Device to Provide Dry Contact on Power-Down (Normally Closed)

[KeepItSimpleStupid]

I am not an aircon guy nor want to play one on TV. Points well taken on your suggestions.
They make load shed modules for prioritizing 4 loads (thermostat dry contacts) but they allow more than one unit to come on based on the watt load on the generator. They also make Power Management module that does similar thing.

I caught that.

I wasn't trying to play thermostat either, but I did have the training to size AC's
If you had two AC's. One for up and one for downstairs, load sheding based on time might give you odd results, comfort wise.
Your idea is better than nothing. I think, in some cases, AC is turned off when the generator kicks in.

 

[EvilGenius]

I caught that.

I wasn't trying to play thermostat either, but I did have the training to size AC's
If you had two AC's. One for up and one for downstairs, load sheding based on time might give you odd results, comfort wise.
Your idea is better than nothing. I think, in some cases, AC is turned off when the generator kicks in.

I am a certified genset guy though. Out in the field we either up-size the gen to meet the load need (AC's) or load shed them based on priority (uneven distribution of time on each AC). In some cases priority one would not allow any other AC's come on because there is a background amp in the house and AC1 is hugging the action.
We overcome the start-up amps with a soft-start. But still on lower power gensets, one AC is all you can manage with everything else running in the background. This device will allow you to have more than one AC (instead of locking it out), and allows time sharing of multiple AC's. I did a field test with ice-cubes and it works like a champ with a manual select switch!

 

[EvilGenius]

See digikey and search for optomos or photomos. there should be hits for some photo led relays with NC and NO contacts.

We discussed these devices in previous posts. I have to read their datasheet more carefully.
What powers the mos part of the optomos (normally closed) and keep it in normally closed position with no power to it? That is the ultimate questions. If it is a passive normally close (low resistance gate) then we are fine. But if it is an active device that requires some sort of power to keep the gate in low resistance (short) then during down-power it might act as a high resistance gate (open) which defeats the default condition of keeping Enable closed.

 

[KeepItSimpleStupid]

e.g. 1 Amp Normally Closed. No power to do that. https://www.ixysic.com/home/pdfs.nsf/www/LCB710.pdf/$file/LCB710.pdf

 

[tomizett]

What powers the mos part of the optomos (normally closed) and keep it in normally closed position with no power to it?

As I described in #16, they use a depletion mode FET. This works like a valve (tube) in that it will conduct with zero gate voltage and requires you to "suck out" the charge carriers by applying a bias to the gate to *stop* it conducting - so no power is required to keep the "contacts closed".

 

[EvilGenius]

As I described in #16, they use a depletion mode FET. This works like a valve (tube) in that it will conduct with zero gate voltage and requires you to "suck out" the charge carriers by applying a bias to the gate to *stop* it conducting - so no power is required to keep the "contacts closed".

I read your post above and post #16, did some study on the thermostat wires and measured volt and amp draw by thermostat.
Common use of the thermostat enable control is done with the red wire. This is typically 24VAC drawing less than 1A from the thermostat transformer to its controller. Occasionally this transformer is not easily accessible and the control wire is the hot line of the transformer (120VAC). In both of these cases a wimpy DIP SSR is not going to handle it, nor will a normally closed cmos output. The reed relay is also out since I need the normally closed contacts for this project.
So my options are now a high powered solid state relay or a small package relay that is properly rated for 120-250VAC 2-3A to be safe. And I also have to add on a disconnect switch and a 1A fuse inline with the on-board power converter to feed the board.
First prototype is designed and sending it to China to be made. (2.5" x 4.5")

 

[tomizett]

Sounds like you would be best with a conventional relay then... You can get some that are pretty tiny (same kind of size as a reed) that will handle an amp or so. Surface mount, too, if you need it.

 

[EvilGenius]

Sounds like you would be best with a conventional relay then... You can get some that are pretty tiny (same kind of size as a reed) that will handle an amp or so. Surface mount, too, if you need it.

Hi Tom
That is exactly the route I took with tiny relays. First set of PCB's are built. Now prototyping and then final testing.
Thanks everyone for your input.
Cheers,
Rom

 

[dknguyen]

You could use a JFET for solid-state (or any other depletion mode semiconductor device) where they are on by default (versus most devices such as MOSFETs which are enhancement mode where they are off by default).

They are a bit trickier to source and are usually used for analog signal processing but if you can find a suitable one with a sufficient voltage rating and resistance find them you might want to give them a shot.