Element Switching Relays

[scoots]

Before i make the leap into a BCS460, I was hoping for a bit of help with an idea.

I have 30A service to my current electric brewery. Not a problem since I only control one 5500W element. My new system will require the use of a RIMs element and a boil kettle element that cannot be operated at the same time. I am hoping to have the BCS make sure that only one can operate at a time.

So my questions are:

1) Can the BCS be easily programed so only one of the elements can be operated at a time? I need to be guaranteed that they will be locked out from firing at once.

2) The option I figured out to guarantee it would be the use of a relay to switch the power. I am not sure of the terminology of relays (Double pole, double throw, I think), but I think I need one with a 5V DC coil that will have one 5V Normally Open and one 5V Normally Closed leg. My thought is to use this relay to control which element is allowed to receive input from the other BCS outputs.

I know I would put a selector switch between the two elements, but I would rather be more automated and have less chance that I will screw the system up by forgetting to flip the switch. So I guess I would want to use one output to control element 1, another to control element 2 and a third to control the relay switch. I think this will work, but I have no idea how to read relays that can work for this type of application.

Thanks for the input,
Scott

[JonW]

Yes, you can easily do this as you outlined. Having a 5V DPDT relay between two SSR's is how I would do it. Output #1 would wire throut the Common & NC terminals of one pole and Output #2 would wire through the Common and NO terminals (and power the relay). This would allow the first element to heat unless power is applied to the second element, at which time the first would be switched off.

[pngaudioguy]

I know I'm reviving a ridiculously old thread here, but numerous Google and forum searches haven't quite answered my question. In the described scenario, you're using the PWM output from the two element outputs to drive the coil circuit of a relay to open a circuit. Do you need to use an opto-isolator, schmitt trigger, LPF, or other circuitry to maintain the relay, or is the signal from the PWM adequate to keep the element select relay triggered? Does it work when your output is only at 20% duty cycle, for instance? I like the simplicity of this solution, but haven't found reference to anyone actually making use of it.

Thanks in advance.

[JonW]

You bring up a very valid point that I did not consider in the original answer. When doing PWM, the relay may in fact not properly latch or may even continually turn on and off at a low PWM. If you have additional free outputs, you could use a different output to drive the relay on/off. By using ladder logic you can tie the state of the relay output to the PWM output so that you would not need to assert the output within your process/state. The benefit of doing it this way is that the logic would also work if you had manually switched on the heating element output from the main control screen.

[pngaudioguy]

Thanks for the clarification. I've done a bunch of research today, and I'm thinking that one could also use a simple series resistor and parallel capacitor to form a low pass filter converting the PWM output to analog voltage. You could then add a transistor to increase the drive voltage to stay above the 2.5V trigger required to hold the relay coil open (assuming Electronics-Salon relay boards) from the 5V output. It would still be extremely low current draw so as not to overdraw the BCS output, and for less than $1 in parts you don't need the extra output from the BCS to trigger the relay. Anytime the PWM is above about 5% duty cycle the relay gets tripped and passes the unaltered PWM signal through the NO contact to the SSR. Easier to implement than describe, I think.

[oakbarn]

I have standard Switches inline AFTER the SSR and before any Element or Pump. Both the BCS AND the Switch MUST be "On" for the Element or Pump to Work. I had built Manual control for my pumps and just wired the BCS upsteam tp the existing wiring. I am in the process of wiring circuits for 4 elements with switches. That way I have "positive" off where I can turn the Brewery totally "Off" when not brewing. The switches are also "Emergency Off" for the Elements.

You could also use Ladder Logic to turn "Off" one Element if the other Element was "On".

[clearwaterbrewer]

thought about this a few years back...

http://forum.embeddedcc.com/viewtopic.p ... 591&p=3554

[pngaudioguy]

Hm - that's pretty interesting! So the NAND gate would trigger the relay selecting the element output? Would you still have to have a low pass filter on the PWM signal to keep the gate triggered?

Edit: The datasheet for the TI 7400 indicates 7-15 nano seconds as the trigger time, and .7-2V as trigger voltage. To maintain an open or closed relay, a low pass filter would not be a terrible idea.

[BrunDog]

I would be careful with any expectation that: 1. software will work reliably to prevent both elements from powering simultaneously, and 2: that any control of said elements be solely managed via the SSR input. SSR's are known to fail in both open or closed position, and hardware relays/contactors should be used to make certain power is cut to the elements. The only thing the SSR input should control is on/off cycling of the elements for duty cycle/PID/etc. control.

If I were selecting elements via software, I would use a SPDT or DPDT relay of sufficient rated amperage to determine power routing. Common goes to source, NO and NC go to two discrete SSR/element combinations.

One should never rely on the circuit breaker to be a backup. It obviously is, but only as a final protection. The wiring and circuits downstream must be appropriately sized for whatever circuits they feed. For example, if you used 10 AWG wire to power the 30A circuit expecting only one element to be on at a time, and both power on because the BCS had a brain fart, that wire, plug, connectors, etc. will see a significant overamp situation. Not saying it will burst into flames, but could very well be damaged, and or put users at risk. My $0.02, anyway.

-BD

[pngaudioguy]

BD - I believe that is inline with what is being discussed, with one little sidebar about the ladder logic option. Most of the posts, including the original and related answer, pertain to a physical relay switching between two circuits. I believe we've digressed to discussing methods of switching the relay, which have yielded what I find to be some very fascinating options for the 5V side of the circuit.

At least for my own circuit (which is looking like parts will be ordered in August after the move to VA) I'm going to have an appropriate contactor inline with each heating element that will be activated according to the result of the element select relay. Both hot legs feeding the element will be completely disconnected at all times the logic circuit is not active. If the select relay fails either open or closed, only the one in the final position would work. That said, I believe most of the relays are rated for several hundred thousand contact closures, so that's not going to be a likely scenario in the immediate future...