Controlling voltage spikes for dc fan

[aasimzee]

I am using relay with flyback diode across the coil to control spikes when relay turns off, relay is controlled by microcontroller depending upon the temperature limit i set and i am using bjt for switching.
Lets come to the problem I am using DC battery to power microcontroller too by decreasing the voltag to 5v using 7805 and same battery is running FAN. question is do i need anything else or this circuit will work fine bcoz i have read fan acts like relay with turned off as its is inductive load. will the fan affects battery. Circuit is in pic.

 

[treez]

sorry i cant see image well enough, it is blurred when i increase its size.
You know you can buy fans with a PWM input which you use to adjust fan speed.
But yes you can pwm the supply to some of them too...but doing it with relay is less usual than doing the fan pwm'ing with a fet......the realy has limit to number of times it can be switched, in theory, and if you switching it regular, then you switch it too much.

 

[dick_freebird]

Most DC fans are brushless with an internal control IC
and their behavior may or may not be inductive at
turnoff; depends on the controller behavior. I'd suggest
trying to measure the response, and make the fan's
power and ground have as little as possible to do with
the uC's power and ground (the ground, especially;
the regulator for uC ought to isolate it pretty well on
the + side, but kicking uC ground can corrupt digital
input states).

 

[aasimzee]

Most DC fans are brushless with an internal control IC
and their behavior may or may not be inductive at
turnoff; depends on the controller behavior. I'd suggest
trying to measure the response, and make the fan's
power and ground have as little as possible to do with
the uC's power and ground (the ground, especially;
the regulator for uC ought to isolate it pretty well on
the + side, but kicking uC ground can corrupt digital
input states).

yes i am worried about ground connection. how to avoid this. any circuit?

 

[chuckey]

I have found that the small DC fans with their internal IC drivers require a very low impedance source for them to work. From this I would guess that at the end of commutation, the driver IC will return the fan's stored back emf into the main supply line. Saves the power having to be stored/dissipated within your $5 fan.
Frank

 

[c_mitra]

do i need anything else or this circuit will work fine bcoz i have read fan acts like relay with turned off as its is inductive load. will the fan affects battery. Circuit is in pic.

The attached pic is, for all practical purposes, useless.

But first we need more information about the fan. Is it a small instrument fan, brushless DC with a built-in driver? Most of them are permanent magnet type and you cannot regulate their speed well.

Using a relay to control speed of a fan is a rather novel idea and you need to turn on and off the relay perhaps 1-2 times a sec. Possible, but will be relay last?

Battery can absorb voltage spikes very well and unless the energy is too much nothing is going to happen to the battery. If you are worried, use a rechargeable battery and the voltage spikes will help the battery getting charged.

But I am just wondering what you will get by using a relay? A simple mosfet can use a PWM relay with excellent control...