Bi-directional DC ELV Control Circuit via 3-core cable

[JohnW2]

@JohnW2: I'm glad you're happy. I don't think it's easy to think about because there are so many different levels of physics involved. From 'conventional' current, to what actually persuades an electron to move. Potential gradient at that 'physics' level is a rather obscure characteristic.

Indeed, but ...

... At a classical level, I think that accepting that all wire has a finite resistance helps somewhat. At least with that in mind, there is always an uphill and downhill.

... except that (as is the case in the 'balanced' scenario) if there is zero current, then resistance becomes irrelevant, and uphill and downhill therefore again become less easy to think about. That's why I moved to the 'unbalanced' situation (with one relay drawing x A and the other drawing y A) to help me understand. In fact, that was just an algebraic generalisation of what I first did to 'satisfy myself', with currents of 1 A and 3 A.

As I said, the silly thing is that it was so 'intuitively obvious' that it would work, that I would simply have 'done it', had I not started thinking!

Kind Regards, John

 

[Harry Bloomfield]

Mind you, on reflection, I'm not sure why the cable length should have a bearing on that potential problem - do you know what mechanism is/was responsible for what you describe?

Kind Regards, John

Cable capacitance, the cable charged up and the charge was enough to hold the relay in. Beware of using a Megger on a long cable

 

[DetlefSchmitz]

Cable capacitance, the cable charged up and the charge was enough to hold the relay in.

Well. Not forever. Needs some sums I suppose.

 

[JohnW2]

Cable capacitance, the cable charged up and the charge was enough to hold the relay in.

Good grief - it would surely have to be countess miles long before the capacitance became high enough to store enough charge to hold the relay in for even just a few milliseconds, wouldn't it?

Kind Regards, John

 

[JohnW2]

Well. Not forever. Needs some sums I suppose.

Quite so. Apologies for any diabolic errors from hasty mental arithmetic, but at a first try ...

... assuming 100pF per metre and a (!2V) relay coil resistance of about 100Ω, I reckon that the stored charge in the capacitance would keep the relay 'held in' for approximately 10 μs per km of cable - so for about 10 mS with a 1000 km cable (London to Edinburgh?).

Kind REgards, John

 

[Harry Bloomfield]

This was some 50 years ago, a multi core cable and for some reason the supply went back and forth several times, passing through several control systems before arriving at the relay. All I can say for certain, is that relay was very reluctant to behave on its DC, but behaved perfectly when fed a diet of rectified AC. The relay powered a very large contactor, feeding an extremely large multi-speed motor, which drove a very large pump at Lynn Alaw (sp??). The motor used cams to set the speed and the Norwich motor manufacturer (Lawrence Scott??) had mucked up the order of the cams. All very embarrassing when it happened on commissioning day.. A massive motor refusing to run on any but full speed and which refused to stop under remote control

It used much higher impedance relays than 100 Ohm..

Electrical controls were so much more basic back then..

 

[JohnW2]

This was some 50 years ago, a multi core cable and for some reason the supply went back and forth several times, passing through several control systems before arriving at the relay. All I can say for certain, is that relay was very reluctant to behave on its DC, but behaved perfectly when fed a diet of rectified AC.

I'm not doubting that it happened as you've described, but I wonder about the mechanism, since cable capacitance does not seem very credible.

DC-powered relays certainly can suffer from release problems, particularly if they are left energiseΩd for long periods, at least partially because they can become magnetised. However, that would be the same with locally-rectified DC as with current that had travelled as DC for very long distances.

It used much higher impedance relays than 100 Ohm.

Yes, that will obviously depend on voltage. For example, if it were 'mains' voltage, it would probably be more like 2,500Ω. However, if my calculations were correct, that would still mean that the energy stored in a 1000 km cable (on reflection, probably "Lands End to John O'Groats", rather than London-Edinburgh!!) would only hold hold the relay in for about 250 mS (quarter of a second).

I therefore suspect that there is some other explanation for what you observed, although I don't know what!

Kind Regards, John

 

[DetlefSchmitz]

For the record John, my spice simulation bears your calculation out. You didn't take inductance into account, but I can't see it making that much difference. Who knows what was going on.

 

[JohnW2]

For the record John, my spice simulation bears your calculation out.

Thanks. However, given the easy numbers I picked, it was far from rocket science - simply multiplying together various powers of 10 (i.e. just adding together the powers), which even I can usually get right in my head ... (pasted in, since the forum software appears not to understand the [sup]...[/sup] BBcode construct!) ...

You didn't take inductance into account, but I can't see it making that much difference.

That would be because no-one had mentioned inductance, but it's effect would presumably be to diminish the effect of the capacitance.

Who knows what was going on.

Quite so. As I've said to him, I'm certainly not doubting what Harry observed, but I do seriously doubt that cable capacitance had anything to do with it.

Kind Regards, John

 

[reds42]

Depending on cable type the capacitance could be higher than 100pF per meter.

Maybe it wasn't 240V cable and the insulation thinner so conductors would be much closer together increasing the capacitance. From a quick google 100pF is ball park for twin and earth where conductors are quite far apart.

 

[JohnW2]

Depending on cable type the capacitance could be higher than 100pF per meter.

It could - or lower. I obviously had to pick some 'ballpark" figure for my rough calculations (and chose a nice round one!).

However, don't forget that my illustrative calculations were using ridiculous cable lengths (1,000 km) and, even then, were resulting in extremely short discharge times. To store enough charge to keep a relay 'held in' for just a few seconds would need the capacitance to be tens or hundreds of thousands of times greater than 100pF per meter (i.e. maybe 1-10 μF/m) with sensible cable lengths (and quite probably an order of magnitude higher than even that for any cable length that was 'sensible' in the context of a DIY forum ).

Kind Regards, John

 

[Harry Bloomfield]

I therefore suspect that there is some other explanation for what you observed, although I don't know what!

Kind Regards, John

As said, a very long time ago and precise memory fades. What I do remember was the urgent need to find a quick fix for the issue, so it was a suck it and see - that worked, with no possibility of experimenting further - those pumps were needed. There might have been some induced voltage on the lines too. I do remember the relay was happy to drop out when the coil was disconnected, but would not pull itself in if reconnected, until the control signal arrived. Likewise with the speed cams on the motors - The manufacturer had been called down all the way from Norwich to see if they could work out what was wrong, they failed so went back to Norwich to consult their design team. I slept on it and developed a hunch that the speed cam angles had been set completely backward, so next morning I took a lump hammer and tapped them round on one motor set first thing next morning and it worked as it should, so I set the other three up the same.

They had timers like fancy electric clocks, which ran the pumps up briefly if they had not run for a day, to avoid them seizing and damage to the bearings from sitting in one spot too long. They also had heating systems built into the motors to keep the motors warm and prevent condensation - heaters are normal on such motors.

All good engineering fun.

 

[JohnW2]

As said, a very long time ago and precise memory fades.

You don't need to tell me about that problem

What I do remember was the urgent need to find a quick fix for the issue, so it was a suck it and see - that worked, with no possibility of experimenting further - those pumps were needed.

Fair enough - that is very understandable.

There might have been some induced voltage on the lines too.

That certainly went through my mind, but that line of thought merely leads to even more curiosity and potential confusion. If anything was getting induced into lines, it would have to be AC, but ....

... I do remember the relay was happy to drop out when the coil was disconnected, but would not pull itself in if reconnected ....

... which sounds very much like a 'DC phenomenon' - I struggle to think how that could happen if induced AC were the problem!

... so next morning I took a lump hammer and tapped them round on one motor set first thing next morning and it worked as it should, so I set the other three up the same. .... All good engineering fun.

Indeed. Tapping with a hammer is a tried-and-tested procedure -) ), but unfortunately not all that often applicable to electrical problems.

Kind Regards, John

 

[bernardgreen]

since cable capacitance does not seem very credible.

When a relay contact is used to switch a voltage onto a long cable the life expectancy of the contact is greatly extended by fitting a resistor between the contact and the cable. This limits the current through the contact when it closes and has to charge or dis-charge the energy stored in the capacitance of the cable.

As to the original question I am not clear what is required. Doe the remote relay have to indicate the state of the mains supply at the near end and also why is the PSU permanently connected to the cable.

Could you use two wires for the A to B circuit and the third wire and Ground for the B to A circuit

 

[JohnW2]

When a relay contact is used to switch a voltage onto a long cable the life expectancy of the contact is greatly extended by fitting a resistor between the contact and the cable. This limits the current through the contact when it closes and has to charge or dis-charge the energy stored in the capacitance of the cable.

That's obviously not got anything directly to do with what we are discussing (I haven't even mentioned what the relay contacts do, and they certainly don't "switch a voltage onto a long cable") and, in any event what you say applies whenever there is capacitance across the load side of a switch, relay contacts or whatever.

As to the original question I am not clear what is required. Doe the remote relay have to indicate the state of the mains supply at the near end ...

Essentially yes. The upper diagram below shows the very simple system I could have used if I hadn't wanted to avoid different phases ending up in the same part of the house. The lower diagram shows the actual situation, with the 230V AC 'converted to' 12V DC for transmission

... and also why is the PSU permanently connected to the cable.

See above (and lower diagram below). Such a 'permanent connection' is an implicit requirement of such an arrangement.

Could you use two wires for the A to B circuit and the third wire and Ground for the B to A circuit

Are you postulating an explicit fourth conductor ('ground'). It is 3-core flex, so has no additional 'ground' within the cable, and the 12V signalling voltages are 'floating'. I suppose I could use the house's CPC network as a common 'ground' (in which case I could have three separate control circuits) - but there are all sorts of reasons why I don't think that would be a very good idea!

Kind Regards, John