Time Delay Switch From Signal

Google "time delay relay", see if anything pops up.

This would be a snap with a PLC, if you already knew how to program PLCs.

--
www.wescottdesign.com

I agree. There are both "delay on make" types (you apply power to the coil, and then there is a delay before the contacts switch) and "delay on break" types (you apply power to the coil, contacts switch right away, you remove power to the coil, there is a delay before the contacts unswitch). Some of them have a fixed delay and some have a knob that allows you to adjust it over some range, like maybe 1 to 15 minutes.

Historically these came in cans with octal bases (8 pin vacuum tube); you can get a socket that gave you screw terminals. These have real relay contacts in them with high current ratings, like 10 A. These "should" be available with UL component listings.

Now they seem to also be available as little bricks or pucks with push- on connectors, and be cheaper as well. These don't seem to be able to switch as much current, though - 1 or 2 A. One brand I Googled up (ICM Controls) has UL component listings on at least some of their products, which should make it more acceptable to an electrical inspector; you'll still need to put the relay in a suitable outdoor enclosure, use the right wire gauge, etc.

If you want to buy it in person, an industrial supply place like Grainger may be able to help. The local heating and cooling supply store may also have some. Otherwise, they are available online from several suppliers. McMaster-Carr has a page with both the 8-pin and puck types:

Standard disclaimers apply: I don't get money or other consideration from any companies mentioned.

Matt Roberds

Thanks, I will look into time delay relays.

Thanks for the info. I don't understand how a time delay relay will fit the bill here though.

My understanding of a time delay relay is essentially as you describe above: it delays the contact or disconnect.

But in this case, that would seem to result in the contact being made during the pump's operation instead of after it.

Here's an example sequence: Pump turns on @ 1PM and runs for 2 hours. Heater turns on @ 1PM (because water is flowing through it) and runs until 3PM (turns off because the water flow stopped from the pump).

I want to flush water through the heater at ~3PM.

So let's say I were to setup a "delay on make" relay. The water flow "signal" will start at 1PM which means the relay will engage at 1PM + the delay. But that means it will open the fresh water valve while the main pump is running.

It seems to me that I need a mechanism which will connect only when the signal has gone from on to off, and then for only a desired amount of time.

Am I missing something with a time delay relay?

something like this:

afaict in one-shoot mode it will be on for a programmed time after turn on

you may need a relay to make pump off turn timer on etc.

-Lasse

If time delay relays don't cut it, look into PLCs. It'll definitely be more work (you'll basically need to write a simple program), but it should definitely do the job.

You'll need a PLC, a power supply for the PLC, sensors that are compatible with the PLC, and one or more relays for the PLC to drive. Fortunately, PLCs are routine things in industrial setups, so it should be quite "inspectable".

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

[snip]

The above page describes a 'one shot' normally-open relay.

If you can arrange for the main pump timer switch to be a SPDT switch, you can do the following: power | | / SPDT o o | | -------- ------- |main | | one |__ valve |pump &| |shot | |heater| ------- --------

When the main pump is 'on', the relay is 'off'. When the main pump is switched 'off', the one=shot relay is switched 'on' for the 10 minutes, then goes off.

You'd have standby current going to the one-shot electronics until the next time the pump runs, but that should be a few mA.

HTH

Here's how you can use the delay relay:

heater on switch / Vac ---+---o o---+--------heat coil-------+--- Vac | | | | +--heat off relay coil---+ | | | | v | | TD relay on signal | | | +-------------TD relay coil---------+ | | | o----flush solenoid----+ | / / +---o o---o HO-nc TD-no

Heater on operates time delay relay with the TD relay on signal. When heater on switch turns off, the relay remains on for X time where X equals the time you want the water flush to occur. The water flush is prevented during the time the heater is on by the "heater off" normally closed relay contact, labeled HO-nc, which will be open while the heater off relay is energized.

Ed

re are two main risks for the heater: (i) salt water; and (ii) standing he ated water (especially salt water) in the unit. In my setup I have a manua lly-actuated valve to control water flowing into the heater or bypassing it altogether. The heater has its own thermostat and various sensors so it w ill turn itself on and off as needed, but only when water is flowing throug h it. [wants to fresh-water rinse the heaterr element]

I'm not clear why you want electronic controls; couldn't you just use a toi let flush mechanism and a float? Mount the heater in a vertical standpipe, so it is wet with s alt water only when the pump is turned on, and when the pump is turned OFF, the sal t-water level drops. Then, at sufficient drop, a float in the salt water toggles the flu sh valve and rinses the heater element with a 1.8 gallon aliquot of fresh water.

Hmm. You may need another plain old relay that gives you the opposite of what the water flow switch is doing. When the water flow switch is on (water is flowing), the plain relay contacts are off. When the water flow switch is off (water not flowing), the plain relay contacts are on. You then wire the plain relay contacts in series with the time delay relay contacts.

With a 10-minute delay-on-break relay, you would have...

relay contacts time main pump flow switch heater time del plain flush

12:59:59 off off off off on off 13:00:00 on off off off on off 13:00:01 on on on on off off [ ... main pump runs according to its timer ... ] 14:59:59 on on on on off off 15:00:00 off on on on off off 15:00:01 off off off on on on [ ... time delay relay waits for its time period ... ] 15:09:59 off off off on on on 15:10:00 off off off off on off 15:10:01 off off off off on off

Matt Roberds

Thanks--I think this is the right track. I don't think I need a SPDT switc h though--for the purpose of this "flush" system I don't have control over the pump's operation. I can--however--install a flow sensor which will be normally closed. So when water is flowing through the heater the switch wi ll open and so the one-shot relay will not activate. When water stops flow ing the flow sensor will close which will energize the one-shot relay which will close the flush circuit.

You're right that this has the small drawback of drawing current while the main pump is not running, which is most of the time. Wondering if I can introduce another relay to avoid it. But considering th e main pump uses hundreds of watts, some mW on this is nothing to lose slee p over.

Thanks!

Thanks Ed. Took me a while to understand your proposal. Unless I'm mistak en, it's similar to using a normally-open flow sensor coupled with a one-sh ot time delay relay. The only difference being that in your design the flo w sensor can be normally closed.

But I think your design helps...

While thinking about the system more I realized that the heater will likely heat the water being flushed through it (afterall it uses a thermostat whi le it's powered on). While it's not as bad as holding heated salt water, I 'd like to avoid it.

The heater has what they call a "fireman's switch" which is basically an ex ternally controllable switch to the control board (open it and the heater w on't run).

So if I have the water flow switch operate a DPST relay--one pole controls the flush and the other controls the heater--then I think all bases are cov ered.

Thanks!

here are two main risks for the heater: (i) salt water; and (ii) standing heated water (especially salt water) in the unit. In my setup I have a man ually-actuated valve to control water flowing into the heater or bypassing it altogether. The heater has its own thermostat and various sensors so it will turn itself on and off as needed, but only when water is flowing thro ugh it. [wants to fresh-water rinse the heaterr element]

oilet flush mechanism

salt water

alt-water level

lush valve

I should have elaborated. The water lines are all closed & under pressure when the water is flowing. The heater uses a heat exchanger (just like a f urnace). So there's no open area to use something like a toilet flush mech . Additionally, when the pump stops, all water ceases in its current posit ion. Unlike a toilet which uses gravity to move water, this system is oper ated only by pressure (via the pump). So when the pump stops, the salt wat er stays in place--it doesn't drain.

Your state table looks correct to me. Any reason I can't use a normally-open flow switch (they seem to come in both forms)?

[snip]

Does the pump timer use a relay to control the pump, and do you have a set of unused contacts on that relay?

You can use a set of NC contacts to route power to the one-shot; that provides the same switching logic as the SPDT version.

[snip]

Or just a Master Off switch for the whole thing.

It's probably less power than a cable modem or network router you leave on 24/7...

Nothing I can think off the top of my head. If you decide you need the negation of the flow switch signal later, you can always follow the flow switch with an xPDT relay.

Sometimes you care about what happens if something gets stuck on, or what happens if there is a power failure, and that drives the choice of normally-open vs. normally-closed.

Matt Roberds

Ok, I think I have a clearer idea of what you want. Your idea is good idea - use a SPDT relay for the flow relay. The flow sensor is normally open, closing when water flows. Here's a new diagram showing what I think you said: (XXX = add whatever interlocks, switches you want) view in fixed font

Flow Relay Contacts Thermostat no / o----XXX--o o---heater---+ | +---------------o->o nc | | | no flush | | Flow | o----solenoid---+ | Sensor | | | / +------o->o nc | Vac ---+---o o--------+ TD relay | | | contacts | | | | TD Relay

The pump's timer is natively integrated with the pump itself and does not provide an external signaling mechanism (short of dismantling it and tapping into its internal electronics).

Great, that seems straightforward.

Well, that would be a bit of work because the pump & heater operate on different circuits. I suppose I can use the flow switch to cut power to the circuit we're designing here, but weighing the pros & cons I don't think it's worth the added complexity.

Thanks.

I'm glad you asked because it forced me to lay it out. Here's what I've come up with. The first section is where I was headed before realizing that the flush will cause the heater to heat the flushing water. The second is my attempt to address that.

(Need to view the rest w/ a monospaced font)

===================================================================== Without Disabling Heater During Flush

Pump On +-------------------+ | | Off ----------+ +------------------------ T0 T1 T2 T3 TN

Flow Switch (NC) Closed ----------+ +------------------------ | | Open +-------------------+ T0 T1 T2 T3 TN

Flush Relay SPST One-Shot Relay (NO) Controlled by Flow Switch Closed +------+ | | Open ------------------------------+ +----------------- T0 T1 T2 T3 TN

Flush Valve Solenoid (NO) Controlled by "Flush Relay" Closing Flush Valve Solenoid circuit energizes solenoid which opens valve and water flows Closed +------+ | | Open ------------------------------+ +----------------- T0 T1 T2 T3 TN

Pump Off T0-T1,T3-TN Vac ---+--------------+ | | | / | / +-o->o------o o------o o---- GND Flow Flush Flush Switch Relay Valve (NC) (NO) Solenoid (NO)

Pump On T1-T2 Vac ---+--------------+ | | | / / | / +-o o------o o------o o---- GND Flow Flush Flush Switch Relay Valve (NC) (NO) Solenoid (NO)

Pump Off T2-T3 Vac ---+--------------+ | | | | +-o->o------o->o------o->o---- GND Flow Flush Flush Switch Relay Valve (NC) (NO) Solenoid (NO)

===================================================================== Disabling Heater During Flush

Pump On +-------------------+ | | Off ----------+ +------------------------ T0 T1 T2 T3 TN

Flow Switch (NO) Closed +-------------------+ | | Open ----------+ +------------------------ T0 T1 T2 T3 TN

Flow Relay DPDT Relay acting as 2 SPST relays, one NO and one NC Controlled by Flow Switch

Flow Relay Pole 1 "Inverted Flow Switch" (NC) Closed ----------+ +------------------------ | | Open +-------------------+ T0 T1 T2 T3 TN

Flow Relay Pole 2 "Fireman's Switch" (NO) Closed +-------------------+ | | Open ----------+ +------------------------ T0 T1 T2 T3 TN

Flush Relay SPST One-Shot Relay (NO) Controlled by Flow Relay Pole 2 Closed +------+ | | Open ------------------------------+ +----------------- T0 T1 T2 T3 TN

Flush Valve Solenoid (NO) Controlled by "Flush Relay" Closing Flush Valve Solenoid circuit energizes solenoid which opens valve and water flows Closed +------+ | | Open ------------------------------+ +----------------- T0 T1 T2 T3 TN

Pump Off T0-T1,T3-TN Vac ---+--------------+------------+ | | | | / | / | / +-o o--+---o->o---------o o------o o---- GND Flow | Flow Flush Flush Switch | Relay Relay Valve (NO) | Pole 1 (NO) Solenoid | (NC) (NO) |

24V ---------o o----+ Heater Flow | Main Relay | Board Pole 2 | Power (NO) | ------------------+

Pump On T1-T2 Vac ---+--------------+------------+ | | | | / | / | / +-o->o--+---o o---------o o------o o---- GND Flow | Flow Flush Flush Switch | Relay Relay Valve (NO) | Pole 1 (NO) Solenoid | (NC) (NO) |

24V ---------o-->o----+ Heater Flow | Main Relay | Board Pole 2 | Power (NO) | ------------------+

Pump Off T2-T3 Vac ---+--------------+------------+ | | | | / | | +-o o--+---o->o---------o->o------o->o---- GND Flow | Flow Flush Flush Switch | Relay Relay Valve (NO) | Pole 1 (NO) Solenoid | (NC) (NO) |

24V ---------o o----+ Heater Flow | Main Relay | Board Pole 2 | Power (NO) | ------------------+