# Voltage activated switch

Discussion in 'General Electronics Discussion' started by d18, Apr 18, 2010.

1. ### d18

5
0
Apr 18, 2010
I am looking for a voltage triggered switch circuit,
for a simple application of turning on a small
vent fan for a solar charging battery bank

The fan has to come on at about 13vDC rising
and go off when voltage drops.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
What you probably need is a comparator to detect the voltage, and with a small amount of hysteresis to ensure the fan doesn't get turned on and off when the voltage is near the trigger point.

3. ### d18

5
0
Apr 18, 2010
I was thinking of a comparator circuit, I'm just
not sure exactly how to accomplish this, when
I was playing with electronics, the 741 OpAmp
was the norm, however I had never actually implemented
one into a usable circuit and built it.

Anyone have a circuit that would work, with a little
explanation of tuning?

Thanks,

Troy

4. ### NickS

367
0
Apr 6, 2010
Op amps can be used as comparators but actual comparators are better suited for the task and they are pretty common.

What is the maximum voltage level it will see?

Here is a link to a datasheet that will get you started. (plenty of example circuits)
I am not recommending this part I am just showing you an example of a dedicated comparator.

Comparator Datasheet

5. ### d18

5
0
Apr 18, 2010
It should never get above 15.5v DC, safely 16 I suppose

6. ### NickS

367
0
Apr 6, 2010
Ok so we know the range you need to cover. Now tell me about the fan you are controlling. What are the voltage/current requirements for it?

I would probably do something simple like have the comparator drive the gate of a low on resistance MOSFET to power the fan(assuming the fan is ~12VDC).

7. ### d18

5
0
Apr 18, 2010
The fan would be 12vDC about 0.25A,
I don't have one yet, but that's about
the size I'm looking at

8. ### NickS

367
0
Apr 6, 2010
Do you still need help here or have to tried building your circuit?

9. ### d18

5
0
Apr 18, 2010
Haven't tried yet, I'm pretty green when it comes
to designing circuits, any help would be great.

10. ### NickS

367
0
Apr 6, 2010
Have you invested any time into reading up on comparators yet? And what are your options for building test circuits?

The part MAX16013 is a specialized part that makes use of p-type MOSFETs to switch power on and off. Look at how they are used in the linked datasheet. All you need is to use the second pMOSFET as your switch and the comparator output to assert the gate level and you can control power to your fan.

You will also need a reference/supply voltage for the comparator(be sure that your inputs do not exceed the supply level). You will probable divide your reference level with two resistors from your comparator supply voltage and two resistors to set your hysteresis on the comparator. Finally its always a good idea to place a 100nF cap between power and ground on your IC's(comparator).