How does the back-to-back FET circuit turns on to close the relay output with zero photons and no input current? I have never seen a FET circuit that can be turned on with zero input energy.

An example part: TLP4176A

Thank you,

Brian

 

How does the back-to-back FET circuit turns on to close the relay output with zero photons and no input current? I have never seen a FET circuit that can be turned on with zero input energy.

An example part: TLP4176A

Thank you,

Brian

Neither have I seen an FET CIRCUIT that is switched on without any energy input. But that is not this device. It requires current to light the LED in the isolator and VOLTAGE to bias the MOSFET photo-switches.

 

Neither have I seen an FET CIRCUIT that is switched on without any energy input. But that is not this device. It requires current to light the LED in the isolator and VOLTAGE to bias the MOSFET photo-switches.

I am using it so I can test it anyway we want to.
The FETs have no gate input to apply any kind of bias voltage, and if you meant the Drain to Source voltage to bias it, that I don't think VDS has anything to do with its on/off states.

With no power applied to the circuit, I measured and the output pins are shorted as spec. That's what they mean N.C relay.

 

I measured and the output pins are shorted as spec.

Then you have faulty device.
With no input the output should be open (high resistance).

 

Then you have faulty device.
With no input the output should be open (high resistance).

Not if the package has a mechanical relay output.

 

Not if the package has a mechanical relay output.

The package is a opto isolator with a MOSFET output.
How did a mechanical relay get into the discussion?

 

Then you have faulty device.
With no input the output should be open (high resistance).

I am lost here. Is this related to NC optical relay?

 

With no input the output should be open (high resistance).

Are you aware that we're talking about a Normally-Closed (NC) relay? It means with no input current the relay output pins are closed as it's designed. Not a faulty device.

From datasheet:

 

From datasheet:

What data sheet?

 

Then you have faulty device.
With no input the output should be open (high resistance).

Only Depletion Mode conduct with 0 bias voltage.
But these are Enhancement Mode.

Both the descriptive words and the timing diagram show as Normally closed.



This is a puzzle to me too.

https://www.mouser.com/datasheet/2/408/TLP4176A_datasheet_en_20211215-1900581.pdf

 

What data sheet?

TLP4176A datasheet

 

This is a puzzle to me too.

I'm glad someone is also interested in such a simple relay. I started using this relay about 6 months ago and I did some google search on the question but got nothing close to what I'm looking. It's almost like they must added a reed relay to the output for this but it's in such a small package so I don't know.

 

As tonySteward wrote, the device uses Depletion mode mosfets and the mosfets conduct between the source and drain with zero bias (no light). The mosfet symbol also indicates depletion mode devices because the mosfet symbols vertical bar from source to drain is a solid line as opposed to a broken line.

 

The package is a opto isolator with a MOSFET output.
How did a mechanical relay get into the discussion?

OK, I was thinking about a proximity switch. My error. BUT there do exist MEMS (Micro ElectroMechanical Switches.) So an actual relay is possible, but not likely.
But consider that most schemes to check for resistance or continuity apply some voltage so that some detected current will flow.
AND, in most cases the circuit shown in in the data sheet is a functional equivalent and not the exact internal circuit. There are exceptions, but not many of them. So what shows in this data sheet is probably a major simplification of what is actually inside the device.

 

I remember the terminologies of Depletion and Enhancement FET, but have never used depletion FET in my 40 yrs of motor servo loop circuit design or switching power supply design. Learned a new thing today. Thanks for all the responses.

P.S. Is there an equivalent device -- normally conducting -- in the transistor NPN and PNP world?

Brian