design help

jamesgangnc · 2009-02-24T13:17:20+00:00

I'm ok with fixing and adapting circuits but not so good with design. I've come up with an electronic switch circuit but I think it could use improvement. The goal is to turn on a 12v relay with a switch that is closed when off and open when on. The switch also has to have one leg grounded. It's a pressure switch with a single terminal. I also want the relay to stay closed (on) for a couple minutes when the switch goes from on (open) to off (closed). Here's what I've got now and it does work. But I had to use a pretty big capacitor and I'm still only getting about a minute of delay when going to the off state. Because I can't make the 10k resistor any bigger. I'm thinking that it is likely possible to get a longer delay and do it with a smaller capacitor if I was better at designing this. The other flaw is that this slowly lowers the voltage to the relay until it drops. The relay stays pulled to about 3 to 4 volts. I also suspect there is a better way to keep it fully on until the rc timer expires. Thanks in advance for any suggestions :-)

J

John Larkin 17 years ago

The point was whether slow release compromises contact life. The answer is "no." Your personality issues are irrelevant.

As the OP stated they are.

Probably not. TO-220 power fets are cheap, and can dissipate a watt or so intermittently with no problem.

I suggested measurement, I designed the experiment, you did the work. When the timebase was such as to conceal the entire shot, you repeated the measurements. Fine by me; I trust your data.

And just how was I wrong? What I did was question the claim that slow release is bad for contacts. You demonstrated that it probably isn't, certainly no worse than diode clamping.

Because I do electronics, and he doesn't. And because I'm friendly with lots of people, and he insults everybody.

Impossible. He has no "defense", just lame insults.

Amateur.

John

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krw 17 years ago

I'm sure they have two Lowe's and three HomeDespots, by now.

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J

John Fields 17 years ago

The answer is "yes". If arcing is the factor degrading the life of the contact, then slow release will do little to minimize the problem, while fast release with no diode will help.

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J

Jim Thompson 17 years ago

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

...Jim Thompson

| James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The first sign of senility is persistently trying to be an asshole The second sign of senility is touting your company\'s wonderful circuit designs as your own, while posting amateur crap on S.E.D The third sign is acting like Polly Prissypants :-)

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J

John Larkin 17 years ago

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

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J

John Larkin 17 years ago

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

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J

John Larkin 17 years ago

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

John

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James Arthur 17 years ago

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

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Jim Thompson 17 years ago

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

...Jim Thompson

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John Larkin 17 years ago

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

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James Arthur 17 years ago

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

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Jim Thompson 17 years ago

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

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John Fields 17 years ago

Yup! 12V | [100R] COIL | [100R] FET | 0V and, yet, you were ready to plug a TO-220 in there before you were forced to look at the numbers. That\'s just sloppy. JF

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John Larkin 17 years ago

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.

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Hell hath no fury...

John

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John Fields 17 years ago

Sorry, but no... What I wrote was: " On top of that is the increased dissipation in the driver, which for a larger relay and a longish time would probably require the extra expense of a heat sink. You must have glossed over the "larger relay" part.

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J

John Larkin 17 years ago

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

John

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John Fields 17 years ago

Yup; I don\'t mess around with married chicks. JF

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