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[oceansoul]

I been tasked at work to redesign a water heating system for a safety shower, so that the heater is shut off when the shower is empty, and also have lights to show that the heater is on and that the mains is healthy i.e. 230VAC. Ive come up with this diagram, but me mentor is off tomorrow and he wants me to finish it by end of the day. Also what rating relay should i use, the thermistor is rater at 16A so that gives a Power rating of 3680W. Im thinking a relay (contactor) rated to 3.8kW.





Just to reiterate im not asking anyone to do my work for me, just check over it and maybe suggest a better way of going about it etc.

Cheers in advance

[STMO123]

 In that diagram, if R1 is closed, then power is still delivered to the element whether the float switch is open or closed.

[ians]

what power is the heater element?

[SteveMJ]

I doubt if the heater element would draw 16 A - that is a lot of current from the mains (I may be wrong there).  The float switch (if it has adequate current capacity) must be in line with the thermostat (not thermistor) and the heating element.  I doubt the float switch will have adequate capacity (my guess) so a relay may be required to provide the switching.

Hope that helps, Steve

[oceansoul]

I doubt if the heater element would draw 16 A - that is a lot of current from the mains (I may be wrong there).  The float switch (if it has adequate current capacity) must be in line with the thermostat (not thermistor) and the heating element.  I doubt the float switch will have adequate capacity (my guess) so a relay may be required to provide the switching.

Hope that helps, Steve

There is a relay in the diagram to do the switching?!?!?!?!?!?!

In that diagram, if R1 is closed, then power is still delivered to the element whether the float switch is open or closed.

Can you explain? Its my understanding that, the water level raises, closing the float switch energising the relay. When the relay is energised the thermostat and element are powered (aswell as the lamp). When the water level drops, the float switch opens, dropping out the relay, thus the element is no longer powered.

what power is the heater element?

I belive (as its writen on the cap of the element) is 13A.

[Ken T]

I would be inclined to stick the heater on light ,L1 directly across the heating element. That way the indication is only on if the heating element is active, not when its powered but cut off due to the thermostat. Makes it a bit more useful, and also you only need a single pole relay. I'm not an expert on ratings, but as its an (slightly) inductive load, you want a contactor rated higher than the heater. See what's available and choose one thatshigher but not too expensive. Oh, if the contactor is made in China, chose one at least 10KW !. They don't know the meaning of ratings over there !.

Ken

[Marks DTM Calib]

I would not use such a broad brush against china manufactured products..!

Any way, a few comments

1) It needs some transient suppression as when the relay coil de-energises there will be a transient which will cause mains born noise and may weld the float switch contacts

2) Similar for the heater coil, they are actualy quite inductive!

3) There should be a seperate stand alone main supply thermal trip/fuse that will stop a fire occuring in the event of the thermostat contacts welding shut which in reality for this case wont be a thermistor as shown, its more likely to be a be metal set of contacts.

Such setups need to be intrinsicly safe!

[STMO123]

In that diagram, if R1 is closed, then power is still delivered to the element whether the float switch is open or closed.

Can you explain? Its my understanding that, the water level raises, closing the float switch energising the relay. When the relay is energised the thermostat and element are powered (aswell as the lamp). When the water level drops, the float switch opens, dropping out the relay, thus the element is no longer powered.

Correct. I was too quick off the mark. Sorry.

[Ken T]

I would not use such a broad brush against china manufactured products..!

Any way, a few comments

1) It needs some transient suppression as when the relay coil de-energises there will be a transient which will cause mains born noise and may weld the float switch contacts

2) Similar for the heater coil, they are actualy quite inductive!

3) There should be a seperate stand alone main supply thermal trip/fuse that will stop a fire occuring in the event of the thermostat contacts welding shut which in reality for this case wont be a thermistor as shown, its more likely to be a be metal set of contacts.

Such setups need to be intrinsicly safe!

Sorry Mark but, I got a weller WSD81 iron that takes 85W, and am running it from a chinese stepdown transformer that is supposedly rated at 100W. It thermally trips after about 10mins. I have tried other similar units they also trip. How much de-rating do I have to use?.

Ken

[Marks DTM Calib]

What I am saying is....there are cheap unbranded items (and this does not just incldue China!) that are crap but, most of the worlds products are made there now and some of the best quality stuff is chinese to!

You get what you pay for!

As for the iron....beware that it is quite an inductive load....which switch modes (I suspect its a switch mode?) dont like...

[Ken T]

The step down is actually a transformer, and OK, there might be something abut a transformer driving a transformer I am not aware of ( I did this theory far too many years ago!), but..
Its a pain knowing you have to finish soldering in 10mins !.

Ken