Detecting a dry contact state

[Hawaka]

Hello,

A circuit I am designing needs to detect the status of a dry contact while being isolated from the circuit.

On my side, I have a connector for the user where he can connect a cable (typ. 5m) with a button that will short the two conductors. Maximum voltage on the connector is 24V, max current through the button <1mA. I cannot change this.

The connector needs to be isolated from the rest of the circuit as the the voltage can reach 700V above the connector (both signal of the button needs to be isolated, no common ground). I am currently using a resistor chain as insulation, and the feedback is done through an opto-coupler.

To improve the design and make it more reusable I was thinking the following: using a small transformer, a square wave is generate on the primary, and the secondary would be floating on the user connector. When the user push the button, the increase in current can be detected on the primary. For cost reduction, the transformer could be done with just trace on different layer of the PCB.

Solution shall be <2$, about 1000pcs per year.

Is there already an IC that does this? Or an easy way to detect a dry contact?

[Manul]

I would say that your transformer idea is nice, but:

max current through the button <1mA.

It might get tricky to detected small currents at the primary. I would also not recommend using very low voltage, cause mechanical contacts require both wetting voltage and wetting current in order to work reliably.

[Hawaka]

Good point for detecting the small current.

The button is a simplification, i think it will probably be the output of an optocoupler.

[Krampmeier]

The Panasonic APV1111GVY can supply typical 45 µA into the isolated side. Its photovoltaic output could be connected series with a really sensitive optocoupler like the 6N138M for feedback, with the external switch closing the connection.

[Zero999]

A transformer is a reasonable solution. If the switch can be an opto-coulpler, then you need a bridge rectifier on the secondary, so the output is DC only.

Do you have a microcontroller?

Apply a square wave to the pulse transformer via a resistor and capacitor. When the transformer is open circuit, then the square wave will be on the other side of the resistor, which is connected to an input. When the transformer is short circuited, the square wave's amplitude will be greatly reduced.

[zapta]

Will this radiate noise on the 5 meter wire?

[Zero999]

Will this radiate noise on the 5 meter wire?  Especially without the bridge and capacitor.

Definitely include the bridge rectifier.

Add one or more Y capacitors between the input ground and C2 to reduce the noise.

[zapta]

One option is to use isolated dc/dc to transfer power to the isolated side and then use an opto coupler, isolated amplifier, etc.

https://www.digikey.com/en/products/detail/gaptec-electronic/1S4E-0505S1-5U/13691708

[Hawaka]

I think the most cost effective solution is the photo-voltaic opto-coupler. The footprint is also quite small.

The transformer might be nice, but with the very limited button current I’m not sure detection will be easy.

DCDC are quite expensive and hard to find the insulation I require.

[floobydust]

I think PV opto-couplers are too expensive and the current available for the switch contacts is too tiny to work reliably.

If you want over 700V isolation and good wetting current and voltage, the transformer approach I think is the way to go.
You can use 10/100 Ethernet, SPDIF, DC-DC transformers etc. - but make sure the safety approvals are there.

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