Current to a Prop input?

T Chap

A device using an optofet switch requires that the current through the switch does not exceed 100mA. On a 5VDC 2amp supply, connected to the opto switch, then out of the switch into a 1k resistor to a Prop input pin, can someone please advise on how to calculate if this meets the requirements? I assume an input to be negligible, but wanted to see if there was any documentation.

Thanks.


The device specs are below:

Solid-state-relay (potential and polarity free)
Max. contact current: 100 mA
Max. contact voltage: 42 V AC/DC

tonyp12

A Prop input-pin have over 2meg Ohm, so if that it's the only thing you are signaling/controlling when you are using micro amps.
If you are using a 10k pull-up/down then you are using a tiny more.

But if you are giving the prop a 5v signal, then current through protection diode is another thing.
So use pull-up to 3.3V on that SSR side to signal a state to the Prop.

But a schematic and product number would be good in case what you are doing is misunderstand.

Duane C. Johnson

Please supply a circuit diagram of what you are planning on doing.

Duane J

kwinn

Based on your description I am assuming this is an Optoisolator with an IR LED as the control input and a fet switch that can switch an AC or DC current of 100 mA as the output. Diagram A in the attachment should be the circuit you describe, and the voltage/current calculations are included. Personally I prefer to use the circuit in diagram B as it is slightly less likely to damage the prop if there was a failure.

T Chap

Thanks for that info. The device is a motion sensor that uses a solid state relay as mentioned, they said it was an optofet. There is a pulldown on the prop pin of 220k plus a .1uf bypass cap.

kwinn

T Chap wrote: »

Thanks for that info. The device is a motion sensor that uses a solid state relay as mentioned, they said it was an optofet. There is a pulldown on the prop pin of 220k plus a .1uf bypass cap.

Does this mean that the switch portion of the device is connected to the prop? If that is the case then most likely one side would go to the prop pin/pulldown resistor and the other side to +3.3V so the pin goes high when the switch is closed.

The motion sensors I work with have a mechanical relay with Common/Normally Open/Normally Closed contacts so you can select either a closed or an open contact to indicate motion.

T Chap

On motion detected, the contact closes(solid state) and the prop sees 5vdc through a 1k. I could not find anything in the manual too see what an input would draw, obviously small and nowhere near 100milliamps.

jmg

T Chap wrote: »

On motion detected, the contact closes(solid state) and the prop sees 5vdc through a 1k. I could not find anything in the manual too see what an input would draw, obviously small and nowhere near 100milliamps.

It is a little unusual for a Motion detector to source 5V, if you are providing that 5V, consider providing 3.3V, or simply switching to ground.
The most common circuit is something like a 10k pullup, and a 10 k series ESD resistor, and the switch connects to GND.

Some designs use ~470R in series with GND, to again reduce ESD effects. Depends how long the wires are to the sensor.

If the sensor does source 5V/1K., you could consider your own opto coupler at the Prop end, for highest immunity.

kwinn

T Chap wrote: »

On motion detected, the contact closes(solid state) and the prop sees 5vdc through a 1k. I could not find anything in the manual too see what an input would draw, obviously small and nowhere near 100milliamps.

You definitely DO NOT want to put 5V on a propeller pin via a 1K resistor. That will most likely damage the prop. Use a 3.3K resistor to limit the current to a safe level.