i have a circuit with an output of 24v when active and 4v when in stand by. i need to power a 12v fan only when the circuit is in the 24v phase. how would i go about creating this additional circuit?

if this has been covered before, please link me to the thread. i searched but couldn't find anything similar.

 

You could use a 5V zener followed by a transistor switch (to control the fan). This would handle the shutoff at 4V. You could use a 12V regulator to drop the voltage from 24 to 12V.

Actually you could probably just use the 12V regulator. When it sees a 4V input, the output won't be enough to run the fan.

 

thanks. is a 12v regulator a single part i can buy or would it be a circuit i need to build?

 

Look up 7812 voltage regulator. It needs a couple of components - capacitors to prevent oscillation.

 

thanks. could you explain to me how it would be hooked up? or link me to some place where i could learn?

 

<SNIP>


The fan wont run with this wired up at 4v as the vdrop across regulator it will only be out putting under 2v or a transistor can be added to turn it off completely.

Moderators note: link contained information that violates the ToS

 

http://www.researchcell.com/electronics/7812-pin-and-circuit-diagram/ . ........

 

For a small fan; all you need is a 12V zener diode connected in series. The dissipation of the zener will equal the power of the fan so make sure you use one with a suitable power rating.

Also, Check the fan specification to see what its maximum voltage is. If the fan is ok at a voltage >12V you can use a lower voltage zener which would reduce the dissipation in the diode and improve the efficiency.

 

Is the fan going to be powered from the 4-24V signal, or is that just used to switch on/off a separate 12Vdc power supply that will power the fan?