EDIT For clarity please see the answer from Richard Crowley. Essentially I want to achieve his circuit with an automatic switch via a pre-set max temp. Really appreciate all the help so far. Thankyou all ******

So I want to vent my home server room as it is getting toasty. I have been running a couple of fans rigged into a homemade vent system to allow airflow. I use a voltage regulator to control fan speed and they have been fine but now cant handle the midday heat- fine rest of time.

What I want to do is keep them wired as they are but add in another power loop with a higher voltage wired through a temp activated switch. Unless I've messed my logic this should allow current cooling for most of the time but when it gets toasty the switch should trip and the fans will get a power boost= more CFM= more cooling till temp drops then back to normal again.

enter image description here

Will this work? thanks

  • \$\begingroup\$ Will the fans work at this higher voltage without smoking? \$\endgroup\$ – Andy aka Jul 31 '16 at 19:36
  • \$\begingroup\$ So you basically want to build a control system that speeds up the Fan when the temp. is higher than a specific value ? \$\endgroup\$ – Elbehery Jul 31 '16 at 19:38
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    \$\begingroup\$ The diagram makes no sense, you're shorting across ground? What's the max voltage rating for the fans? What voltage are they running from now? \$\endgroup\$ – John D Jul 31 '16 at 19:57
  • \$\begingroup\$ Yes Abdo.Sorry I'm really not that technical if you hadn't guessed. The % refer to how much power I will allow through the variable resistors, so continuously @ 50% resistance (so 50% max cfm) and then when the switch trips the fan will ALSO get power from a zero resistance source. clearer? \$\endgroup\$ – mary Jul 31 '16 at 20:01
  • \$\begingroup\$ Take a look at using a fan control IC + temperature regulator such as my company's MAX1669 \$\endgroup\$ – MarkU Jul 31 '16 at 20:26


simulate this circuit – Schematic created using CircuitLab

Assuming that your lower and higher voltages are compatible with your fan (which only you know based on the facts in evidence). You can use a double-throw switch (or relay, etc.) to connect the fan to EITHER the lower or the higher voltage.

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  • \$\begingroup\$ Sorry yes the fans are speed controllable via voltage. And while I really like this solution I does require me to set the higher speed manually, something I'd rather automate. Is there an easy way to incorporate my temp activated switch? (I have one already as hubby keeps snakes....) \$\endgroup\$ – mary Aug 1 '16 at 21:12
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    \$\begingroup\$ SW1 is your thermostat. You need one with a changeover contact as shown. \$\endgroup\$ – Transistor Aug 1 '16 at 21:36
  • \$\begingroup\$ You can connect your thermostat to a SPDT relay which will replace SW1 in the circuit above. When the thermostat is "off", the relay will connect the fan motor to V1. And when the thermostat turns "on", it pulls in the relay to connect the fan motor to V2 (as shown in the diagram). \$\endgroup\$ – Richard Crowley Aug 2 '16 at 1:17
  • \$\begingroup\$ Richard thankyou for piercing through my ineptitude and finding the solution for me. If I could be just a little bit more needy- finding a SPDT (thankyou google) is a simple case of checking the max load of the SPDT is above the draw of my current fan system correct? \$\endgroup\$ – mary Aug 2 '16 at 15:56
  • \$\begingroup\$ There are several factors to selecting a proper relay. What is the nature of your thermostat? Is is a contact-closure? What is the rating of the thermostat contacts? That will help select what coil rating for the relay. How much power (volts and amps or watts) does your fan operate on? That will help select the contact ratings of the relay. \$\endgroup\$ – Richard Crowley Aug 2 '16 at 16:15

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