Small SCRs with 1mA or less in holding current?

There used to be some nice SCS made, that would work, but I bet they're long defunct.

The idea was to add R to your load to give a minimum additive current. This would only been noticed when the SCR is on with external load.

What is the highest V and I that is to be around this?

Jamie

I've used C106's back around the dawn of time- are they actually available in small SMT packages? The GE ones used to come in a horrible package with wide (0.053" max) stamped leads on 0.1" pitch eg. C106B1 but that's been replaced with some nicer packages.

Oh, and sensitivity goes hand-in-glove with dv/dt problems so high volume applications usually don't want too much sensitivity.

Best regards, Spehro Pefhany

Or learn to be more clever with your designs ?:-) ...Jim Thompson

That used to be a good reason for using SCS.

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Actually in this case dv/dt is even desired :-)

It's supposed to come on when a voltage rail comes on and then remain on, unless a uC deliberately "extinguishes" it but briefly shunting a FET across the SCR.

I just can't have several mA wasting away all day long.

That would deplete the battery. There are periods of time where the load draws very little and I can't have the SCR path burn off power that whole time.

V can be up to 60V and I should be

Yeah, if I have to ... thing is, most contract mfgs shy away from 0201 and then you have to use 0402.

For this sort of thing I almost always use networks rather than individual resistors. Considerably less space and less mounting cost at very little additional parts cost.

Best regards, Spehro Pefhany

That is a good idea. If I can find uses for the remaining resistors in there as pull-ups or something.

Probably I'll build some other self-latching gizmo for this one, looks like there really ain't no

We can't use 0201s (yet) but 0402 is the norm. We will soon be forced into

Less space? Lower production cost (when that's counted), sure. Routing can be a problem, though.

If it's "extinguished" does the rail stay on? This implies original triggering was due to the "desired" dv/dt?

Only shallow thinkers have "several mA wasting away all day long" ;-) ...Jim Thompson

T'is the idea, intentional dv/dt. But since the up-slope steepness is not guaranteed there will be an extra "regular" trigger. And the rail does stay on.

Since I won't that means I can't be a shallow thinker, I assume.

Specs? I know that's a naughty word around here. Minimum dv/dt required to trigger. Further definition of 'extra 'regular" trigger'? "Un"trigger specs?

;-) ...Jim Thompson

Can't rely on dv/dt of the rail coming up. It can be a milliseconds or, if contacts get dirty, tens of msec. My goal is that it shall work up to

So the extra trigger uses a capacitor and R feeding current into the trigger input for a second or more, to be on the safe side.

"Untrigger": A digital circuit shall be able to issue a pulse as short as 100usec which needs to open the SCR. This would be done via a FET that goes in parallel to the SCR with its DS path. Of course this must happen after the trigger current has petered out to below the trigger threshold.

Maybe I'll just build a poor man's flip-flop. ICs are pretty much out because of the 60V requirement.

These are the little things that make engineering fun :-)

Ok, to save on space, how about using a GTO ? ETO's would work but I find them mostly in large applications.

At least with a GTO, if you keep the gate voltage in the on state, it'll keep the device switched in at low currents. Unless you have a pulse transformer in there?

In any case, this could remove your need for the shunting fet and free up some space to maybe enhance the GTO operation.

Jamie

You can make the holding current as small as you want by injecting a bias current into the gate.

why not use a sol single package type RS flip flip to drive an open Drain higher voltage Nmos? It almost appears to me that you are doing a design that does not involve a uP ?

I've done such cheap designs using a 555 timer but only for its trigger and threshold latches to control a device. The trigger snapped it on and threshold input turned it off. It also works well for a ripple detector in circuits where rush currents or voltages need to be acted on.

Jamie

Well, I've got a design now. Just a transistor-based flip-flop. Logic chips are not so suitable for this function because of the 60V rail.