design help

Not everything related to hardware is that simple.

A single-module program may be fine for a PIC/AT8 which is doing the work of, say, a dozen 7400/4000 logic chips.

But consider a couple of levels above that, where you might be performing complex DSP, providing a substantial user interface, interfacing with USB, BlueTooth, TCP/IP, etc, possibly needing to write a Windows driver to interface your hardware with a PC.

At this level, it's still at least as much electronics as it is programming. Someone with no h/w experience is going to have difficulty doing this. The lowest level most Comp.Sci. courses touch is "systems" programming, and even that doesn't get into details on IRQ handling, bit-banging and the like (no assembler, either).

--- That's my plan for today.

I've got a nice Aromat HC#-HP-DC12V-F

which is rated to hot switch 3A at 30V, so what I'll do will be to hook it up like this:

.+12VDC>---------------+-------+ . | | . +-----+ | . |K | O--> | . [DIODE][COIL]- - - - -| +--------+ . | | O-----------+----------------------------+ | | [10k] [COIL] | | | d ____ | diode | O O---+----ak--+-----+----------g-| nfet | | | | | 100uF ~1M s | | | | GND>---+---------------+-----+-------------+

I'll post the results sometime today.

JF

That depends on how the 'opening event' mechanism is designed into the relay by the designer. One would think that the whole industry follows a paradigm or standard.

I would say that the contacts are against each other, and the length of the strip between the pivot and the contact determine how much flexure can be placed on the strips when closed. A closure has to pass beyond mere contact to the point where the pressure against the contact faces flexes those strips a specific amount. Those strips and their length determine contact point pressure.

The release event is a release of a magnetic capture, as we know. We also know that said captures can require less holding energy than the original clamping energy, yet still hold the same weight. The faces have to mate very well. There will also be a spring that would be placing pressure in the normally open position direction. The gap between the contacts when open determine the working AC voltage of the relay. I would say it is very hard to open them slowly and that if there is such a window of control, it will be a very tight window.

I think they snap open, but the gap is pretty small, and how do relays which have contacts being made when they are 'off' as well?

Most of mine are KXP-1124, the 8.5" version, but I do have a pile of untested wide carriage tractor feed printers in storage.

Used wide carriage printers usually get tossed because no one wants them. I've bought them for 50 cents each at auctions, when no one bid against me. After a while, I quit bidding on any of them.

Have you ever used a high speed drum printer? They will print that entire box in a few minutes, and the paper will hit the ceiling. I paid $1 for the last one I had.

So are Rich's newest jokes.

Wrong. It's at *least* one step to change fonts. ;-)

Mea culpa. I "saw" NPN when I glanced at the drawing.

How so? The base current is modest and limited by a resistor.

Anyways, I agree that the low-side PNP emitter follower as suggested is the better solution, requiring fewer parts.

^ or transistor

What resistor? I don't see no steenkin' resistor! ;-)

...but I might mention that the high-side version has the advantage of a slightly higher voltage being left for the relay. The emitter follower version guarantees that the voltage across the relay will be one forward junction short of the supply, about 0.8V, whereas the high-side version "steals" only the collector-emitter drop for the saturated transistor, about 0.1V.

This probably makes no difference for this application (unless the supply voltage is low enough to make several tenths of a volt matter), and the fewest-part solution remains preferable.

My only point is that I won't predict relay behavior without hard, quantitative facts. How does slow coil current dropoff affect contact velocity? A lot? A little? How much does a clamp diode affect release velocity, as opposed to release delay (which I have quantified for some relays, for my tach/overspeed-trip module.)

Trouble? I only expressed uncertainty, exactly the opposite of being sure I'm right. You are apparently sure about how relays behave. Subsequent facts may be a hazard to you, but all I can do is learn.

It's strange that when I ask questions, expressing uncertainty, you take it as expressing certainty, and then take offense at my acting like I'm always right. Which means that *you* think you know everything and have nothing to learn. Downright bizarre behavior for an engineer.

John

No, I am asking if the conventional flyback diode significantly reduces contact opening velocity. Does it? If so, power-switching relays might be more reliable if coil current is made to drop very rapidly. Rapid drop/no diode certainly reduces release time, but I don't know how it affects release velocity.

We actually use a lot of relays in our products (BIST switching, mainly, but some customer outputs like overspeed trips) so I'm interested in them, especially as regards reliability. Spec-wise, they are phenomenal components.

You are saying that time proceeds slowly, so don't worry about the future. Interesting philosophy.

The problem isn't defined, other than GOOD and NOT GOOD. And the associated figure is clearly absurd and unreal.

I never take appnotes and eval boards at face value, and am wary of datasheets. I know some of the guys who write this stuff, so I test anything that looks fishy. Like the Variac thing; "trust but verify", as Reagan (?) said.

John

It's not especially my circuit. A couple of others have posted very similar things. It is sort of obvious.

You can measure actual delays and contact transition times by connecting the contacts like...

o---------+-----1K----V+ / | gnd---------o | | o---------+-------- to scope

which makes a positive pulse when the contacts are in flight.

Dual-trace that with coil voltage or coil current.

John

I sort of doubt that there's any serious community-wide theory or culture of relay design, especially considering how many different countries design relays. Some relays are reliable, some aren't.

That all sounds about right, but we'd need numbers, on a variety of relays, to quantify what's important.

John

Just keep adding parts until it works.

How many iterations is this?

John