How to turn a NO relay into a NC

[rmeldo]

Hi all,

I am trying to control a relay with an 08M2. The intent is to power cycle a broadband router (12V, 1A power supply)

I would like to buy an off the shelf solid state relay because I haven't got time to make one and for other reasons I have no control over. Ideally I would like the relay to be normally close (NC) so that I can disconnect the Picaxe circuit for tinkering/maintenance and still have the router on.

However there is much less choice of NC relays than Normally open (NO)

Is there a way to setup a control circuit (using power from the load side, I an not worried about separating the two circuits) to turn a NO relay into a NC one? Intuitively this should be possible with a combination of a transistor and resistors but I couldn't find a fitting example on the internet or on the forum.

I am not good at thinking in "transsistor language" so any advice would be very welcome.

Thanks in advance
Riccardo

 

[techElder]

The real question is why do you want a solid-state relay to begin with?

There are plenty of hard contact relays available that will just sit there powering your router for decades until you want to operate the power cycle. Then they'll just sit there for more decades waiting for the next power cycle.

Easy.

 

[Jeremy Harris]

I agree wholeheartedly. Just use an off-the-shelf changeover relay and leave it de-energised so the NC contacts power the router. When you want to power cycle it, just activate the relay for a few seconds (some routers need the power off for up to 30 seconds to properly reset, some don't).

 

[sghioto]

If you must use a SSR this circuit should work but keep in mind the SSR is drawing a small current continuously. May be an issue if battery powered.

Steve G

 

[hippy]

It would seem easier to use a mechanical relay but I would guess you could probably use a resistor pull-up to a suitable supply to bias a NO solid state relay to keep it closed when the PICAXE circuit is disconnected.

 

[rmeldo]

Great thanks to all.

Changeover will do the job and it explains why there ate so few NC relays on offer on the market.

I had thought about SSR because of the non moving parts but as pointed out relays are very reliable anyway.

Thinking about it the application of SSR would be for fast switching?

Thanks also to sghioto for the schematics.

Riccardo

 

[erco]

How about a latching relay, which stays in the last position with power removed?

 

[cypherf0x]

How about a latching relay, which stays in the last position with power removed?

Why add complexity? There are loads of cheap SPDT relays that are exceptionally reliable. To reset the router just send a signal to the relay to open it from the NC terminal for a few seconds. There are ways to do latching with a MOSFET, BJT, IGBT, etc., but why sit there and use power when a simple cheap relay can just sit unpowered and do the same job using the NC terminal?

 

[rmeldo]

Sorry for the late reply.
My main concern regarding the latching relay is that if you lose count (i.e. an unespected reset, etc.) you don't know if your relay is close or open.
Riccardo

 

[fernando_g]

You could always use a depletion mode Mosfet.

Depletion mode are similar to Jfets, whereas the channel is always enhanced, and the gate voltage turns it off.
Because of its structure, DM-Mosfets can be made to switch high voltages and currents.

Check it out:
http://www.ixys.com/ProductPortfolio/PowerDevices.aspx

 

[cypherf0x]

Sorry for the late reply.
My main concern regarding the latching relay is that if you lose count (i.e. an unespected reset, etc.) you don't know if your relay is close or open.
Riccardo

What you're looking for is a SPDT relay. SP for single pole, one contact, and DT for dual throw meaning a NO and NC connection. I don't know why you would think NC relays are rare because it's more difficult to find a single throw relay than a dual throw one. They come with both connections and you just wire it to NC. When the relay is activated it will switch and open the contact.

 

[eggdweather]

But he's referring to latching relays where the concept of NC or NO does not exist unless you know what state their in.

 

[cypherf0x]

I'm saying a latching relay isn't needed. The object is to power cycle a router. A SPDT relay will do that wired to the NC connection. It meets all of the requirements initially stated.

 

[erco]

Agreed, a SPDT relay is the best choice. Seems like those are actually more common than SPST types anyway.

 

[fernando_g]

And many times, cheaper.............