Advice when buying relay boards

[clearwaterbrewer]

I just replaced my two 2-channel boards with a nifty 8-channel model, and when I connected it, all the relays came on... Apparently this one turns the relay on when you ground it, not take it to 5V.

I suggest you make sure it has something like "IN_x: Control signal input, Low: relay OFF, High: relay ON." in the description

I desire to not have the relays on all the time, I can either buy a new board or hope that there is a 'invert output' checkbox next to the 'enable' checkbox of the system settings page in the BCS roadmap

-mike

[Baron Ken]

...I desire to not have the relays on all the time, I can either buy a new board or hope that there is a 'invert output' checkbox next to the 'enable' checkbox of the system settings page in the BCS roadmap

Though that might provide you a workaround, it means you have to have the BCS powered up and providing the 5v signal(s) before turning on the equipment that has the relay board. And in the event your signal cable becomes disconnected (from tripping over it, etc.), all your relays would kick on as well. Granted, if you have the BCS mounted inside your panel (I do not), you won't have the opportunity to trip over cables.

[clearwaterbrewer]

You do bring up an interesting point... Thanks!

Mine is directly connected via ribbon cable in the control box, and is attached to the same power source as the BCS, originating from the 'convenience' 120V in the panel. so that would not be a problem.

My 240 and 120 (or both legs of 120) for 'process power' are switched on via a contactor and an E-Stop, so I would have the ability to see outputs before turning on process power.

-mike

[Baron Ken]

Ok, try not to take this the wrong way, but how did you wire it up? Looking at the board it looks like each relay has 3 contacts with the middle one being the one that moves between the outer two. Connecting to one of the outer and the middle is a normally open (NO) connection and connecting between the other outer and the middle is a normally closed (NC) connection.

Hopefully that helps.

EDIT: ok, I see that didn't really help with the input as the board uses PNP transistors to control the relays and the input needs to go low (as you said originally ).

[Baron Ken]

from other subforum:
Would really appreciate an 'invert output' checkbox next to the 'enable' checkbox of the system settings page. Not sure how many folks would use it or how much code it would really take to have the input show as 'OFF' but be at +5V , and show as 'ON' but be at 0V.

When you connected the BCS and the output was 'off', did that cause the relay to turn off? Meaning, did 0v out from the BCS cause a ground condition on the board causing the relay to turn off?

[clearwaterbrewer]

The issue is not the relay contacts, it is the voltage that activates the relay coil, these relays, grounding the lead activates the coil.. which is non-intuitive to us BCS users, but PIC/8051/audrino folks may think differently...

I can hook up the relay contacts to accommodate what you mention, and the system will work, but then, as an example, if the BCS were to die, 8 relay contacts would shut off and then turn on everything connected to them.... not pretty

relay board will go in the spare parts box and I will buy another... I have learned that if you look at the schematic (and they are right) you can see if it is the sort we want, it will have one of the coil leads going to Vcc, will possibly have a 'pull-down' resistor on the input of the transistor, and transistor being NPN..

http://www.rason.org/Projects/transwit/transwit.htm is a good link

The first image, the 2-relay boards that worked fine for me, has the coil lead connected to Vcc, a 100k pull-down resistor, A NPN transistor(darlington pair, actually), and the transistor is more closely connected to ground than it is Vcc.

D112-D114-D1181V1SCH[1].png (44.84 KiB) Viewed 7681 times

the second picture, which is the wrong one I got, has a relay coil lead to ground, the transistor is PNP, and the transistor is more closely connected to Vcc than it is to ground.

DSC06246[1].jpg (81.05 KiB) Viewed 7681 times

[Baron Ken]

Ok, I'm trying to visualize the board's operation.

If you connect an input from the BCS and it is 'off' (0v), the relay is 'on', correct? So connect your heater, pump, etc. to the NO side of the relay. When you turn on the BCS output, the voltage goes to 5v and the relay turns off, connecting the NC connection and passing power to your heater, etc.

If you lose power to the BCS, the voltages drop to 0v and the relays turn on, but since your connections would be NO, there is no problem.

You said you did not want energized relays always on, but other than the power used to keep them on, I would think it would be fine.

EDIT: of course the problem lies in that if you lose power to the relay board, all the relays de-energize and pass power, not safe, no go. *sigh*

[clearwaterbrewer]

yup, save this one for the BCS Christmas light project where nothing gets hurt if it fails 'on'

[clearwaterbrewer]

Ok, I'm trying to visualize the board's operation.

If you connect an input from the BCS and it is 'off' (0v), the relay is 'on', correct? So connect your heater, pump, etc. to the NO side of the relay. ....

that NO above should be NC

[Baron Ken]

Ok, I'm trying to visualize the board's operation.

If you connect an input from the BCS and it is 'off' (0v), the relay is 'on', correct? So connect your heater, pump, etc. to the NO side of the relay. ....

that NO above should be NC

Academic, but correct