Does this board have a flyback diode built-in? Can it switch low amp 24v AC power?
The relay markings say 10A @ 250VAC and 10A @ 30VDC so that should answer one question.
The data sheet for the actual relay does not show a diode but the pic shows a diode near the green LED. With no circuit available this could be anything. The black blob immediately above this diode looks like a transistor or optocoupler so it would probably be the flywheel diode. It would be oretty slack of the board manufacturer not to include this. But it IS Chinese…
When I received my relay didn’t come with headers as shown in the photograph. I tried soldering on without success. Is this normal should it come with headers for power, ground and data?
Soldering or headers, it should not make any difference unless the soldering job is a bit flaky or suspect. Headers would probably be supplied loose so you would have to solder them anyway.
It is still a bit slack though when the pic shows headers fitted. A good philosophy would be if they are not included don’t show them. A little bit of misrepresentation.
If you mean that you couldn’t get the solder to wick onto the headers, so it was always a dry joint, then this is normal. These headers usually need a good scraping with a sharp blade before they can be soldered, even when using a good flux.
Can I control this relay with a Raspberry Pi, even though its GPIO only output 3.3V?
Welcome to the forum
The manufacturer’s specs have designed these modules for 5V operation but we’ve found during our bench testing here that they’ll work fine at 3.3V as well.
Keep in mind the power supply to the module should also be 3.3V if you’re trying to run it at 3.3V logic levels.
Also, this relay is active low, so pulling the signal pin to 0V engages the relay.
That might be so but I think it is a pretty bold statement to think that ALL of these modules will be reliable at 3.3V.
The Songle data sheet says pull in is 75% Max and release is 10% Min. Some relay manufacturers quote Min pull in voltage That is the minimum voltage guaranteed to operate the device and Max voltage the relay is guaranteed operate before release. From that I assume this unit “must operate” voltage is 75% of rating (3.75V) and release max is 10% (0.5V)
Now admittedly 3.3V is not far below the “must operate” point but you must factor in the voltage drop across the opto transistor which could be anywhere between 0.2V and 0.3V or even higher. This may or may not vary somewhat with opto LED intensity if the transistor does not switch hard on (darlington opts are better in this area) but as the info regarding the circuit and opto type are unknown and look like staying that way accurate forecasting is a bit difficult.
The point I am making is that when you subtract this voltage drop from 3.3V there is not much left and it would be a bit hard to guarantee reliable 5V relay operation under these conditions.
I realise you stated you tested some units to be OK and you haven’t guaranteed anything but as I read it the implication is there and you could be misleading anyone inexperienced who may not realise there are all these little things adding up and don’t realise that their particular case is sailing a bit too close to the wind and suffers from the worst kind of fault. The ones that come and go for no apparent reason.
I’ve just been experimenting with one of these modules that I bought about 18 months ago.
I found that 3.3V wasn’t enough to switch the relay. When testing with a power supply, I needed to power it with at least 3.4V.
With 3.3V it did switch the indicator LED, but not the actual relay itself.
The PCB markings at the left size are in Chinese only. Is there English marking at the bottom?
Sort of verifies what I said above. It was a bit of an iffy claim that these devices worked oK at 3.3V just because a few might have been OK.
It did not go unnoticed that Core did not see fit to reply to my comments. Also the statement
" * While marked as a 5V supply voltage & input signals, we have found this to work fine at 3.3V"
is still there. As I said this may be true for some but not all and this figure of 3.3V is actually outside the relay operating specs.
Thanks for bringing this to our attention. This is the first instance we’ve come across, but that’s all it takes to get us to change our product description accordingly. I’ve made a note to get that fixed up ASAP.
Don’t know if this has been answered, haven’t read all comments.
The answer is YES.
I have one of these and the diode is connected across the relay coil.
I converted my 3.3V signal (from a raspberry pi pico) to 5V using one of these logic level converters - Logic Level Converter Bi-Directional | Core Electronics Australia - and that worked fine to control the relay for me.