How to sense a when a mains changeover thermostat operates

[brian_mk]

The problem seems simple enough:-

I have a thermostat that operates a 230V AC mains pump.
The thermostat is a changeover switch: The pump can either be connected to permanent live or to a switched live.
I want to add an indicator that shows which way the switch is connected i.e. is the thermostat ON or OFF?

The only way I can think of doing this is to sense the current flow in either the permanent live wire or the switched live wire to the thermostat.

Searching the internet for AC current sensing throws up possible solutions using a current transformer or Hall effect sensor.
There are the inevitable Chinese modules available on eBay - some are intended to connect to an Arduino ADC and provide a voltage proportional to current.
I don't need anything that sophisticated or complicated.

I could design something using a ZMCT103C transformer and an op amp to drive a LED but that means including a power supply.
There isn't much space available next to the thermostat and I don't want external wires if I can avoid it.

I should add that the pump has three speed settings. The current for each speed is :-
1=0.12A
2=0.27A
3=0.42A
An indicator that shows when the current is above, say 0.1A should suffice.

I did come across a suggestion below that may work, but it looks potentially dangerous!

Is there a better/safer option?

[Gyro]

This is going to sound low tech, but a good old mains Neon lindicator across the thermostat contacts would be the simplest and, almost certainly, safest solution.

I'm not quite clear about the various possible states (PL vs SL, where the SL is switched, etc.) but you have the options of putting multiple neons across both sides of the changeover contacts, SL to Neutral, and across the pump itself, depending on what makes most sense. The ~1mA current of the Neon indicator won't affect operation of the pump.


P.S. Regarding the schematic that you show, yes it will work and I have seen it used before. I would add a ceramic capacitor (maybe 10nF) across it to avoid any possibility of EMI generation from the diodes switching though.

Sensing current through the pump itself doesn't really buy you anything unless you have the need to quantify it.

[themadhippy]

Or put the neon across the switch contacts in an old skool type of way,neon on switch open,neon off switch closed,no neutrals required.

[Gyro]

Yes, sorry, that's what I was suggesting for the relay thermostat contacts (not quite clear).

[brian_mk]

Something like a neon would be perfect but, I don't see how that can work it this case:-

In normal operation, permanent live (L) and switched live (SL) are both at 230V.
If you connect a neon between L and SL, then the neon will always be OFF irrespective of whether the thermostat is ON or OFF.
If you connect a neon to the pump then the neon will always be ON irrespective of whether the thermostat is ON or OFF.

To clarify the problem, the thermostat controls the pump overrun function in a heating system.
It is intended to keep the pump running for a while after the burner switches off to prevent the water temp in the heat exchanger from rising after the water circulation stops.
The pump overrun thermostat switches over from SL to L when the water temperature in the boiler exceeds a threshold.
I think this is usually set at around 70 degC.
This normally happens some time before the room thermostat switches off the boiler so that when the boiler does switch off, the pump is already connected to L.
The pump therefore continues to run after the boiler turns off until the water temp in the heat exchanger again drops below the threshold.

Because the overrun thermostat is electro-mechanical, the switching temperature set point drifts over time.
If the threshold becomes too high (e.g. 90 degC), the pump switches off at the same time as the boiler.
This higher than normal temperature can result in a bubble of water vapour in the heat exchanger suddenly erupting up the expansion pipe causing a loud 'bang' in the pipework.

Some sort of indicator will allow me to monitor if the overrun thermostat is working correctly.
What I want is to be able to view the thermostat ON/OFF state before SL switches off.

[edavid]

Use the thermostat switch to drive a relay instead of the pump.

Use one set of relay contacts to drive your indicator.

Use another set of relay contacts to drive the pump.

[brian_mk]

Use the thermostat switch to drive a relay instead of the pump.

Use one set of relay contacts to drive your indicator.

Use another set of relay contacts to drive the pump.

Brilliant. That should work. A lot simpler than current sensing and no power supply needed.