I'm working on a system which can interact with a home automation system called KNX, which can just be attached to the 30V bus of KNX in exchange information. The system will be in charge of controlling a fireplace, which is controlled by a 0 - 5V. The protocol will also let the fire place keep burning when 0V is applied, so for safety reasons, I also included a relay which is normally open.
In this setup I am starting of a NanoBCU, which is a KNX bus coupling unit, allowing for communication over the KNX bus. To this I have coupled
an arduino Beetle
a MCP4725 to output the desired output voltage
a ot8520, which is a logical level converter 3.3V to 5V
a OT961-C105, which is an arduino relay module
The following setup works when I power the arduino from the USB 5V, all communication is ok and the relay switches on demand. When I attach the board only to the KNX bus, which powers the arduino board, it boots fine (it starts sending out information on the KNX bus as expected). However, as soon as I switch the relay, the arduino board reboots.
Can someone help me to figure out what is happening here and how to resolve this?
I'm not familiar with any of these components, but to me it sounds like the NanoBCU cannot provide enough current at 5V to run the circuit when the relay is energised. When connected to USB, the USB cable can provide enough current and there is no problem. I reviewed the link to the NanoBCU posted and there was a maximum current rating of 100mA given, but it wasn't clear whether this was the the maximum current the NanoBCU can provide to an external circuit, or if it is the maximum current the NanoBCU consumes for itself. But if it is the former, then I think your circuit will be pulling close to that maximum when the relay is energised. This will be causing the 5V output from the NanoBCU to drop and that in turn will cause the Beetle to reset.
The relay, when energised, will be pulling far more current than the rest of the circuit together, so if you can eliminate that, the circuit won't get near to the maximum current available.
I don't quite get what the relay is for. You seem to be saying that the fireplace that is connected to the output of the DAC behaves differently when the output voltage is zero compared to when the output is disconnected, which doesn't quite make sense to me. Can you post more technical details of this fireplace?
First line states that at 0 VDC flamesize is set to 0% and state of fire is on. In reality, the fire controller will put the flame to minimum put it well be kept on. This means, that if the arduino is attached, the fire will stay on. Only when I disconnect the wires or put 3VDC on it, the fireplace goes off. I found this a bit of a security risk, so wanted to have a relay in between.
About the other remark of maximum available current of 100mA:
do there exists relays consuming less which would stay in this limit?
I did try to power the relay module directly from the 30V bus from KNX, by using the LM2596 module allowing me to step down from 30V to 5V. For some reason, in this setup the knx nanobcu module is however not able to connect to the bus anymore. I didn't really yet get to the reason of why this setup doesn't work unfortunately.
Hi, @consec
Do you have a DMM?
If so measure the 3V3 and the 5V when you operate your system.
Can we please have a circuit diagram?
An image of a hand drawn schematic will be fine, include ALL power supplies, component names and pin labels.
The current diag is hard to follow with different colours and diagonal wires, you have labelled all connections which is good.
Hint: Draw your components as boxes with connections coming out and labelled, refrain from cutting and pasting images of the actual component.
@PaulRB
Good to know, thanks. I will order a reed switch like this one https://www.allekabels.be/zwakstroom/7372/1072065/dil-relais.html and try it out.
I didn't really like that idea as you could get into the situation where the power of the KNX bus gets lost, which results in a 0V on the input, allowing the fire place to stay on. In that case you can only put voltage back to it to power it off. I like to have this shut down in case of any issues on the board side.
That is correct, the bus does deliver both power and data. Didn't think about interference. I could make use of the auxiliary power lines in the EIB cable. But would like to first try with the reed switch approach.
I think that looks ok. The coil has 500R resistance, so the coil current should be only 10mA, which is within the Beetle's output pin limits. BUT YOU MUST add a flyback diode across the coil terminals, otherwise the Beetle could be damaged. (Your large relay will have a flyback diode on its PCB already, but I don't think this reed relay will have one built-in).
EDIT: the internal wiring of the relay shows no flyback diode, so you must add one externally
