Volt Free Relay

[CarlosF]

Hi. I have a problem with an ATAG boiler. ATAG can supply their own tank sensor to accompany their boiler or alternatively you can use a volt free contact. The electrician who originally installed the boiler has correctly installed a 240v 8 pin relay of which the coil is energised via programmer and tank stat and the terminals that require a volt free contact are connected across the N/O poles of the relay. In my opinion this should work fine. BUT the boiler keeps giving a error code of E14 which according to ATAG is unwanted voltage on the terminals. I went to site and checked and with all cables disconnected and the coil energised there is indeed between 1.5 and 2.6 volts on the contacts to earth. I can only think this is being produced by magnetic effects of the relay coil. I have since changed the relays for Schneider mini contactors and bar 0.2 volts the affect is the same. I have been told to try solid state relays by a colleague. Any other suggestions?? Carlos

 

[Marvo]

I have no experience with the actual boiler but if they specify a volt-free contact then I doubt a solid state relay will work. I'd check things like all other control board connections are correct and supply polarity and maybe the earth impedance of the supply circuit etc. Could the 1.5-2.6v you're seeing on the volt free contact wrt earth be a sense voltage originating from the control board.

 

[tazz]

The insulation in most voltage free contact relays is over 30megs, so I cannot see this votage coming off the relay...
suggest disconnecting the nc and com cables and testing the relay itself .....if ok this voltage is coming from boiler pcb.

 

[gnuuser]

most ssd relays are opto-coupler type and are not influenced by a magnetic field.
but the conductors connecting to it are subject to inductive pickup
some ssd's may be semiconductor type and you would have an errant voltage from them.
if you want to use an ssd refer to its manufacturers website for the relevant info on it
because you would want the opto-coupler type

 

[CarlosF]

The insulation in most voltage free contact relays is over 30megs, so I cannot see this votage coming off the relay...
suggest disconnecting the nc and com cables and testing the relay itself .....if ok this voltage is coming from boiler pcb.

I have tested the relay/contactor in normal (non energised state) with nothing connected to common or N/O poles and I have zero volts. When the coil is energised I get the 2 volts. So the stray voltage is coming from the relay and not the boiler pcb.
I have purchased a solid state relay and will try next week to see if this solves my problem.

 

[Lucien Nunes]

Measuring the voltage from an unconnected contact to earth does not tell you anything, except that your test meter has a good high input impedance. When the relay coil is energised, there will be a minute amount of coupling from the coil circuit to the contacts that will give you some sort of reading, as between any two cables or parts that are close together. Under working conditions that should not produce a troublesome voltage if the boiler input circuit is correctly designed.

If the cabling on the volt-free-side of the relay is run alongside that of the 240V, try separating the two circuits physically as much as possible. It might be important that both cores of the volt-free circuit run together in one cable, i.e. that they don't form a loop of single-core anywhere, as that is a common way for stray voltages to be induced in control circuits. If you can't improve the wiring layout, try reversing the connections to the boiler input, as one side of the input is probably much more sensitive to stray pickup than the other.

We had another thread recently where a boiler control input was being much too sensitive to ghost voltages. I have seen more of this recently, where simple design errors make it difficult to interface a machine to the real world because they haven't allowed for contact resistance, stray voltages, reverse polarity etc.

The pickup across the SS relay might be better, or worse. It's not especially an electromagnetic effect, unless you are winding the volt free circuit around the relay many times to make a pickup coil!

 

[CarlosF]

Measuring the voltage from an unconnected contact to earth does not tell you anything, except that your test meter has a good high input impedance. When the relay coil is energised, there will be a minute amount of coupling from the coil circuit to the contacts that will give you some sort of reading, as between any two cables or parts that are close together. Under working conditions that should not produce a troublesome voltage if the boiler input circuit is correctly designed.

If the cabling on the volt-free-side of the relay is run alongside that of the 240V, try separating the two circuits physically as much as possible. It might be important that both cores of the volt-free circuit run together in one cable, i.e. that they don't form a loop of single-core anywhere, as that is a common way for stray voltages to be induced in control circuits. If you can't improve the wiring layout, try reversing the connections to the boiler input, as one side of the input is probably much more sensitive to stray pickup than the other.

We had another thread recently where a boiler control input was being much too sensitive to ghost voltages. I have seen more of this recently, where simple design errors make it difficult to interface a machine to the real world because they haven't allowed for contact resistance, stray voltages, reverse polarity etc.

The pickup across the SS relay might be better, or worse. It's not especially an electromagnetic effect, unless you are winding the volt free circuit around the relay many times to make a pickup coil!

Well it's my last try with the SS relay anyway. We are also running the contact circuit in a screened pair beldon type cable to avoid any interference from other voltages.
Ill let you know.