Ripple harmful in this circuit?

No, as long as the transistor voltage rating is compatible with possible transients. Like switching the transformer primary off.

But it gets interesting when your sensor closes.

A filter cap of about 22uF is needed to reduce ripple to an usable value - you can find the minimum value by experiment.

The cap will also supress transients from the transformer when the AC supply is disconnected.

Best go for a sealed relay or the contacts may not work when you need them - depending on how many volts you are switching.

.... Phil

You might want to consider a diode from base to emitter to prevent reverse b-e breakdown.

If the transistor just closes a circuit, why not use a latching switch (an SCR) instead of a relay? Those are available in optoisolated versions, if the relay has to be isolated. Adding a capacitor or two won't hurt, and they DO routinely last decades.

It only has to operate once every 10 years at which point the whole thing will be replaced, but you need the flyback protection diode because....

I'd sure love to know what kind of application this requirement is for

I am mostly involved with project with 10 years of at least guarantied time,

I would be mostly concerned with critters entering the relay contacts rather than the transistor driver life time.

Where does the negative base voltage come from?

Ya, or moisture incursion, or repeated thermal expansion/contraction of the relay housing over temperature swings, or...

Sounds like the kind of project one would have a bit more budget on than average, it seems if OP is concerned with the lifetime of electrolytic filter caps (since apparently it won't be energized most of the time how much will electrolytics degrade if they're basically just experiencing a continuation of their 'shelf life'? idk really) they could just parallel up some MLCCs.

Ones in the single digit uF range aren't actually that expensive anymore and you'd get some inherent redundancy

Good point.

You mention the relay will only ever close once. Really so? Does it get tested during manufacture, during installation, etc? If yes, you might need that extra reverse diode across the relay contacts after all, to protect the npn.

On Sunday, July 23, 2017 at 8:15:32 PM UTC-7, Phil Allison wrote: ...

The diode across the relay may smooth the relay current enough to avoid any contact chatter - depends upon the inductance of the relay.

keivn

Your sketch shows the sensor contact switching 2-3mA but you write the relay coil only demands 25mA. Just wondering if you really need the transistor?

Have you checked the minimum switching voltage and current specs on the sensor contact - some need at least volts and milliamps to clear oxide films - especially critcial if the contacts are going to be open so long.

Seems to me if this contact closure is a one-time event you might actually prefer the contacts to weld on contact!

piglet

Excellent point!

All good points, piglet!

Actually, how about a 24VAC relay and remove the bridge, resistors, and transistor?

What happens if it fails? How many people will die?

Please ignore the troll.

AC relay coils usually draw big inrush currents.

Yes, but that may not be a show-stopper. Depends on the transformer and contact. Also, OP did not indicate how long the relay must be active after it triggers, only that it happens once.

A momentary switch from the base 10K (sensor side) to ground is used for testing the circuit.

Thanks, Ed