I'm looking for a solid state relay that has an input of 240V a/c and that can switch DC on the output side ie switch 12v DC to ground. I have been using the old style relays (Omron MY2NJ) in the past but these dont seem to last to long. So far I can only find solid state ones that have A/c on the out put side.
I think you need to specify what current you want to switch and whether or not it is an inductive load before any sensible advice it possible.
One company I knew who changed to using solid state relays inadvisedly managed to have their kit catch fire with monotonous regularity.
You should be asking why are the classical relays are failing first.
Allow me to clarify. The current I want to switch is negligible because CCTV camera works on only 12V DC. I want to switch the cameras continuous 12v D/C output signal to ground by having the relay switch from NO to NC. The relay on the other side will be activated by a 240volt a/c input. So a 240v input will switch a 12v D/c to ground
Er, that's a non-sequitur, or even worse. If it's "only 12v" then the current you need to switch is going to be a *lot* more than the current you would need to switch if the device was powered by a 240v supply, to be exact about 20 times as much current!
Not to mention that switching DC is harder on contacts than switching AC.
That makes no sense at all. Are you switching the *signal* from the camera or the 12 volt power to the camera?
I *think* I understand that you want the relay to be operated by a
240v supply, not unusual/impossible but 240v relays are not the norm.Chris Green laid this down on his screen :
A suitable diode across the contacts will help.
The "matchbox" sized solid state relays are readily available for mains or LV operation.
first.
Say 5 W for the camera, half an amp. Any IR LEDs in the camera sharing this supply? They could easyly double or treble that 5 W load. So 1 A or more is quite possible and not "negligable".
What do you mean by "12v D/C output signal"? Is it an auxillary 12 V power source for other kit or, as "signal" implies, a 12 V output that is set by events in the camera (motion detect or WHY).
Shorting a power supply is not normally a good idea and has a tendancy to release the magic smoke out of the power source? Shorting a PSU through relay the contacts could be why the relays are failing...
If it is a signal *output* why do you want to connect it to ground? A signal *input* I could understand to tell the camera about some external event.
"switch from Normally Open to Normally Closed." Mutally exclusive. I think you mean you want a Normally Open contact pair that closes when the relay coil is energised.
Still worried about this 12 V being connected to ground...
ITYM to reduce back EMF one would use a diode reverse-biased across the coil of the relay. To stop sparking across contacts a suitable capacitor would be a better bet.
How often does it operate? I've got 240v relays working here that are 40 years old.
These are referred to as "snubber circuits" and can be tuned for good results. The people in sci.electronics know how to do that. There was at least one guy over there, who wrote a textbook of electronic design. A kind of "resident perfessor".
And you "tune" the snubber to the thing to be snubbed. It's not a random thing. Someone who knows what they're doing can reduce the ringback to almost nothing. If the characteristics of the load are not known with any precision, you can breadboard the circuit and change values until it's tame. Snubbing a relay coil should be a slam-dunk, as the relay composition will not vary too much from unit to unit.
Paul
Really you can pretty much BUILD that out of a mosfet, and some high voltage diodes....
You haven't made any sense at all. Why do you want to short 12v to ground?
That is sure to degrade both the relay and the camera.
Some of the answers the OP has already given suggest that he really doesn't know enough to start messing around with diodes or other circuit components (no criticism of him intended).
newshound used his keyboard to write :
It's to switch DC.
It would take a true optimist to guarantee a solid state relay will have a longer life than a traditional one.
If mechanical relays are failing it suggests that shorting a 12V output to ground is not a sensible thing to do. I'm not sure what the OP is attempting to achieve by shorting out a 12V supply rather than just switching it off.
I think he doesn't mean switching an zero impedance 12v supply to ground, but switching a 12 v supply via a load, to ground
anyway this sort of thing works
Or even for very low currents this works...
Apologies for not explaining the electrical details in a professional manner and maybe I should have found a forum for CCTV matters. I'm not an electrician but an elderly and pretty capable DIYer.
As I said earlier I have been using old style (coil) relays specifically the MY2NJ type and they have worked with the 5 cameras I have. I use a 240v PIR sensor that sends 240v to the relay when it is activated. The relay, when activated, makes or breaks contact in the camera. This triggers the camera to start recording. Here is an extract from the manual.......... """Trigger This is the cause or stimulus which defines when to trigger the Network Camera. The trigger source can be external digital input devices. This option allows the Network Camera to use an external digital input device or sensor as a trigger source. Depending on your application, there are many choices with digital input devices on the market which help detect changes in temperature, vibration, sound, light,"""
is this volt-free contact on the camera, or a 12V input to the camera?
if the latter, you could feed the 240V to a 12V DC PSU, and use that to trigger the camera
All the mains coil relays that RS supply, can switch DC as well as AC
If that relay is a specified one, it sounds like they've got it wrong.
Go to CPC etc and buy one of the spec you need - perhaps a higher current rating.
And if the contacts are giving trouble, add a diode across them (or buy one with it built in) An inductive load at DC can shorten the life of marginal contacts.
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