Solid State Relays

[john.b]

Hi all, can anyone tell me if you can buy a solid state relay that works in the opposite way, what i mean is when the ssr has no dc power connected to the input side the output terminals are not connected no flow but when dc power is connected to the input side the output side is connected (electricity flows) what i'm wanting is the opposite, when i have dc power to the input side i want no flow then when the dc power is removed i want flow.

john

[bcroe]

when I have dc power to the input side I want no flow,
then when the dc power is removed I want flow.

What are the power requirements for the output, voltage, current, AC or DC? Bruce Roe

[bob-n]

High-power SSRs with inverting inputs are rare but they do exist. For example: https://www.digikey.com/product-deta...1089-ND/221782 Sensata Crydom D2425-B is a 25A normally-closed SSR rated to 280V AC. Check out the data sheet for details. Another option is to use two SSRs, a small one to invert the input and a larger one to switch the signal itself. But that takes a bit more thinking and wiring.

[john.b]

Hi Bruce, output power is DC (small wind turbine) 500w 24v the SSR i have is 24v-220vdc 40A it may be i need something bigger but just wanted to know if it's possible to have one work in the opposite direction.
What i'm trying to do if possible is grid tie the turbine and use the SSR for when the grid power drop out to stop the turbine spinning up, the SSR would be permanently powered by a dc power supply which would plug into the mains then if the grid drops out the dc power supply would also drop out and the SSR would default and redirect the turbine power to a dump to stop it spinning up.
So when the SSR is under power i need it to be open (no flow) rather than closed like the one i have.
This setup may not be possible i was just trying to think of how i could do what i need to do.

john

[bob-n]

I don't know of an SSR that does exactly what you need, but it can be done, and may be on the market. Check distributors like alliedelec.com, digikey.com, and newark.com.

SSRs come in two varieties: AC switching and DC switching. It sounds like you need a DC switching SSR. The one I mentioned earlier is only for AC use.

As I suggested, another approach is two SSRs where one turns off the drive to the second when it gets power. That's very simple to hook up, depending on your exact situation. One of us here can give you a diagram when you are ready to build.

A third approach is a traditional mechanical relay (also called a contactor). These are good for DC and available both normally open (like your SSR) and normally closed (opens when power is applied).

As you shop around for SSRs and relays, look for the phrases "Form B", "Form C", "Normally Closed" or "NC". Form B means one set of contacts that is normally closed and opens when power is applied. Form C means two sets of contacts, one that is normally open and one that is normally closed. You wouldn't use the normally open contacts.

[bcroe]

I woud be inclined to power a relay from the line, could have a line voltage coil
or rectify some line for a DC relay coil. Use a NC contact to short the turbine.
Bruce Roe

[gbynum]

Hi all, can anyone tell me if you can buy a solid state relay that works in the opposite way, what i mean is when the ssr has no dc power connected to the input side the output terminals are not connected no flow but when dc power is connected to the input side the output side is connected (electricity flows) what i'm wanting is the opposite, when i have dc power to the input side i want no flow then when the dc power is removed i want flow.

john

One trick I've used is to connect a positive control signal through a resistor, then take the input as opposite polarity, same level, same value resistor. In op-amp circuits it is called resistive summing.

[john.b]

Hi guys, many thanks for the link and info much appreciated, i'll have a dig on the net and see what i can find

john

[john.b]

Hi Guys, just a quick update and another thank you for all the help, it turns out the grid tie inverter i have my eye on will transfer the power to a resisted dump load as soon as the grid drops out so there will be no need for me to set up a ssr or relay so all should be ok but again i thank you for your info and help
Take care
john