Electronics help wanted

[Tony Brooks]

I am way out of my depth and Mr Google and Yahoo are not much help.

 

The Alde 292x room stat has what I take to be an arc suppressor of some sort across its contacts. I am modifying the wiring to incorporate a cylinder stat but all the readily available ones are only AC rated at around 3 to 5 amps with no obvious arc suppression so I want to fit a capacitor across the contacts or at a push a diode. The Alde motor draws up to 200mA.

 

So brainy people can you give any idea for the value or designation of the part I require.

 

I do not want to use a relay because it is likely to be energised for long periods and, although small, demand an extra current draw.

 

For completeness I have already fitted a valve so I can direct the boiler out put to calorifier only or radiators and calorifier. It works well except to heat just the calorifier I have to turn the room stat to maximum and then rely upon the boiler stat to shut the boiler down as required. This produces water rather hotter than ideal so probably uses more gas than necessary Hence my desire for a cylinder stat.

 

 

Thanks all

[nicknorman]

So do we understand this is a 12v system? And you want to suppress arcing caused by the (presumably) inductive load of the gas control coil?

 

If so I would suggest a capacitor is not the best solution since it tends to cause ringing. Rather I would use a TransZorb which is a bit like a zener diode but with an exceptionally fast turn-on time and high short term current capability. You would need to choose one with a voltage knee that is higher than the max possible system voltage (say 16v). You then fit it so it's reverse biased across the contacts. Job done.

 

Ed: maybe something like this:

 

http://uk.rs-online.com/web/p/tvs-diodes/6255414/

 

With RS you can set up an account on line, order stuff for collection at your local RS trade counter with no minimum order or delivery charge. You do have to buy 5 of those diodes but at 8p each that's not too bad.

 

 

Edit again: actually the voltage rating of that one is a bit low, try this one:

 

http://uk.rs-online.com/web/p/tvs-diodes/6255240/

 

More expensive but chunkier and you only have to buy 1.

Edited February 29, 2016 by nicknorman

[dmr]

Maybe a fair bit higher than 16v. 16 could possibly be reached during an equalisation and I assume a transorb with significantly higher voltage would still do the job????

 

..............Dave

[Tony Brooks]

Thanks Nick but, no, it is not the gas control coil. I think on the 292x series that is mechanical. This is the inductive load caused by the motor being turned off and the official room stat definitely seems to use a capacitor or something very like one (no easily visible markings).

 

I did find a suggestion that something like a diode 1N4001 to 4007 could be used but I do not altogether trust what you can find on the internet.

[bizzard]

Would not an old 12v car coil ignition distributor contact breaker capacitor, we called em condensers do the trick. Most were rated at around 500volt.

[nicknorman]

Maybe a fair bit higher than 16v. 16 could possibly be reached during an equalisation and I assume a transorb with significantly higher voltage would still do the job????

 

..............Dave

But don't forget it's just across a switch so worst case scenario is that a volt or two is sent to the load, hardly catastrophic. I think I would be more concerned about the effect of 16v on other equipment left on (which is why it's best to isolate everything before equalising).

 

but ultimately yes, you could select one with a higher voltage without it making it less effective.

Thanks Nick but, no, it is not the gas control coil. I think on the 292x series that is mechanical. This is the inductive load caused by the motor being turned off and the official room stat definitely seems to use a capacitor or something very like one (no easily visible markings).

 

I did find a suggestion that something like a diode 1N4001 to 4007 could be used but I do not altogether trust what you can find on the internet.

Ok motor load then, same difference! I would suspect that the thing that you are seeing as a capacitor may be a varistor, but who knows!

 

Anyway yes you could just use a freewheel diode that allows the current to decay (relatively) slowly but I have found transzorbs to be better in terms of RFI supression (which, I accept, is not what your concern is). Bearing in mind the rating of the proposed switch and the motor I doubt you need anything but if you have a diode to hand use that, if you are going to buy something it might as well be a transzorb of equivalent transient supression diode (transzorb is a trade name, sorry!).

 

Ps a capacitor is not the best idea because with the switch open, the capacitor will have 12v across it and when the switch closes it will short the capacitor and put a large (though very short duration) current through the contacts.

Edited February 29, 2016 by nicknorman

[Sir Nibble]

Couple of small magnets to blow out the arc? I don't think a dc motor can be considered "reactive" in the conventional sense the issue being more with back emf. Is it a series motor? That should collapse electrically as the contact opens.

[dmr]

But don't forget it's just across a switch so worst case scenario is that a volt or two is sent to the load, hardly catastrophic. I think I would be more concerned about the effect of 16v on other equipment left on (which is why it's best to isolate everything before equalising).

 

but ultimately yes, you could select one with a higher voltage without it making it less effective.

Ok motor load then, same difference! I would suspect that the thing that you are seeing as a capacitor may be a varistor, but who knows!

 

Anyway yes you could just use a freewheel diode that allows the current to decay (relatively) slowly but I have found transzorbs to be better in terms of RFI supression (which, I accept, is not what your concern is). Bearing in mind the rating of the proposed switch and the motor I doubt you need anything but if you have a diode to hand use that, if you are going to buy something it might as well be a transzorb of equivalent transient supression diode (transzorb is a trade name, sorry!).

 

Ps a capacitor is not the best idea because with the switch open, the capacitor will have 12v across it and when the switch closes it will short the capacitor and put a large (though very short duration) current through the contacts.

 

I've never used a Transorb, they were not really around when I was doing lots of electronic design. How low is the resistance after they turn on?

Surely with the switch off it will see the full supply voltage so if it turns on it will effectively be a short circuit till the motor starts turning?

 

Its good to turn stuff off when equalising but as I am sure you know, if an accident can happen then it certainly will!!

 

..............Dave

 

 

and how much energy can irn on

[nicknorman]

I've never used a Transorb, they were not really around when I was doing lots of electronic design. How low is the resistance after they turn on?

Surely with the switch off it will see the full supply voltage so if it turns on it will effectively be a short circuit till the motor starts turning?

 

Its good to turn stuff off when equalising but as I am sure you know, if an accident can happen then it certainly will!!

 

..............Dave

 

 

and how much energy can irn on

Transzorb-type things are great because the turn on so quickly, less than 1nS for the one I linked to but for some it's measured in 10s of pS. They don't really have an "on resistance" as such, but the spec shows it starts to conduct around 18v and the voltage doesn't exceed 25v at 60A.

 

Many years ago I had a Subaru without remote central locking (just central locking from turning the key) so I made a 8 pin PIC microcontroller circuit to lock and unlock the doors, flash the indicators etc when triggered by a Maplin remote control set. The switching of the lock motor and the lights was done by relays driven by the PIC but it was hopeless, the transients from the relays just crashed the PIC. I tried various supression techniques until I filtted Transzorbs - problem solved and it went on to work flawlessly for many years.

[steamcompound]

Might I ask how many wires go into the thermostat? 2 or 3?

[Tony Brooks]

Might I ask how many wires go into the thermostat? 2 or 3?

Two

 

 

Couple of small magnets to blow out the arc? I don't think a dc motor can be considered "reactive" in the conventional sense the issue being more with back emf. Is it a series motor? That should collapse electrically as the contact opens.

 

Almost certainly permanent Magnet

 

But don't forget it's just across a switch so worst case scenario is that a volt or two is sent to the load, hardly catastrophic. I think I would be more concerned about the effect of 16v on other equipment left on (which is why it's best to isolate everything before equalising).

 

but ultimately yes, you could select one with a higher voltage without it making it less effective.

Ok motor load then, same difference! I would suspect that the thing that you are seeing as a capacitor may be a varistor, but who knows!

 

Anyway yes you could just use a freewheel diode that allows the current to decay (relatively) slowly but I have found transzorbs to be better in terms of RFI supression (which, I accept, is not what your concern is). Bearing in mind the rating of the proposed switch and the motor I doubt you need anything but if you have a diode to hand use that, if you are going to buy something it might as well be a transzorb of equivalent transient supression diode (transzorb is a trade name, sorry!).

 

Ps a capacitor is not the best idea because with the switch open, the capacitor will have 12v across it and when the switch closes it will short the capacitor and put a large (though very short duration) current through the contacts.

 

Thanks agin.

 

I had intended to put the capacitor between the motor positive switched by the stat. and negative so, in my rather hazy idea of things, when the inductive spike tried to form it would flow into a non-polarised capacitor to quench it. Then hopefully the arc on the contacts would have stopped so the higher induced voltage would not have ionised air in the gap to continue the arcing. Then the capacitor could discharge thorough the motor. I will probably use a diode because the nearest RS Components is Watford or Southampton.

 

Anyway I have no worries over 16 volts as I do not equalise apart from when the solar does it at about 15 volts. the only other thing that might cause a higher voltage spike woudl be the water or shower pump turning off.

[WotEver]

My guess is that whatever is fitted in the room stat is there for EMI/RFI suppression as opposed to arc suppression to protect the contacts.

 

Ofcom got a bit sniffy with suppliers of room thermostats a few years back and most manufacturers now proudly state that all of their thermostats are suitably suppressed.

 

Tony

[Jon12345]

Does it have to be electrical? How about a Drayton Tap Stat/

 

A bit pricy new but there is a used on on ebay:

 

http://www.ebay.co.uk/itm/Drayton-tap-stat-/262301102464?hash=item3d125d3180:g:oR0AAOSwezVWyeyo

[Tony Brooks]

Does it have to be electrical? How about a Drayton Tap Stat/

 

A bit pricy new but there is a used on on ebay:

 

http://www.ebay.co.uk/itm/Drayton-tap-stat-/262301102464?hash=item3d125d3180:g:oR0AAOSwezVWyeyo

 

Thanks but that will not control the pump motor.

[Arthur Brown]

Simple contact spark suppression used to be 0.1uF in series with 100 ohms all paralleled across the contacts.

[Hairy Animal]

I do not want to use a relay because it is likely to be energised for long periods and, although small, demand an extra current draw.

 

It's significantly more expensive than a capacitor/condensor, but have you considered a latching relay?

 

It only requires a brief energisation pulse ( < 1 sec.) to switch states, and then stays in that state drawing no power, until another pulse switches it back. Having said that, it might be more complicated to arrange a suitable pulse instead of a continuous voltage/current.

 

Just a thought ....

[smileypete]

I take it this is a DC pump motor on a boat, so...

 

Could put a reversed biased diode across the DC motor, in the same way is done with a relay coil when driving it from a transistor:

 

(http://www.davidbridgen.com/coil%20diode.htm)

I'd rate the diode to a reasonable margin above the max current drawn by the pump.

 

More and more small DC pumps these days are brushless with built in transistors driving the phase coils, so for these types I doubt they'll cause any arc.

 

cheers, Pete.

~smpt~

Edited February 29, 2016 by smileypete

[smileypete]

Having said that, for a 200mA DC load I doubt the thermostat contacts will wear out within several lifetimes!

 

(Microswitches in freshwater pumps are an entirely different matter!)

 

cheers, Pete.

~smpt~

[WotEver]

Having said that, for a 200mA DC load I doubt the thermostat contacts will wear out within several lifetimes!

Yup, as I said above I believe that whatever Tony has on the thermostat has little to do with arcing and everything to do with RFI suppression.

 

Tony

[Tony Brooks]

Thanks all once again. Maybe I will stop worrying and try it without any extra gubbins.

[bizzard]

If its the tall skinny Alde Comfort boiler the motor is only about the size of an 00 gauge 12v model railway loco.