Latching relays

Why use a latching relay, if you plan on keeping it energized?

Thanks for the answer, even if it doesn't answer my question.

It's very simple, because I want a stable condition when the power is cut off. Latching relays guarantees that the contacts don't change their position when the mains power goes down.

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My point was, there is no reason to continue to power a latching relay. Look at the current draw, vs a non latching version. It takes more power to set or reset a mechanical latch, It doesn't matter if the relay is single or double coil. Why waste power and heat up the relay? That will shorten it s useful life.

I am currently designing a replacement 'A23' Attenuator board for the HP

3325A/B function generators. The original relays are long obsolete so rathe r than making adapter boards, I am designing a smaller board that uses mode rn relays. It will also allow you to select the supply voltage for the rel ays, to allow the use of more common voltages.

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If you're getting a bunch made, I'd chip in for three of them. That's the o nly thing that seems to go bad on those things. So far Deoxit has always fi xed it, but I expect it's just a matter of time.

Cheers

Phil Hobbs

On Tuesday, September 24, 2019 at 9:00:06 AM UTC-4, snipped-for-privacy@gmail.com wrote :

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only thing that seems to go bad on those things. So far Deoxit has always fixed it, but I expect it's just a matter of time.

I need at least three for myself. I was looking at making ten boards, wi th a choice of Phono or SMA connectors. If you have a failed board you can reuse the resistors, but I am planning to use SMD for complete boards. I ma y also make a replacement for the HV output board, and the 10 MHz OCXO as w ell. I have a pile of them that are missing one or more of those three boar ds.

I have two reasons.

The first is the software that must be more complex to generate an impulse instead of setting/resetting an output pin level forever.

I know, it's simple to create a pulse too with a blocking delay:

activate_coil1(); delay_ms(10); deactivate_coil1();

however in my software, that is cooperative multitasking, I can't block for more than 100usec. I need to implement a small state-machine to generate a pulse.

The other reason is: how long should be the impulse? Datasheet says maximum 3ms, just to be sure I would generate a 10ms pulse. Could I be sure the relay has switching after my pulse? I don't have any feedback from relay.

In my application it's much more important to be sure that the relay has switched instead of saving some power.

8.3mA vs 12.5mA

It's a pity the datasheet says nothing about expected life vs coil activation time.

Look at 'Pulse stretcher' circuits.

Either you trust the relay, or you find another way to design the item. If the datasheet says the maximum is 3ms, you don't need 10ms.

Implement the timing in the driver circuit. How cay you trigger the second coil. if you are still powering the first coil?

Looks OK to me. The coil power is not excessive and the data sheet doesn't restrict on time.

We like to blip latching relays to reduce thermoelectric offsets.

I don't think that keeping one side powered forever will work.

Cut and paste from the Teledyne RS-170 bistable relay data sheet:

%< ----- %< ----- %< ----- %< ----- %< ----- %< ----- %< ----- Since operation depends upon cancella- tion of a magnetic field, it is necessary to apply the correct polarity to the relay coils as indicated on the relay schematic. When latching relays are installed in equipment, the latch and reset coils should not be pulsed simultaneously. Coils should not be pulsed with less than the nominal coil volt- age and the pulse width should be a minimum of three times the specified operate time of the relay. If these conditions are not followed, it is possible for the relay to be in the magnetic neutral position. %< ----- %< ----- %< ----- %< ----- %< ----- %< ----- %< -----

... etc. I can't paste the graphics on usenet.

regards, Gerhard

Am 24.09.19 um 18:41 schrieb Gerhard Hoffmann:

RF-170, not RS-170

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Am 24.09.19 um 18:47 schrieb Gerhard Hoffmann:

The Mouser web site points into nirvana.

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(Ingrid was someone in the German sub-internet. You could see her thinking in endless series of posts.)

Oh good, you brought it up! I read this on the data sheet, "Low thermal electromotive force(approx.

5 ?V)" Where is this developed? is it between the contacts? Is it heat from the coil getting to the contact arms?

Mikek

From the application point of view, it looks like between the contacts.

In reality, there are thermal gradients and dissimilar metal junctions in multiple places and the value indicated in the data sheet should be a sum total for the relay as a whole.

There is likely a bimetal junction at each contact: the arms are made from a flexible material and the contact points are made from another, typically softer non-oxidizing material (and sometimes plated too).

Since the arms tend to be at unequal temperatures (one arm is nearer to the coil and the thermal interface between arms is poor because they can only touch at a tiny spot), each touching contact pair ends up being a reverse-series of 2 thermocouples at different temperatures.

There are also other pairs of bimetal interfaces at the solder joints between the relay and the circuit board. The relay partially heatsinks itself through its pins, and since the pins have unequal values of thermal conductivity from the heat source, they end up at different temperatures at the solder joints too.

Also, the arms might not be made from one piece of metal internally.

The method of mounting can also make a difference, such as soldering one pin into a large plane area and another to a tiny trace. This will heatsink the pins at different thermal conductivities to the ambient, and the result can drive a temperature difference between the joints.

Regards, Dimitrij

An energized coil gets hot and makes thermal gradients everywhere. Thermoelectrics can happen at the contacts, all the internal conductors and hinges and stuff, leads, and solder joints to the PC board. Small telecom type relays can make 10s of microvolts of offset.

This board uses one dual NOR gate per relay. There are lots of relays+nor gates arranged in a matrix. We set or clear 8 at a time.

HP wants the RF relay to be electromagnetically dead when it passes signals to avoid low level nonlinear distortion that may result. But that will be the least of your challenges in a fitting a new board, depending on how much of an instrument you want.

Bit bloody easier to use a modern approach, like

Clifford Heath.

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r to set or reset a mechanical latch, It doesn't matter if the relay is sin gle or double coil.

life.

he HP3325A/B function generators. The original relays are long obsolete so rather than making adapter boards, I am designing a smaller board that uses modern relays. It will also allow you to select the supply voltage for the relays, to allow the use of more common voltages.

Ummmm, this operation is very strange. What CPU are you using? Almost all of them have a timer capability. Write a '1' to the DO port associated wit h the relay and at the same time, set time to count down x ms. The ISR as sociated with the timer can write a '0' to the DO port. Doesn't matter if you have an RTOS, a roll your own task scheduler, or a cyclic exec or a big loop. or, make the entire system event driven. No hw timer on the cpu? u se the timer or RTC function associated with the RTOS. Busy-waits are inef ficient - there is almost no reason to use them. Also, a loop with a gazill ion no-ops to implement time related events makes for non-portable code. Sorry, but your reasons are invalid.

No feedback from the relay? So this is a fundamental design issue. If it is imperative that you know the state of the relay, then feedback should ha ve been incorporated into the design. The relay state could be run into a bit on the interrupt register. Can the state be inferred from some other state variable? Connect a 555 as a monstable one shot to do the pulse function. Don't *have* to know the relay state, then energize for 2x what ever the da ta sheet says, 'just to be sure.' Since you have a multi tasking OS, run a thread to turn the relay on when n ecessary. Make the worst case execution time = period = 2x the data sh eet pulse width. Create a on/off state machine in the thread that services the relay. Make the thread dispatch based on a relay_on event. Do schedu alability analysis to make sure all the threads are scheduable with the inc lusion of a sporadic event thread. Check to make sure priority inversion ca n't happen. Any embedded micro designed in the last 30 years most always has a hw timer . Good luck J

I didn't say I can't implement a software that generates a pulse, I said that this implementation is more complex compared to set/reset a pin. I know how to code a software that generates a pulse, however I like to keep the code simple if possible (bugs are everywhere).

This is a 'Who's on first. joke, in reverse.

You can only guarantee a powerless logic state in hardware by starting your design with that physical state. Even then, you can't count on such a configuration to provide user safety under single fault conditions.

If you want to 'control' with no power, you need a second power source, even if it's only an unlatched spring.

Latching relays sole function is as memory of their last controlled power state, so you can't use them for a fail-safe.

If you want a safe power-off, you need physical guards and a dedicated kill switch that provides that feature.

RL