Mains current flow indicator

[nick]

Have I got the right term? I'm looking for something that might fit between an electric water pump and the mains socket and light up whenever the pump is in operation.

Most of the time - for days at a time - the pump runs completely normally but there are some anomalies in the communal water supply and I think this is what's causing the pump to occasionally get stuck into a phase where it cycles between running and stopping continually with less than a second between cycles - a hysteresis loop, maybe? Or could it be something to do with the installation? It's only about three months old.

I can hear the pump when it gets into difficulties but the hearing-impaired householder can't - that's why I think a visual indication would be helpful. I can't think that this cycling is being very kind to diaphragm and switchgear and if there's an indication that something is going wrong, the pump can be temporarily disconnected.

Any suggestions?

Thanks,

Nick

[Rod Speed]

nick [e-mail] wrote

Sounds like cavitation is happening.

I'd try a different pump.

[John Rumm]

What causes the pump to switch on in the first place?

Here is one:

or something like:

Could be simplified to drive a lamp or a relay to give an on / off indication. (having the smarts in the microcontroller would allow you to sense the particular problem condition, since it sounds like it should be able to see and interpret the pulsing load when it gets into its error state)

[Tim+]

If the pump is triggered by a pressure switch I think I’d be looking for a leak, either a non-closing cistern valve or perhaps a failing non-return valve somewhere which is allowing the pressure to drop.

Tim

[Pancho]

Not quite what you asked, but possibly helpful....

Athom smart wifi plug, web interface, reports power over wifi, acts as wifi switch, allows multiple switch timers.

Runs Tasmota, preinstalled, also send data to a message queue (mqtt) for use by software.

Combined with free software like Domoticz you can graph power usage, when it turns on and off, how much power is being used.

Pretty good for just over a tenner.

[The Natural Philosopher]

Sure. The easiests way to monitor current flow is a resistor in the pump feed. If you calculate that carefully and then use a larger resistor and a diode to get a sort of current limited DC out of that, and pop a LED in there it will light up when the pump draws current. There are many many ways to do this and the way you select depends on you knowledge skill and exactly what you want to measure!

[Nick Odell]

A little black box attached to the pump :-) I can hear a microswitch inside so I presume it uses a diaphragm to actuate it. It switches on when a tap is opened and water flows through the heater and switches off when it doesn't

Both very interesting ideas but for the immediate purpose a little high on the sledgehammer/nut scale, in my opinion.

I know that saying this may lead to my immediate expulsion from the forum but I think I would prefer something ready-made in this instance. I'll explain why when I reply to TNP's post below.

Am I using the correct terms to describe what I want?

Thanks,

Nick

[Nick Odell]

It's in an apartment building with an ancient and failing water supply and the problem is almost certainly related to pressure variations but initiated elsewhere. The correct answer is to fix the building but that's easier said than done with so many interested parties who fail to agree on so much. A local solution such as unplugging the pump when the system is misbehaving would appear to be the short-term solution.

Thanks,

Nick

[Nick Odell]

Interesting but again, I think overegging the pudding in this instance

Thanks,

Nick

[Nick Odell]

That's exactly the sort of thing that I mean. All it has to do is light up an indicator of some sort when it is working. Is there a ready-made product anyone can suggest?

It's exactly the sort of thing I would knock-up myself from bits in my spares boxes if I were at home, but I'm not. I'm not aware of any physical component shops around (which doesn't mean they don't exist) but everything seems to happen on an on-line market-place which I can't access.[1]

Thanks

Nick [1]Not because I'm dumb[2] but because it's only possible to access on-line shopping here with a local identity number and local bank/card accounts neither of which I have.

[2]Although I probably am too.

[Chris J Dixon]

That sounds very like the same sort of situation that can arise with my caravan water pump, which is complicated by the voltage supplied depending upon the state of the battery and whether I am on mains hook-up.

In my case the pressure switch adjustment is accessible, so I can do my best to optimise matters. Rapid cycling (where the pump shuts off as soon as water flows) can be damped with a pneumatic accumulator. The worst state is if the pump cannot produce enough pressure to switch off the supply, and to ensure I notice this I have a tiny sounder in parallel with the pump supply.

I guess you don't safely have access to the actual pump electrical feed?

Chris

[Nick Odell]

It's plugged into the electricity supply with a regular connector but I don't have access to the black box it goes into, no. The pump was an add-on to the water heater and is under guarantee so I don't want to mess around with it - especially as my instinct says the pump is doing what it shoould do and merely responding to the situation in the water supply which is out of our control.

Nick

[Clive Arthur]

Just a thought which others who know more about these things may pick up. Essentially, an RCD contains a transformer and a relay. The transformer has three windings, one to power a relay and two to carry current, live and neutral. The current windings are arranged such that under normal circumstances with live and neutral currents the same, the flux cancels and there's nowt to power the relay.

If you connect just one of the sense coils, say in the live wire, the relay should operate when there's live current flowing, but of course not when there isn't.

None of this takes into account the mechanical latching and switching stuff or wiring, and it may be too difficult, but I'm sure someone here has had these apart.

[Brian]

Hmm, checking for current flow may give a misleading result.

If the pump ‘stalls’ it may well still draw current, depending on the design, possibly more than normal.

What you really need to detect is a change in water flow. You can get simple ‘paddle wheel’ flow detectors which give an electronic signal - some give a pulse, others a sine wave.

[Tim Lamb]

In message [e-mail], Nick Odell [e-mail]> writes

Point of interest rather than a solution.. the farmhouse was mostly gravity fed water from a loft header. The shower unit was fed by a pump. Opening the shower head valve created enough gravity flow to move a magnet in the pipes and operate an external reed switch. This was used to energise the pump circuit. The new owners are changing to a pressurised system so it is likely long gone in a skip!

[Clive Arthur]

Can you measure the pump current when working?

Let's say it's 1A. A 12V 50W car bulb has a hot resistance of about 3 ohms, much less when cold, lets call it 1 ohm. So with 1A through it, it'll drop 1V. Not enough to light, but in the right town, if not ball-park.

So, try a selection of low voltage lamps in series with the pump live. You may need some in parallel. If you can get a lamp to glow significantly dimmer than intended, it will last for a long time. Old

6V bulbs may be even better.

It's not easily calculable because of the highly non-linear lamp resistance, but should be a fairly easy experiment.

[Cursitor Doom]

You only need some indication that mains voltage is present across the pump, by the sound of it. In such a case I just use an LED. Do a few simple sums to work out the resistance required to supply the LED with its rated current and chain some resistors together so you don't get too much potential across any one resistor. Then heat shrink a tube around the resistors. Yeah, I know it's not 'up to code' but I'm old school and it works for me so I don't give a shit.

[nothanks]

Get yourself an old shower pump and repurpose the sensor

[Tim Lamb]

Going further back.. the pump is intended to overcome a shortfall in water pressure. Could it be that other flats have adopted the same solution and occasionally there is simply insufficient water to feed the pump?

[Animal]

afaik no ready made product exists. So some homebrew options:

1. Relay. Measure pump current, pick low voltage ac relay with suitable coil current, use it to light a lamp.
2. 12v (or less) LED in series with bridge rectifier. Pump limits current, pick suitable LED.
3. Ammeter in series. If electronic it must stay powered up. I'm sure there are more