555 relay current reducer

I think so, because ultimately they operate the embedded S-R flip flop. Once the input goes low, the flop 'flips' with no possibility of flipping back, as long as the other terminal has a valid logic level (which it does here). So I don't see any problem with metastability, slow edges, etc.

They're not mine though, and I've never used one. Those are from David Johnson's site. He uses them for all sorts of clever micropower circuits.

Pulling PR* and CLR* low at the same time makes Q and Q* both high, e.g., on the 7474 IIRC.

But I'm surprised if an RC delay to PR* or CLR* didn't work. The CMOS parts are buffered, so it'd be a lot harder or maybe impossible to get an HCMOS flop's guts linear (and acting ill-logically, so to speak).

I appreciate and second your concern in general. However, John didn't say "I guarantee," he said that most relays are guaranteed.

That's very clear to me that he means it's a possibility that any given relay--even most--may have a reduced holding current that is guaranteed by its manufacturer.

The fact that he said "most" is more than enough qualification for me, a flaming flag that some types may _not_ guarantee, etc.

I'd _always_ check anyhow, whenever doing something tricky (even though I've never seen a relay that wouldn't hold at half current).

Well, if up to 50% off is good, it's even better if there's more!

:-)

Cheers, James Arthur

All that is true, and widely known. Any reduction in holding current will always result in a reduction in holding force.

So if shock and vibration are a possibility, reducing holding current may not fly. Strong enough jolts could unlatch a mag-latching relay, for that matter.

OTOH, mobile devices might not permit reduced holding current, but fixed locations might like it fine.

It's really pretty amazing what a wide range of applications exists, and how specialized the optimal solution for each one is. That is, there is no universal 'electrically-operated' switch, AC/DC, micro-to-megawatt, etc.

Cheers, James Arthur

Here's a David Johnson example...

I *think* this works, but no guarantees. More discretes than I'd hoped.

Vdd .---|--|D Q|-- | .--|D Q|----+-----. )|| | | | | | | | _)|| ON+ >-+----|> | | +--|> | | Q1 | | | Q*|----+ | | Q*|----. | ||--+---. | | CLR | | === | CLR | | | ||

that is true at all coil v&i. Whether it's a problem in the product depends .

it depends. On the size & nature of the shock, the springiness of the relay mechanism, and at what voltage the relay actually does pull in. And whethe r the drive circuit notices & corrects. One possible way to implement reduc ed current is to supply full voltage when coil energised but contacts not y et closed, and reduced power when they are closed, effectively using the re lay to provide its own drive delay. It can reduce component count when it's workable.

NT

John Fields

Oh pshaw, I'm not pretending anything. I'm just another designer who's used a few relays. I looked into PWM'ing coils in 1985 for a low-power product that used a coupla handfuls of 'em.

You echoed other's cautions that lowering holding current reduces holding force, I pointed out it might not always matter. Usually doesn't, IME.

My most recent relay experience was dabbling with a solid-state replacement, designed for my car (posted here). But after vetting a pile of automotive relay datasheets for that, the electro-mechanical beasts are still pretty hard to beat for toughness, resistance, and value.

Cheers, James Arthur

Sounds indeed like nonsense to me. As long as the metal part that moves the contact is tacked onto the coil's core, the contact force is sufficient and as it should be. As soon as the metal part gets 'untacked', the magnetic force already decreases and it will just release the contacts. What matters is whether the relay is holding, not with how much force it does that.

joe

Fields, like Sloman, only posts to s.e.d. to deliver coarse, ritual, droning insults at an increasing circle of targets. Neither makes technical contributions, or seems interested in the topic. They are bitter at the world and everyone in it, and I bet they whine in a lot of other venues beyond this one. Everybody gets old, but some people have enough intelligence and guts to do it gracefully.

I love it, bunch of old farts gumming it!

Jamie

I already did. At lower holding current, the relay can drop out if subjected to shock. Probably very critical in a car, probably not at all critical in most fixed installations--they don't bounce.

My point was that, after recently examining the state of the art in both solid state and mechanical relays, the electro-mechanical beasts are still pretty hard to beat for toughness, contact resistance, and value.

Cheers, James Arthur