Fan run-on timer

[Jay666666]

Hi all,

Been searching through the net and im a bit lost and could do with some pointers. Im trying to build a circuit that would have 5v power to run, a switch input +ve (either 3.3v, 5v or 12v) with an output of 5v@1amp. The output only turns on once the input turns off and stays running for 3 minutes.

It is to run a fan at 5v for 3 mins once the item has been switched off, for cooling purposes. From what ive read i should be able to use a 555 timer, im just unsure how to tigger/delay the output.

I guess a bit like a car fan on the radiator once the car got very warm, its runs on.

Any help would be appreciated!

Thanks,

James.

 

[Jay666666]

wow this forum is busy!

 

[Jay666666]

I think this maybe what im looking for but im unsure how to trigger it without a switch, it would then output to a relay to turn the fan on. How can I send a grounded input into this once power is off? there are no switch grounds. Some sort of transister?

**broken link removed**

 

[Hero999]

Well you haven't really said what you need assistance with.

 

[Boncuk]

Well you haven't really said what you need assistance with.

Here's my interpretation:

A device is turned off and the same moment a fan should kick in and run for three minutes.

Boncuk

 

[Mike_2545]

Maybe you should consider a thermal circuit, if it's for cooling purposes.

Like the LM34

 

[Boncuk]

Maybe you should consider a thermal circuit, if it's for cooling purposes.

Like the LM34

A combination of a thermal and a timer circuit would be a perfect solution. Especially motors build up heat when shut down after heavy duty when the built in cooling fan also stops working.

Accumulative heating occured to my rotary saw when my son in law used it to cut trees. After a break of two hours it wouldn't run anymore because of burnt up field windings.

Boncuk

 

[Jay666666]

Hi and thanks for the replys,

The fan runs at 12v or 9v all the time when the power is on, havnt decided which yet. Once the power is turned off from the fan, I want the fan to run for a further 3-4 minutes at 5v then power off.

Once power is supplied to the circuit and fan i want the circuit to reset and start again with the 4 minute timer ready to go once power is turned off again. I want to keep it real simple and already have a few 555 timers and transistors to play with.

Hope this is more understandable, im not one for explaining! Im going to design a box chart of some sort on paper.

Thanks,
Jay

 

[crutschow]

The 555 delay circuit shown should do what you want. You can replace S1 with a transistor that is on when you want the fan to run. The output can be connected to another transistor that controls the fan. The output remains high and the fan on as long as S1 is closed. When S1 opens (power off) the output will remain high and the fan on until the 555 times out.

To get the long delays you need you may want to use a CMOS version of the 555 such as the LMC555 which allows the use of a 10MΩ timing resistor. You want to use a tantalum or other low leakage capacitor for the long delay.

 

[Jay666666]

That is amazing mate! I only have the NE555N timers but will have to get the LMC555.

So if i have a switched voltage going into the transistor (in place of S1) this will switch on and remain on until the switch voltage is turned off? thats brilliant.

Only thing i need to figure out is how to run the fan at 9v whilst on and allow the 5v to take over once the power is turned off. Would this be just a case of having to supplie to the fan? 5v (from the timer) and a switched 9v? then use a diode in series?

Thanks for all the help! Looking forward to building this, should be this weekend if i can get the componants.

 

[Jay666666]

also would a ICM7555IPA be suitable replacement for the LMC555?

also, found this one on ebay TS555CN if that will do? is it just any 555 cmos timer?

thanks

 

[crutschow]

Only thing i need to figure out is how to run the fan at 9v whilst on and allow the 5v to take over once the power is turned off. Would this be just a case of having to supplie to the fan? 5v (from the timer) and a switched 9v? then use a diode in series?

also would a ICM7555IPA be suitable replacement for the LMC555?

also, found this one on ebay TS555CN if that will do? is it just any 555 cmos timer?

What power supplies do you have? Can you just have a separate 9V power to power the fan all the time? The 555 can also be powered by the 9V. Then you wouldn't need to do any transfer of power.

If you post a diagram of what you are trying to do, I may have more suggestions on how to control the various voltages. It's a little unclear from your description. (You know, a picture is worth a thousand words).

As far as I can tell the ICM7555IPA and TS555CN are the same as the LMC555. Be sure to read the data sheet of the one you decide to use (just Google the part number).

 

[Jay666666]

Hope this helps

oh and missed off that the 5v out from the timer should switch off when the 9v comes back on within 3 mins, or wont that matter if using some sort of diode for both the 9v and 5v to the fan? stopping power going back through either?

**broken link removed**

 

[crutschow]

So you are planning on running the fan from 5V for the last 3 minutes? Will that give you sufficient speed on the fan motor? Do you have any 9V available for that time?

 

[Jay666666]

I just have 5v at the mo constant once power is off, the fan will run at 5v i just want it too run quietly. Its more a safety measure than anything else. 9v is quite noisey. The fan runs at 6v normally, they have been modded to run at 9v with a 2A regulator using a 12v switched source.

 

[crutschow]

You could isolate the two voltages with a diode in each line to the fan. The problem may be the voltage drop of the diode, particularly in the 5V line. If you used a schottky diode, the drop would still be about 0.4-0.5V. Will the fan run on 4.5V?

 

[Jay666666]

I think it should be ok, ive got the Timer now, just got to get the caps and resistors the play with. I got 4 of the TS555CN from ebay for £2.20 delivered, seems a good price. Will be giving it a go this week.

Thanks for all the help!

 

[Jay666666]

oh one more thing, in calculating Tsec, what is R and C values?

1 meg ohm = 100,000 ohms

so, Tsec = 1.1 x 100,000 x C (how is 1uf used?)

Im a bit thick!

 

[Boncuk]

oh one more thing, in calculating Tsec, what is R and C values?

1 meg ohm = 100,000 ohms

so, Tsec = 1.1 x 100,000 x C (how is 1uf used?)

Im a bit thick!

You are a bit "thin":

1MΩ equals 1,000,000Ω, 100,000Ω = 100KΩ

Regards

Boncuk