Simple solid state time delay

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dumpystig

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Hi, I'm new here and I'm hoping someone with a tad more knowledge than myself can help me with a circuit I want to build.

I'm adding a small 12VDC fan to my LCD TV. I want a 12V supply from the TV PSU to power the timer input and the output for the fan. After turning the TV on the 12V would go to the timer delay and after maybe 30 seconds it would then output the voltage to the fan. If possible I would also like the output voltage to be around 9V so the fan is running slower and therefore slightly quieter, although this isn't paramount. The circuit needs to be solid state as I don't want to use a relay.

Any pointers or advice will be appreciated.
 
If you also want the fan to run while the TV is on, then a diode and a bank of capacitors would be the simpliest.
 
the delay can be achieved with R-C circuit, transistor being used as a switch and you can make the fan slower by connecting it in series with a resistor to limit the current.
 
Yes, after switching on the TV a 12V supply from the PSU would start the delay timer, after 30s the fan would kick in and then run until the TV was switched off. I want a power efficient circuit so a series resistor is not an option (I'm installing the fan to dissipate heat so don't want to add any more heat sources if I can avoid it). I'm thinking of a circuit based around a 555(delay), a MOSFET(?)(12V power switch), and a LM7809(12V>9V fan) with associated components, but I'm not that savvy at designing such a circuit. The fan is only small, rated @ 0.2A.
 
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Just thinking maybe I'm going a bit OTT in my last post - if there are simpler ways then I'm all for it. I've used series resistors on several fans in my PC and even higher wattage resistors can dissipate a surprising amount of heat, so this type of circuit would also come in handy for those fans too.
 
Yeah, I found that page a couple of months ago when I was doing my PC mods. But if you read my previous posts you will see I'm not wanting a temp. controlled fan, I just want the fan to run 30s after the TV is powered on. Thanks for your reply though.
 
Ok, this seems like a good option for a timer: **broken link removed** 30 seconds is a LONG time for RC time delays. The PWM method is a good way to control the speed of a fan. With that method, the torque is constant with varying speed.
 
Is there a current rating on the fan?
Why the 30 second delay?
 
@ KeepIt... - thanks but that's way overboard for my needs.

@ ronv - yeah, the fan is 0.2A, although I may want to use one slightly larger so I am looking at allowing for 0.4A. Reason for the delay is because the simultaneous TV switch on and start of fan is causing the Protect_Detect circuit of the TV to operate, whereby the TV then goes into fault mode. After a little experimenting I reckon a 10s delay will be fine.
 

Using a 7809 isn't that much better than using a resistor in terms of energy efficiency. You'd need a switching style converter to gain back some of the efficiency but that's going to be too mant parts for your design.
 
ronv said:
Is there a current rating on the fan?
Why the 30 second delay?
Click to expand...

Why delay after the TV turns on? Most people want the fan to runn after it turns off, to help with cooling the system down after it's done being used. I have often thought of doing something similar for my PC.
 
@ KeepIt... - I will have a look at what this device does.

@ ADW... - Please read my last post, re: TV Mainboard Protect_Detect circuit; the fan load @ initial TV switch on causes this circuit to operate, sending TV into fault mode.
The main chassis Microcontroller which the fan is above will not reach its usual high temp. during TV operation as the fan will be keeping it much cooler, thereby negating any need for fan over-run - when the TV is switched off it will already be much cooler than it would be at stock w/o the fan. I have already performed test runs and the fan does it's job with the results I want. I realise the simplest way to reduce fan speed/noise is to use a wirewound res. and after looking at things in more detail have decided to go with this in order to keep things as simple as possible.
 


**broken link removed**
I hope you will like this. C and VR will do the delaying and set schmitt trigger inverter output high to make the MOSFET conducts after a delay period. I guess you can find the value of C, VR, R1, R2 and appropriate MOSFET. Good luck
 
Fan delay

I think this will work. The diodes could be replaced with a resistor, but you may have some problems getting the fan to start.

AWT, That is a nice circuit, but the NFET won't work good in that circuit.
 

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  • slow fan.PNG
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Thanks for the above circuits, I'm well impressed with the expertise displayed on this site.

@ ronv - I'm no expert myself but are the voltages displayed at the top of your attachment the wrong way round? Is gateV not the blue trace and fanV the green trace?
 
No they are correct. The gate rise time is very slow - this creates the delay, but the turn on time once the threshold is obtained is still fairly fast as in the simulation.
 
OK thanks. Guess I will have to do some extra reading on Mosfets, gates etc.. Although I do have electronics knowledge (self taught/learned) I'm only at a basic level but it does help me enjoy building my own circuits/mods for whatever the project may be. I've just repaired the TV in this project (lots of bad/dry joints, leaky electrolytic on PSU) and after seeing how hot the main processor was getting (more dry joints on adjacent SMD components) I decided on this fan addition.

Anyways, would you mind giving me a brief description of your circuit so maybe I can try to get a better understanding?
 
R2 charges C1 untill Vgs > Vth of M1 (See datasheet)

M1 turns on rather slowly. D4 absorbs inductive spike from the motor.

D1, D2, D3 reduces the motor voltage by 3 diode drops (3*~0.6V)

R1 allows a way for charge to deplete and aids in discharging C1 at power off. at~ 5*R1*C1 seconds, the cap should be totally discharged.
 
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