my fan is 0.7 amps 85 watts , what size of variac do i need?

one amp should do you the 0.75 ones from maplins is pushing it a bit

nice one , ta.

one of these http://www.maplin.co.uk/searchtemplate.asp...a=8AMP%20VARIAC

thats the fella.
now , is it possible to wire them up so that when lights are off they are live and run the fan at half speed , then , when lights come on the variac gets py-passed and the fan is on full pelt?

what do i need to do this?

whazzup would be able to answer your question, where are you whazzup?

the more i think about this , the more im thinking 'switching relay' triggered by a plug in thermostat but i know nothing about 240 volt systems.

that's the 0.75 amp one i said not to use

you can switch it using just a thermostat, but you need to make sure it's the right sort, some thermostats need live on a certain terminal to operate properly.

i suggest getting the bellows operated thermostat sold in b&q it's the most boggiest standard one they do, and it has both call for heat and satisfied (cooling) terminals, and is ideal for what you want

oops, sorry, see you said at lights on and off not temp, easy enough, and yeah you'll need a relay and a timer (and a lead socket and a few other bits n bobs)

so , if i use the b+q thermostat , this will switch from one live feed to another when the temps rise above the temp it is set for?

this means i can have it powering the variac when the temps are low , then , when the temps rise above the pre set it will switch over to the other live feed and power the fan direct?

that sounds much better.

i'd get a terminal box to mount the thermostat on and a couple of 20mm plastic cable glands and some 1.5mm 3core flex

cool ,but just to complicate things, do you know of any thermostats that have a fly lead sensor?
id like to keep the unit out side the grow area but at the same time read temps inside it.

not off the top of my head, they're not really used for ambient temp switching, and not likely to find one from a domestic supplier. google it

i found a remote sensor from honeywell ,its the ''Honeywell CM901 / CM907 Remote Sensor'', the sensor alone is 20 odd quid and its only compatible with
CM901/CM907 thermostats, which are programmable but weigh in at 40-50 quid.
i will keep searching , im sure there is a cheaper option.

i found this in a reptile shop.





my fan (150mm ruck) is only 80 odd watts , would it be compatible do you think?

Is 85w really 0.7A? Doesn't sound right. Would give my 13A sockets a rating of only 1500w.


To have a lower speed during lights off all you need is a relay (mains powered solenoid) plugged into the ballast socket.

With no power to the solenoid A+B make contact, When the coil is energised (lights on) current can flow through contacts B+C.

Live wires only, all neutrals common. Mains to variac to A. Fan to B. Mains to C.

sorry , my bad , its 0.3 amp , 65 - 80 watts.

if i cant find a cheapish thermostat with a remote sensor then i will go down the relay road , im not really fussed on having a thermostat in the grow area with the high humidity levels in there.

hehe so the 0,75 is good after all

There are some pretty good topics here how to build a fan controller with a variac. For the more "advanced" low-budget builders: you can actually build a two speed controller with one variac. All you need is one 2 way switch and one 2 way thermostat (with heat and cold contacts).

There are two ways of doing is. First one you have two selectable low speeds, and a continuously variable high speed. Second one is continuously variable low speed and 2 high speeds, whatever suits your cabinet or purpose best.

You just tap the desired high/low voltages directly from the coil. That's a precise work I would not recommend to any non-technical person btw. Better spend an extra 35 quid for an extra variac if you want it two way. You use the 2-way switch to select the low/high max and feed that to the thermostat, together with the output of the variac. The common of the thermostat goes to the fan.

That electronic thermostat is no good. It's ok for 100 wat resistive loads (a heat mat) but your fan is a inductive load. You would need an extra relay. Might well be that the contacts are not even suitable for inductive loads but scraglor knows a lot more about that.

Not too sure on the size of the Maplins variac but if small enough an old computer psu will make a cool project case for it. You'll need the old AT type that has a monitor socket as well as the mains in socket. Could do with a Dremel type tool or some patience with a hacksaw blade.

Rip out all the electronics and use the 2 sockets for in and out. With the lead that goes from power supply to monitor, cut off the kettle plug bit and wire it to a mains socket.

edit, just read the second post!