240v fan controller from 12v?

[guandi]

i'm looking to whack 2 120mm 240v beast's on my heatercore, but i want to be able to controll them with my hardcano rheobus.

i have had a proffesional electrician tell me it cant be done. i dont belive him. i reckon it might be possible with some kind of relay, so that the voltage accross the 12v line is directly proportional to the voltage accross the 240v line (12v=240v, 10v=200v, 6v=120v ect.) i have no clue how to do this :shrug:

any ideas guys? come on, i know your all genius's... or genii?

[Levish]

Off the hardcano it'd be rather hard since i doubt it can handle the voltage/wattage/amperage of those big 240v fans

[AntiBling]

If big fans is what you're after you might try using some 24Vdc 170mm Comair Rotrons, or 170mm Pabst like I have. It would be much easier to rig that up. My Pabst fans are spec'ed from 12-24Vdc and will run all the way down to 7 volts although they need closer to 10 to start. ( I have a seperate starter circuit, but thats another story ) The only advantage I can see to having the AC powered fans is greater flowrate and pehaps mains power vs a power supply. But I can tell you that you wont need more than the DC powered 170mm fans have to offer. I have 3 in my system, 2 push/pull for the heatercore and 1 more to ventilate the case. When they are turned up to 12V my computer room sounds like a walk in cooler. I leave them spinning at a leisurely 1000RPM at about 8 volts, which keeps things nice and quiet and still plenty ventilated. You're going a bit over the top with the AC fans IMO but whatever, its your project. You might need a variac or something similar. I wouldnt try a light switch dimmer if thats what you are thinking... that could be bad.

[guandi]

i have been thinking about his at work all day and its driving me nuts, i know the rheo on its own cant do it (duh ) and i know i cant put 240v through the rheo, but if i controll the voltage through a 12v circuit, maybe i can use that curcuit to control some kind of relay. i know a light dimmer switch wouldnt be any good becouse a lighbulb is a constant flow and a fan pulses. gah. this is driving me nuts! (oh wait, i said that allready) :shrug:

[guandi]

i think what i will have to do is use the rheostat to control the voltage to a series of resitors and relays. it would only have a few "steps" as it were of speed for the 240's, but i could live with that.

oh well, back to the drawing board

[pdf27]

Something along the lines of a circuit that has say 6 transistors set to trip at various input voltages would probably work. Say if you put the voltage from the fanbus into these transistors at 6v and set half of them to turn on. This could then turn on 3 relays.
If you've got each relay in series with a really big resistor (say such that it will turn on with 1 resistor in series - just make sure they can take the current), and wire all the relays up in parallell then increasing the number of relays turned on will reduce the resistance across the control circuit, and hence increase the current through the fan.
If you can be bothered to sit there and work out all the resistor values, you could extend the number of transistors/relays until you get a reasonably linear control system.

[guandi]

yeah, thats exactly what i mean

i wont bother working it out, i found a web applet a while ago that does that sort of thing all for me (i flunked electronics at school) all i have to do is find it again.

thanks