Electrical help, please

[youngrichard]

There seem to be a number of electronic-savvy members of this forum, and I wonder if they could help.
I have motorised rockers for my trays (see Thread: Motorised Tray Rockers in this forum). Currently they are connected to clockwork timers, and they are powered separately by 115 volts AC from a single transformer.
Problem is they only time up to 60 seconds, and I now want to lengthen that time, because the cellar darkroom now gets cold in winter (new efficient boiler) and one minute is no longer enough for dev and fix for RC when the ambient temperature is down to 16 degrees C.
Now I have 2 electronic timers which can switch 240v AC, standard UK voltage. But they don't work on 115 volts so I can't use them directly to control the rocker motors. But it occurs to me that I should be able to switch the 115 volt motors on and off using separate relays switched by the 240 volt timers. The timers are enlarger timers and therefore should, I imagine, be able to switch up to 300 watts at least. The motors are quite powerful, about the size of a DIY electric drill motor, rated at 0.3 amps.
I could alternatively, I suppose, double up on the transformer, but I would like to explore the possibility of using relays.
Can anyone help me by saying 1. Whether it is possible? 2. If it is what sort of spec should I be looking for for the relays? 3. A circuit sketch would be helpful.
I can use a soldering iron, drill holes in cases, and have a pretty good idea how to avoid electrocuting myself.
Richard

 

[Chris Douglas]

Richard,

You say you are using a 115 volt transformer to power the rocker motors. Why not power the transformer from the 240 volt timer? If you need the transformer for other stuff, your relay idea is fine. Ice cube relays with 5 amp contacts will probably work fine. Get relays with 240 volt AC coils.

 

[Chan Tran]

It's definitey possibe. I think the easiest way is to plug the transformer into the switch output of the timer and power the rocker with the output of the transformer.

 

[youngrichard]

Richard,

You say you are using a 115 volt transformer to power the rocker motors. Why not power the transformer from the 240 volt timer? If you need the transformer for other stuff, your relay idea is fine. Ice cube relays with 5 amp contacts will probably work fine. Get relays with 240 volt AC coils.

Because I want to be able to control the rockers independently, a timer for each.
What's an ice cube relay?
Richard

 

[youngrichard]

It's definitey possibe. I think the easiest way is to plug the transformer into the switch output of the timer and power the rocker with the output of the transformer.

See above
Richard

 

[Chan Tran]

You have 2 rockers, 2 timers and 1 transformer?
Ok, In that case you can use the relay. I got it. It's possibe. But let me find a way that you can do it very easily

 

[youngrichard]

That would be great.
Richard

 

[Chan Tran]

After doing some research I don't find anything premade. So you will have to make it yourself. Get some small relay (they call them ice cube), socket, wire, male plug and outlet and some enclosure then you're good to go.
Something like this relay http://www.ia.omron.com/product/item/mys_2005d/index.html and the correct socket would do.

 

[polyglot]

Yes, this is very doable but I (as an electrical engineer) have to say that where mains electricity is concerned, "if you have to ask how to do it, you should not be doing it". The risks for fire and electrocution are significant and there are subtle things that can go wrong that you won't know you've done wrong until they kill you.

Plugging a transformer into the output of a timer module is completely fine. Once you start cutting and soldering and decide you need to put stuff in a box though, that's a recipe for trouble and I would suggest that it's much cheaper, easier and safer just to buy a second transformer to run off the output of your second timer. Hell, the second transformer costs on eBay about what you'd spend for a decent box.

 

[tkamiya]

I agree with polyglot. This is not something you want to take lightly. It can kill you. I am a licensed electrician.

Well.... it's technically possible but it's not worth the trouble. It's far easier and cheaper to just buy another transformer. It's also safer. You said yourself, you are not families with electronics. You will be using your creation near water and at 240 volt potential. It's not safe - worse, you won't know it when something isn't right - until you get a shock. That may very well be the last shock you get....

With that said...

I understand you want to control a 115 volt circuit using TWO 230 volt timers. (or is it just one timer?) Then what you do is... buy two relays that works on 230 volts AC (not the DC kind). Connect each relay's coil windings to its respective timers. Now you have two relays that goes on and off via two timers.

Now, take those N.O. (normally open) side of relay switches and tie them in parallel. Put that in series with the 115 volt circuit. This way, if one of the timer sends ON signal, the motor starts.

If you just want one timer, then forget the parallel part.

Please PLEASE be careful with this. You'll need to carefully ground all human contactable metal surfaces. None of the circuit should be at or near water area.

 

[youngrichard]

Done it

Well, it's up and running and I am delighted. Perhaps a second transformer would have been a simpler solution, but I have never incorporated relays in a circuit before, and I am so pleased with the result.
I appreciate your concerns about my inexperience in electrical/electronic ways but perhaps I undersold myself. I may not be able to design a circuit (I have followed Polyglot in his quest for an F-stop timer with admiration, and know T Kamiya also understands electronics). But I think I have a pretty healthy regard for the dangers of mains electricity especially when next to water, and understand insulation and secure connections. I am a recently retired Consultant Surgeon and wouldn't dream of switching anything on until I have been over it thoroughly with my AVO meter. So I know enough to understand your concern about amateurs dabbling with electricity; I thank you for those concerns and also your input which gave me the confidence to go ahead.
Richard

 

[polyglot]

Excellent! Perhaps you could post a photo of the wiring?

 

[Peter Simpson]

If you haven't got one already, a Ground-Fault Circuit Interrupter is a must for using AC near water. They can be had as short extension cords at your local DIY/"home improvement" store. Best, though, is installed as the wall outlet.

Running the whole setup off a transformer gives you line isolation, which is safer, should you ever decide to redo it. (you couldn't get shocked to earth ground, because the transformer isolates your circuits from AC line, which is referenced to earth ground)

 

[bobwysiwyg]

I second Peter's comment on the GFI. They are 'code' here within 6' of any water source.

 

[John Weinland]

All the electrical warnings are good advice. 120VAC here in the states is bad enough, but 230 sounds like dangerous territory indeed. Why not obtain a hobbyist step-down transformer to something like 6 or 12 volts, and obtain a motor to suit? A lot safer all around, and hobby stores (don't know what you call them in the UK) have rafts of wonderful low voltage gadgets, timers, motors, train gear, airplane gear, and so forth just waiting for non-standard applications.