design help

John "Sugar Ray Weasel" Larkin has become down-right self-destructive.

...Jim Thompson

Hi John, I remember from my old SxS telephony days, that they did all sorts of tricks with relays and coils, like put brass slugs on the armature end to delay operate or break, have more than one coil, so that one operated the relay, but another just held it. They did things with timing, so that 'this' would happen because relay B was still held, even though the operating current had been released, while relay A operated quickly, and the a pause in pulses on A allowed C to operate, etc.

But, yes, some contacts arced more because of slow opening and closing of the contacts, while others just eroded because they operated more.

Charlie

And all the senile Californians move to Palm Springs!

Wait, that is where I live! 8-)

Charlie

Peak fet power dissipation will be 360 mW for the equivalent of a few seconds. Not very scary. Even a TO92 can do that.

John

Yep. I have in-laws living there ;-)

...Jim Thompson

The Austin-Healey overdrive solenoid had a contact that opened when the armature was fully seated, adding some turns and resistance to the winding so it wouldn't fry. Healeys were sometimes known to vibrate.

John

Cool. In some cases, rapid coil current cutoff will be significantly worse for relay contacts.

John

Cool. It's servo'd.

I measured the di/dt of a solenoid whilst the armature was in flight, and extracted acceleration and dynamic inductance from that. I suppose you could deduce something about the pull-in / hold-in ratio, magnetic reluctance of the two cases, forces, yadda yadda yadda, if you had a need. Plug results into fancy model, etc.

Or you could just power the coil and yank on the armature with a scale to see how much force was keeping it pulled in... :-)

Cheers, James Arthur

Vaguely recalling my mechanical engineering electives, can't you calculate all that from inductance versus armature position... Hamilton's principle?

...Jim Thompson

For solenoids that bottom hard and go to essentially zero air gap when seated, the holding force per amp is way, way higher than the pull force.

Ac-coil relays automagically reduce their coil current as needed.

John

Beats the heck outta me ... I was going to derive it from basic physics.

But then why model--probably inaccurately (or wrongly)--when you can measure? On the current waveform it was easy to see where the armature started to fly, to watch di/dt slow as the armature entered the solenoid, and flatten out abruptly once the armature slammed home.

The 'scope capture gave a great time-of-flight record too.

Cheers, James Arthur

Yes, I prefer experiments myself. But I also like to understand. I didn't do much with Hamilton's principle in school, but have since used it several times when calculating the power requirements of various solenoids versus core position, AND how fast they'll move.

...Jim Thompson

Wrong again. You said the fet power dissipation would be a problem, and I responded that a TO220 could be used if it were.

You are really obsessed, hanging on my every word. That's sorta sick.

Hell hath no fury...

John

Yeah, you've been that way since I mentioned that I'm married.

John