppresses that.  That's why you get a big spark if you try to switch a motor, transformer, anything inductive through a mechanical switch.  As soon as the switch contacts separate, you get a big voltage across the coil, big enough to jump the gap in the switch contacts making an arc.

[tonyv2842]

When working with triacs (like in the LE) and SCR's, a designer needs to consider how fast the voltage is changing across the triac or SCR because of that inductive load. These devices have limits after which they will no longer function as expected.  An extremely rapid change in voltage across the device, turns on the device without it being told to or, won't allow it to shut off. There is also the possibility that the inductive load will allow that voltage to increase to beyond what the device can handle, resulting in destruction of the device.

[tbone321]

so when you energize a magnetic field, you want your circuit to be protected from the possibly crazy amounts of voltage that can happen once that field fails?  am i getting this right?

Chris,

You're on the right track.  When using a relay (inductive load) the "protection" is to place a suitable diode in parallel with the coil of the relay.  I've got some small relays I'm contemplating using for some things.  They have just such a diode built into the DIP package and that was one of the key things I looked for when shopping for them.  Here's a schematic of the particular relay I've got and you can see the diode connected across the coil.

Alan T

While this works well for DC and doesn't increase the load on the circuit, it doesn't do the trick for an AC circuit.  That is why I suggested a light bulb as it will work for either one at the cost of a slightlyincreased load.