solid state replacement for NC SPST relay?

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Hi there - I'm looking for the functionality of a normally closed, single pole single throw (SPST) relay.

Now - I know of the existence of SSRs. However, I think they may all be NO. See - I need this relay to be closed when this circuit is not powered, and then I need it to open when the circuit is powered. As in - there will be no power available on this board when the relay needs to be closed.

A normal SPST NC relay would work great - but I don't want to use one due to the size, weight, driving current, vibration sensitivity, and cost of the part.

If it matters, current will only flow in one direction through this part.

Anybody know of any component with these properties? I can't think of anything.

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If your SSR can bleed some power when your AVR is switched off, you only need to supply a passive pull-up resistor. The AVR would then switch it off with an active-low output.

Otherwise you are stuck with a mechanical C/O or NC contact set. You can get two-state relays, that are just powered to switch state. Neither is satisfactory in high vibration surroundings.

What current / voltage do you want to switch ?

David.

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Crydom DO/DMO series SSRs offer n.c. function.

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david.prentice wrote:
If your SSR can bleed some power when your AVR is switched off, you only need to supply a passive pull-up resistor. The AVR would then switch it off with an active-low output.

Otherwise you are stuck with a mechanical C/O or NC contact set. You can get two-state relays, that are just powered to switch state. Neither is satisfactory in high vibration surroundings.

What current / voltage do you want to switch ?

David.


There's nothing to pull up to though when the system is off.

However, a colleague suggested something that I had forgotten: using a depletion mode FET. I believe that should do it.

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Can you use a DC solidstate relay in series to source juice to a load? If this trick works, then can I run the juice both ways once its on? In other words, does it really work like a metal relay? (heaven forbid I should actually do an experiment to answer this question myself)

Imagecraft compiler user

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kfaria wrote:
Crydom DO/DMO series SSRs offer n.c. function.

You're right - I wonder how that works... Maybe depletion mode FETs?

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bobgardner wrote:
Can you use a DC solidstate relay in series to source juice to a load? If this trick works, then can I run the juice both ways once its on? In other words, does it really work like a metal relay? (heaven forbid I should actually do an experiment to answer this question myself)

A lot of them allow bidirectional current. I believe it's typically done with two back to back mosfets. However, many don't offer the isolation that a true relay gives you.

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Quote:

I know of the existence of SSRs. However, I think they may all be NO.

" Results 1 - 10 of about 13,000 for ssr "normally closed"."
and the first page of results gives at least 6 pertinent hits.

If the power is off, what is being conducted through the NC contacts? :twisted:

One quote looks particularly interesting--it appears that you are looking for those that spec "Form B" contacts:
http://www.eeproductcenter.com/e...

Quote:
Clare Inc., a wholly owned subsidiary of IXYS Corp., offers a family of 4-pin solid-state relays (SSRs) as a replacement for reed relays. The 1-Form-A (single pole, normally open) and 1-Form-B (single pole, normally closed) SSRs in SIP packages are pin-for-pin compatible with reed relays.

Armed with that, a trip to DigiKey indicates 192 choices for SSR, NC (Form B). Given non-stock items and separate listings for bulk quantities it looks like about 100 selections just at DigiKey form about half a dozen different manufacturers.

Lee

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