EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: pplaninsky on March 21, 2022, 08:17:58 pm
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Hello,
I am hacking a home automation product that drives a 230 VAC load via a relay.
The product has 2 PCBs - one is PSU + hot mains side + the relay, the other is a digital PCB connected via pin headers to the PSU board.
I want to have a visual clue for when the relay is on. For some reason the manufacturer has missed to incide such feature.
I don't want to mess with the mains voltage, so I thought of adding a LED to the GPIO of the MCU (EFM32 Gecko).
With 20K resistor to ground, the current through the LED is about 60-80 uA, which is enough to illuminate an efficient red LED.
I think this will not affect the switching of the power transistor, but wanted to double check and ask for a second opinion.
Please, check the circuit diagram below.
Thanks!
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Interference with the transistor is not to be expected.
But usually the uC should be able to source a couple of mA, so you could afford more current for your LED.
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Thank you!
I will try then 10k or 5k resistor.
I don’t need the LED too bright as well.
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Hello,
I am hacking a home automation product that drives a 230 VAC load via a relay.
The product has 2 PCBs - one is PSU + hot mains side + the relay, the other is a digital PCB connected via pin headers to the PSU board.
I want to have a visual clue for when the relay is on. For some reason the manufacturer has missed to incide such feature.
I don't want to mess with the mains voltage, so I thought of adding a LED to the GPIO of the MCU (EFM32 Gecko).
With 20K resistor to ground, the current through the LED is about 60-80 uA, which is enough to illuminate an efficient red LED.
I think this will not affect the switching of the power transistor, but wanted to double check and ask for a second opinion.
Please, check the circuit diagram below.
Thanks!
Are you sure your schematic is correct?
The 1N4148 diode is in the wrong place to protect the transistor against the relay's back EMF. The diode should be across the relay coil not the transistor.
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Why diddling in that place?
Two simple solutions:
1: neon lamp plus resistor directly over the load. That will give you the best information.
2: LED plus resistor across the relay coil. That will give you the 2nd best information.
Very old (and seemingly forgotten) rule: "Annunciators/indicators should show what's actually happening. NOT what the controller thinks is happening."
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Hi Benta,
as I mentioned, these were the first options come to mind - well not the neon lamp exaclty.
However, the relay is on another PCB/Module, which is connected directly to mains.
I don't want to mod this encapsulated module. Reasons: high voltage, fire, liability, etc.
So the relay is not very accessible, also the power transistor that drives the relay as they are both on this mains module.
The load itslef - it is a floor heating valve - located inside the wall in a box in my antre. So, again not feasable.
That is why I have chosen just to modify the harmless 3V digiatal board.