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Normally closed solid state relay

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Sep 27, 2007
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normally closed relay

Need a SSR that is NC in the off state (to pass 10A AC) then opens with gate drive. This will be across a 1.5uF as a short, then short removed and the AC current will pass through the 1.5uF capacitor.

Is this possible ?

solid state relay

It would require a depletion-mode FET which are not
commonly available in high power, except maybe RF
exotica. No high power FET, no DC SSR (ACs are
often SCRs / TRIACs, which always need a trigger current,
often photocurrent based for isolation)

Question is, does it have to be NC with -absolutely no
energy input of any kind-, or does it just have to be
normally closed when there's something to pass?

Because if there is a supply or an input AC line to "sniff",
you can steal a little and make an inverter for the control
sense that also defaults to "on" in whatever conditions
(like lack of input control current / voltage).

A rechargeable battery backup might be able to give you
enough "hang time" in a cyclic application, but not an
indefinite-no-power, now-jump-up one.

solid state relay nc

The application is a relay driver, The short (S2) allows full pull in current, then about 200mS later the short is removed to limit the current to the solenoid.

So it has to be as "passive" as possible but I suppose a close/open sequence might work. The relay inrush is several amps though.

Below is a simple diagram to explain the preferred operation.

Thank for the response..

normaly closed state relay circuit

Here's an idea for you - what about splitting the full
wave bridge into two half-wave ones, give the 'noid
a freewheel diode and energize both switches to get
the full effective voltage (for pull-in) and then turn
one back off (for hold-in) getting about half the
applied voltage (sort of a crude PWM)? You could
lose the cap, that way.

dc solid state relay normally closed

That is an interesting approach. Would work except I need to cut the voltage way down to the solenoid. It's an AC winding but I'm banging it with DC, need to limit to about 3 Watts across the coil..

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