Is this article a Red Haddock ???

[ericmark]

There's your diode, should not use a three port valve with a type AC RCD I assume, but never seen a warning about it! But think the two port valves also stall the motor, so that does not seem to be a problem, the problem is micro switches sticking.

 

[JohnW2]

<diagram> .... There's your diode, should not use a three port valve with a type AC RCD I assume, but never seen a warning about it! But think the two port valves also stall the motor, so that does not seem to be a problem, the problem is micro switches sticking.

Thanks.

For a start, I have never had any dealings with a 3-port ('mid-position') valve, so have never really had any reason to discover how they work - so everything I've said has related to the simple 2-port ones with which I am very familiar. Having now looked, I have learned how the 3-port ones work, as outlined here (seemingly probably from the same source as your diagram):

The Honeywell V4073A three port, mid position, spring return valve, is a masterpiece of clever engineering. It manages to move to one of three positions using only a cheap non-reversible AC motor, a spring, a couple of micro-switches, a resistor and a diode, and act as a relay for the boiler into the bargain! However, it has obviously required quite a bit of lateral thinking to conjure up, and its operation is thus not easy to understand. Here's how it works.
..... The spring pulls the valve to open the flow through port B (traditionally connected to the hot water cylinder's heating coil), while the motor winds it towards opening port A (feeding the radiator circuit). If the motor is left continuously powered, it will stall in with port A open, but if it is fed with DC (produced with the resistor and the diode), then it will stall in any position. Two micro-switches, operating just either side of the 'A+B' point, are used to define this position.

That's pretty clever [and it subsequently goes on to explain in greater detail]! However, I'm still learning, and, in particular, haven't yet worked out exactly how/why the motor behaves in this way in response to AC/DC.

However, it seems fairly clear that the diode is only there to facilitate the 3-port functionality, so I presume that there is never such a diode in a 2-port one (which, as above, is what I'm familiar with and was talking about). Furthermore, contrary to what Harry suggested (and I couldn't understand), it does not seem that it is the diode which 'causes' the motor to stall - as usual, it seems that it probably stalls because it is (mechanically) stalled, the use of the diode being to determine what position it can stall in. Is that all correct?

It therefore seems as if, always in the case of an 'open' 2-port valve, and also in some situations with with a 3-port valve, the motor is, indeed, 'continuously powered but stalled (mechanically)' - which is the situation which several of us considered to be 'daft', and which presumably has some #negative' consequences.

 

[Harry Bloomfield]

That's really just a repeat of what you wrote before(which is something I hadn't previously heard of).

What is the diode meant to achieve, and in what sense is the motor 'stalled' when the diode is in-circuit?

John - might I suggest you try inserting a diode, in series with a clock motor, and see what happens to the motor?

 

[JohnW2]

John - might I suggest you try inserting a diode, in series with a clock motor, and see what happens to the motor?

I presume that it would stop working, as would happen with most things designed to run on AC when they were supplied with DC.

When supplied with DC, the motor would presumably continue to draw current (which would end up as heat) even though the motor was not 'working'. If that's what you meant when you said that the diode causes the motor to 'stall', then fair enough, the only issue then being of semantics/terminology.

However, none of that is relevant to the 2-port valves which I was thinking and talking about (which I presume never have diodes), and nor even to 3-port ones when in the 'heating only' state (when the diode is irrelevant) - so in all those situations we still have the situation of a 'powered but mechanically stalled' motor, which I think most of us are inclined to regard as rather daft?

 

[Harry Bloomfield]

the problem is micro switches sticking.

I have long/lot of experience with our three port actuator problems, and I only ever had a problem with the microswitch failing the once. More often, the problem was the mechanism simply jamming, because the motor power was only just adequate to turn the valve, and at the same time, wind up the spring return. Plastic parts, running against plastic, and brass, in a hot environment, with congealed grease - all under heavy stress from a strong spring.

The MOMO, I now use, was a vastly better design, little stress involved, and no reliance on a spring, and so much less wear.

There is a very knowledgeable discussion of such valves to be found here - https://www.seered.co.uk/sunvic.htm (towards the bottom).

The one problem, the slightly more complex PCB of the MOMO seems to suffer from, is that of water, if the actuator is not mounted above the valve.