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substitute for mechanical relay

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geboy

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I have got a circuit with a mechanical relay. The mechanical relay used is a normally closed one. The problem is that I need to replace the relay with a some solid state device which behaves very much similar to this mechanical relay. That is, even when the circuit is not powered by any supply, since the mechanical relay is a normally closed one, the solid state device should act as a normally closed one. for example, consider a p channel mosfet. The mosfet is a normally closed one if Vgs=0 and when there is some Vds. In my case I need a solid state device which acts as normally closed even with no Vds and no Vgs
 
the current is DC. But I still got doubt about NC solid state relays. Will it work as NC with no voltage. I mean if we use a multimeter to check continuity will it show as short circuited as mechanical relay?
 
I didn't notice any high-voltage DC SSRs, but you can try the search and filters on the element14 website. Or perhaps digikey has some.
 
The NC SSR seem pretty rare. Are you allowed to include a small lithium cell to power the SSR? It should last >10 years.
 
no. I am not allowed to use a battery. But the ratings specified can be amended. Can I use a n channel depletion mosfet with opto isolator as a substitute?
 
no. I am not allowed to use a battery. But the ratings specified can be amended. Can I use a n channel depletion mosfet with opto isolator as a substitute?

hi,
How is the present relay being controlled in the existing circuit.?
 
hai i have uploaded a jpg image showing the position of the relay. The application is actually the field coil of a generator. The relay is shown as a switch in the diagram and the field coil is shown as an inductor. As the rotor rotates the generator produces a small voltage due to residual magnetism in the field which is regulated and supplied to field coil and thus the magnetism is strengthened and in this way the generator produces its rated current and voltage. for this process to work the relay must be short circuited so that the field coil is connected to ground for free flow of current. Mechanical relay is used for this purpose, but now a solid state device is needed. I need to find a solution
relay.jpg
 
Digikey depletion mode mosfets: https://www.digikey.com/product-sea...ee=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25
Digikey isoPower driver: https://www.digikey.com/product-search/en?x=-1147&y=-71&lang=en&site=us&KeyWords=adum5240

Datasheet for first 200V/16A fet in list: https://www.electro-tech-online.com/custompdfs/2013/05/DS100260AIXTH-T16N20D2.pdf

If you use both channels of the isoPower IC to drive the gate-source, you can have +/-5V to control the gate. This means 1 Gigaohm in the off position, 40 milliohms in the on position, and <1ohm when unpowered (these are rough estimates based on Fig 2 of the datasheet).
 
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