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Run a fan from 10-60 seconds when power is applied (noob)

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TippyG

New Member
Hi.

I studied electronics 13 years ago and haven't done it since. I have a schematic below which I know is wrong I just don't know how to make it right. I am also hoping you fine peeps can tell me what each components value should be. This is a once off project and would take days to figure it out by myself.

The purpose is to run a fan from 10-60 seconds once power is applied then stop. I would also like to be able to vary the speed of the fan. The fan is a small 80mm computer case type fan. Not sure what the current draw is but can test if needed.

I know the problem with my schematic is the fan will be off until the RC turns on the base so I've actually created the opposite of what I need. I also am guessing VR1 would be better at the base of the transister?



I am a noob and this is my first post. Your help would be hugely appreciated. Many thanks.
 
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dougy83

Well-Known Member
Swap C1 with VR2, this will give a turn-off delay. also, put the motor where VR1 is and dispose of VR1. You may wish to use a darlington transistor.
 

TippyG

New Member
Thank you Dougy.

I read up on your comment and thats exactly what I need. Well done. :)

I have a revised schematic below. I dont see how the motor can be variably controlled though.

I have added the values in I believe are required for the components but I am not at all confident. The formula of t=RC seemed a little too easy. Never have the words "use it or lose it" been so true.

I have added R1 so there is a minumum of 10s and with VR1 at full resistance will give 60s. R2 is based on a 63% charge of the RC circuit and to supply 100mA at the base of the transistor. I read 63% was the value to use so I have. Let me know if it's wrong.

Thanks again for you help and a prompt reply.

 

fernando_g

New Member
Many HVAC units use a 4541 oscillator timer. It will do exactly what you need
 

dougy83

Well-Known Member
Yeah, sorry, I missed the bit where you mentioned you wanted it variably controlled. Have a look at the attached cct. It's a variation on yours.
 

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TippyG

New Member
Thank's very much for your efforts Dougy. I'll be building this on the weekend and will let you know how it went.

Thanks again.
 
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