Latching relay circuit to protect still

[Yummyrum]

Nothing new here but it was asked about in a recent topic so I thought I’d throw up some diagrams if others are interested .

The idea uses the simple latching circuit often seen where the big Red and green buttons are used to control a Contactor or heavy duty relay .

Although I’m using the Schnieder Contactor unit , a multipole relay can also be used if it has changeover contacts able to handle the element current .

A Contactor has sets of contacts that are moved when current flows in its Actuator coil . It is an easy way to switch 10s or 100s of Amps with just a small control current .

Many contactors also have a set of smaller Auxiliary contacts that are used for latching and control purposes .

You will see that the Contactor shown has three sets of 38 amp contacts ( the Six big arse screws on the base ) labeled L1 ,L2 , L3 then there are two Auxiliary contacts ( the smaller screws on the top left and middle ) . Finally , there is the Actuator coil connections on the right labeled A1, A2

The two Auxiliary contacts are different .One is normally Closed (NC) while the other in normally open (NO) .When the Actuator coil is energised , the contacts change.

Here is the basic latching relay circuit with a start /stop switch .

It is also worth pointing out that the big red switches are normally available with “Normally Closed “ contacts while the green ones have “Normally Open” contacts .

There is a good reason for this .

So getting back to the Schematic .
When the green Start button is pressed , the circuit through the Actuator coil is complete and it “pulls in “ .

This is where the Auxiliary NO contact snaps closed . Now , if the Green button is released , the Auxiliary NO contact “holds” the relay ( Contactor) on . It is now latched on

It will stay latched on until the Red “Normally Closed” STOP switch is pressed . This resets the relay to it’s original state .

(If you click on the diagram , you will see a dotted line from the Actuator coil through all the contacts . This means that all contacts in that line will move accordingly)

This is where we can use the basic circuit and add extra “NC” circuits in series with the Red STOP switch to switch off the relay ( Contactor) if say , the coolant flow is too low …..or…. The temp at the too of the RC is too high ….. or…. the level in the boiler is too low ….etc .

It is also a quick check when you attempt to “fire up” the elements .
Unless all the safety checks are OK , the element won’t switch on .
IE , if the boiler isn’t full or the coolant flow isn’t sufficient .




OK , a bit more detail . The big red and green switches usually have a light in them as well . So when you hit the Green button , its light glows Green . When you hit the STOP button … or something triggers a shut off , the Red button glows .

These buttons have lamps built in and powered via seperate terminals . The Lamps can be of a variety of voltages , but 24 , 48, 120 and 240 are common .

The Contactor coils also come in a variety . Again , 24, 48, 120 and 240 are common .
Note that the Contactor coils are generally AC . This is very important . If you hook them up to DC , they will burn out in a matter of minates . ……Be warned …. ( Coils have inductance and that limits the current flow . If it says AC , it means AC).

Getting back to the lamps in the switches .

The Auxiliary contacts have been setup so that they can also switch the supply to the Lamps as well as providing the latching function .

All this could have been done at higher voltage . But if we are going to have cables running to flow sensors and Over temp sensors , having a nice low 24V AC is a great safe option .

I have bot shown connections to the Main contacts . It is intended that this unit goes before any controller or elements and the main contacts would feed such circuits .

[shadylane]

I also use a latched relay for the power to the still.
There are two STC-1000 temp controllers with the contacts in series with the relay coil to shut off the power if the output end of the product condenser gets hot or the boiler exceeds 210'f
The reason I did it this way was to make the still as stupid proof as possible. I figure anything that limits drama is good. 120v is used for the control circuit, a 24v stepdown control transformer like your using would have been better. But my rig doesn't have any remote wiring like switches to worry about. There's power into and out of the box but the other wires are just coming from low voltage thermocouples.
I did it this way based on simplicity and safety.

[NZChris]

When doing multiple stripping runs, I record the wash temperature when I finish the first stripping run, then program the still to shut down at the same temperature for the subsequent stripping runs. The ABVs of the strips are very close to the ABV of first strip.

Overly warm product at the spout will also shut the still down.

[Yummyrum]

I also use a latched relay for the power to the still.
There are two STC-1000 temp controllers with the contacts in series with the relay coil to shut off the power if the output end of the product condenser gets hot or the boiler exceeds 210'f
The reason I did it this way was to make the still as stupid proof as possible

Thanks Shady , that is the sort of things that I was thinking about . STC1000 are a perfect fit .

When doing multiple stripping runs, I record the wash temperature when I finish the first stripping run, then program the still to shut down at the same temperature for the subsequent stripping runs. The ABVs of the strips are very close to the ABV of first strip.

Overly warm product at the spout will also shut the still down.

Great idea Chris , it was you saying recently about it that started this . Thanks for the elaboration .

BTW , it’s going to become part of my new controller so looking for suggestions on what you all have done .

[Millbo]

Nice!
In the motor control world we call that sort of setup a collapsing circuit.

[Black Bull]

This be ok Yummy ?
https://www.ebay.com.au/itm/22586581605 ... ZZPWHTB0VE
and the switches, latching or momentary ?

[Tammuz]

Now that solid state relays have dropped in price would that be a better choice for your controller.

[NormandieStill]

Now that solid state relays have dropped in price would that be a better choice for your controller.

SSRs are a great solution for power control and switching applications but they do not create the air gap necessary to declare something as off. A contacter will completely cut the power while an SSR can / will leave a small current flowing.

When I get my dream controller built (Probably not before the summer) it will be based around YummyRum's burst fire controller and will incorporate the safety features from this thread.

[amh71]

In my experience an SSR will always fail on, so you may not ever be aware that your safety device is broken.

They are great for doing the control switching but need to be backed up by a good old fashioned relay on their input power.

[Yummyrum]

This be ok Yummy ?
https://www.ebay.com.au/itm/22586581605 ... ZZPWHTB0VE
and the switches, latching or momentary ?

Blackbull , that is just a standard 2pole contactor .
While it is suitable for switching the main power , it does not have the extra auxiliary contacts to do the latching circuit .

It could be used , but you would need to using a second relay wired as a latching relay to drive it’s actuator coil .

[Yummyrum]

In my experience an SSR will always fail on, so you may not ever be aware that your safety device is broken.

They are great for doing the control switching but need to be backed up by a good old fashioned relay on their input power.

I agree with this .
Infact while using my SSR a couple of weeks ago it did a weird thing where I had been running it at 100% for several hours while doing a stripping run . When I went to switch off the power , I could still hear the boiler running . So the SSR had somehow kept running even though there was no drive to it . I had to switch it off at the wall to stop it . This happened twice . Once it had cooled down it works normally .

But main point is that if I had been relying on that device to kill power , it could have ended bad .

[amh71]

In my experience an SSR will always fail on, so you may not ever be aware that your safety device is broken.

They are great for doing the control switching but need to be backed up by a good old fashioned relay on their input power.

I agree with this .
Infact while using my SSR a couple of weeks ago it did a weird thing where I had been running it at 100% for several hours while doing a stripping run . When I went to switch off the power , I could still hear the boiler running . So the SSR had somehow kept running even though there was no drive to it . I had to switch it off at the wall to stop it . This happened twice . Once it had cooled down it works normally .

But main point is that if I had been relying on that device to kill power , it could have ended bad .

Not sure i would trust that SSR again.

It's a lesson I learnt some years ago, I replaced a failed relay in a kiln controller with an ssr, no safety relay on the controller. Big mistake, ssr failed eventually and kiln continously heated, was over 1100C when I spotted it, cost me a fortune to replace the elements but could easily have gone on to cause a fire had i not been there.

Only use controllers with a safety output now and that always drives a contactor relay.

These are PID type temperature controllers rather than the power controllers we use here but the safety principles are no different.

[Tammuz]

I've used contact relays when switching high power. I have had more than 1 weld the contacts so it was locked on. I would think that if ssr were such a problem companies that build controllers like Auber Instruments would avoid using ssr's in their controller.

[amh71]

I've used contact relays when switching high power. I have had more than 1 weld the contacts so it was locked on. I would think that if ssr were such a problem companies that build controllers like Auber Instruments would avoid using ssr's in their controller.

A mechanical relay as a safety only sees one activation per session, hundreds or thousands less per session than a control relay so has a very much longer life, when it does go it will fail either on or off, a SSR will always fail on, certainly in the case of the dozens I have used or made for others.

The point was, whatever you use, have a safety device, be that a relay or a big red switch but an SSR is unsuitable to be that device.

[shadylane]

By default mechanical relays win the latching job because they can have an extra set of normally open contacts.
99.9% of solid state relays don't.

As to SSR's failure mode it can fail on or off or somewhere in between.
The majority of the time it will fail open.
Mechanical relays fail closed more often than SSR's due to broken springs or the contacts welding together.