fan on demand circuit

peace brainerd · 3rd July 2008, 02:00 PM

What is the simplest proposition for achieving 'fan on demand' with a thermistor?

I've got a handful of likely thermistors I can use. It's a noncritical application.. just turn on a 120vac muffin at about 100F and off again when it gets back to room temp.

I looked about for kits/boards and came up pretty empty. Is there a really simple circuit posted somewhere?

thanks

Christer · 3rd July 2008, 02:08 PM

The simplest is to use one of those thermoswitches (whatever they are called). It is a switch in a metal case which is designed open (or close) at a certain temperature. You can mount it on the heatsink and use it to switch the fan on and off. Of course, it gives no regulation, there is just on or off.

wakibaki · 3rd July 2008, 03:12 PM

A simple thermostat...

Make your thermistor one of R3, R4, and make R3=R4. Any single-rail opamp will do, make the supply voltage = relay voltage.

w

wakibaki · 3rd July 2008, 06:12 PM

If you put a positive feedback resistor or preferably a pot from the output (1) of the opamp to the noninverting input (3),you will get some hysteresis, i.e. separate on and off points.

A signal diode (1N4148) may be adequate if the relay is small, in fact you might get away without it, but if the coil is big you might need a power Shottky. The deterioration in performance of the opamp due to reverse voltage when the coil is switched off may not manifest itself immediately...

w

wakibaki · 3rd July 2008, 07:55 PM

With hysteresis...

The feedback is just a starting value. R3/R4 may have to swap, depending on the tempco.

w

I_Forgot · 4th July 2008, 01:27 AM

Instead of a thermistor that reacts to an already elevated temperature, you can monitor the load current and use it to trigger the fan. Since it takes a while for heat to build once the high current condition occurs, triggering the fan based on the current will reduce the maximum temperature rise.

I_F

peace brainerd · 4th July 2008, 01:43 AM

Damn,

Yer gonna hate me for this after all the excellent suggestions. I may use some of them on something in the future. But this is really a simpler need.

I've got a 1940s Ambassador record changer that is a bit of a treasure. All in one little lift-lid box with walnut veneer and little tube amp. Everything is completely original. It was hardly ever used. I appreciate it especially for spinning ancient 78s. There's just something heart warming about this music on this player on a pedestal table in the corner of the room sometimes.

But it tends to rapidly overheat. The tubes and old motor with less than adequate ventilation just build up pretty fast. I don't want to do any real mods to this. But I'd like to put a little fan inside to boost movement out of the vents. The added fan noise and possible mechanical noise is not much concern at this level of fidelity.

So I think I'll just put a five minute delay on break relay before the fan and call it good. That'll make the ol girl happy.

I_Forgot · 5th July 2008, 03:05 AM

Why bother with a delay? Just run the fan whenever the power is on.

I_F

peace brainerd · 5th July 2008, 05:41 PM

Mostly because I'm compelled in most cases to making my life difficult (as recent threads would attest).

But i'll give in. I'd like it to be cooling the motor (a quite bulky, heavily laminated job which stores tons of heat) between frequent cycles. There is no 'main' on/off switch. The knob which initiates the platter/changer cycle turns the motor and amp off each time a record or a stack finishes. I could add an on/off switch with a little indicator lamp. But that's a bit of a major modification.

I'll just go ahead and run the fan when the motor is running. Originally I was hoping that a quick two or three element circuit with thermistor might be applicable to just adhering the thermistor to the motor laminations and it would be a neat little soulution. I have so many projects laying about right now waiting for exactly the right fix that I can't afford (for sanitys' sake) to add this to the pile!

Thanks again for the suggestions.

In fact christers' suggestion might be in the offing next time I order a box of parts from digikey. They have a little canned thermoswitch that operates at 40C for about six bucks.

Nordic · 5th July 2008, 05:59 PM

How does one use the NC (Normaly Closed) ones?
I understand to use a normaly open one is easy... you just wire it all up and wait for the temperature to close the circuit...

But I can't get my head around the Normaly Open ones... which are the common types in the temperature ranges usefull for an amplifier...

3rd July 2008, 02:00 PM	#1
peace brainerd diyAudio Member Join Date: Nov 2004	fan on demand circuit What is the simplest proposition for achieving 'fan on demand' with a thermistor? I've got a handful of likely thermistors I can use. It's a noncritical application.. just turn on a 120vac muffin at about 100F and off again when it gets back to room temp. I looked about for kits/boards and came up pretty empty. Is there a really simple circuit posted somewhere? thanks

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3rd July 2008, 02:08 PM	#2
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	The simplest is to use one of those thermoswitches (whatever they are called). It is a switch in a metal case which is designed open (or close) at a certain temperature. You can mount it on the heatsink and use it to switch the fan on and off. Of course, it gives no regulation, there is just on or off.

3rd July 2008, 03:12 PM	#3
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	A simple thermostat... Make your thermistor one of R3, R4, and make R3=R4. Any single-rail opamp will do, make the supply voltage = relay voltage. w

3rd July 2008, 06:12 PM	#4
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	If you put a positive feedback resistor or preferably a pot from the output (1) of the opamp to the noninverting input (3),you will get some hysteresis, i.e. separate on and off points. A signal diode (1N4148) may be adequate if the relay is small, in fact you might get away without it, but if the coil is big you might need a power Shottky. The deterioration in performance of the opamp due to reverse voltage when the coil is switched off may not manifest itself immediately... w

3rd July 2008, 07:55 PM	#5
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	With hysteresis... The feedback is just a starting value. R3/R4 may have to swap, depending on the tempco. w

4th July 2008, 01:27 AM	#6
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	Instead of a thermistor that reacts to an already elevated temperature, you can monitor the load current and use it to trigger the fan. Since it takes a while for heat to build once the high current condition occurs, triggering the fan based on the current will reduce the maximum temperature rise. I_F

4th July 2008, 01:43 AM	#7
peace brainerd diyAudio Member Join Date: Nov 2004	Damn, Yer gonna hate me for this after all the excellent suggestions. I may use some of them on something in the future. But this is really a simpler need. I've got a 1940s Ambassador record changer that is a bit of a treasure. All in one little lift-lid box with walnut veneer and little tube amp. Everything is completely original. It was hardly ever used. I appreciate it especially for spinning ancient 78s. There's just something heart warming about this music on this player on a pedestal table in the corner of the room sometimes. But it tends to rapidly overheat. The tubes and old motor with less than adequate ventilation just build up pretty fast. I don't want to do any real mods to this. But I'd like to put a little fan inside to boost movement out of the vents. The added fan noise and possible mechanical noise is not much concern at this level of fidelity. So I think I'll just put a five minute delay on break relay before the fan and call it good. That'll make the ol girl happy.

5th July 2008, 03:05 AM	#8
I_Forgot diyAudio Member Join Date: Jan 2005 Location: Phoenix, Az.	Why bother with a delay? Just run the fan whenever the power is on. I_F

5th July 2008, 05:41 PM	#9
peace brainerd diyAudio Member Join Date: Nov 2004	Mostly because I'm compelled in most cases to making my life difficult (as recent threads would attest). But i'll give in. I'd like it to be cooling the motor (a quite bulky, heavily laminated job which stores tons of heat) between frequent cycles. There is no 'main' on/off switch. The knob which initiates the platter/changer cycle turns the motor and amp off each time a record or a stack finishes. I could add an on/off switch with a little indicator lamp. But that's a bit of a major modification. I'll just go ahead and run the fan when the motor is running. Originally I was hoping that a quick two or three element circuit with thermistor might be applicable to just adhering the thermistor to the motor laminations and it would be a neat little soulution. I have so many projects laying about right now waiting for exactly the right fix that I can't afford (for sanitys' sake) to add this to the pile! Thanks again for the suggestions. In fact christers' suggestion might be in the offing next time I order a box of parts from digikey. They have a little canned thermoswitch that operates at 40C for about six bucks.

5th July 2008, 05:59 PM	#10
Nordic diyAudio Member Join Date: Sep 2005	How does one use the NC (Normaly Closed) ones? I understand to use a normaly open one is easy... you just wire it all up and wait for the temperature to close the circuit... But I can't get my head around the Normaly Open ones... which are the common types in the temperature ranges usefull for an amplifier...

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