Looking for a fan control circuit that does this...

Rory Buszka · Post by **Rory Buszka** » Mon Dec 12, 2005 12:02 pm

What I'd like is a fan controller that lets me select the temperature where the fan kicks on (it would be off until this point), and when it turns on it applies a starting voltage for about a second (set by a potentiometer) and then changes the transistor output voltage to a different voltage at which the fan would run, and then adjusts the voltage along a continuum (there's another potentiometer) between the specified operating voltage and a maximum voltage as determined by a thermistor, and then the fan would shut off when the temperature falls below the threshold set by the first. In addition, there would be a small pushbutton that would defeat the thermistor and turn the fan on for as long as the button is pressed, for purposes of checking the starting and running voltages.

I'm looking into what would be involved in designing such a circuit (including a small 741 op-amp setup to drive a transistor). There are a couple places I'm hitting a snag, though:

1) I need some way to make a one-second timer. I'm thinking that a simple RC or RL network might be a way to do it (building a circuit with a one-second time constant. Once the capacitor is charged, it would act like an open.

2) Once I have my one-second timer, I need some way to switch from the starting voltage to the running voltage. These won't actually be the full starting or running voltages, but instead they will simply be low voltages that are the input to the op-amp. This "switcher" would need to make the switch when there is no longer a voltage on the input to the switcher. The current on the output of the RC network would be slowly decreasing

Do any of the electrical engineers on this board have any idea of how I might accomplish these two things?

datapappan · Post by **datapappan** » Mon Dec 12, 2005 12:20 pm

Check for ideas at www.cpemma.co.uk

/ datapappan

Rory Buszka · Post by **Rory Buszka** » Mon Dec 12, 2005 1:30 pm

Ok, a more focused question: In a comparator, does the output voltage equal the input voltage (if it's above the reference voltage) or does it equal the rail voltage?

Bat · Post by **Bat** » Mon Dec 12, 2005 1:34 pm

The rail voltage, or as close to it as the op-amp can drive its output.

frankgehry · Post by **frankgehry** » Mon Dec 12, 2005 2:18 pm

R,

I would suggest using a microprocessor - this would allow you to implement kickstarts, delays, and other controls using a simple program intead of circuitry. My development environment consists of a $2.50 pic16f684 mcu and free downloadable software.

This sketch shows a thermistor based temperature sensor and a simple digital to analog converter that could supply low voltage to an op amp and drive circuit. With pwm fans, the drive circuit is built into the fan, however it shouldn't be too hard to drive 2 or 3-wire fans. A button could be connected to one of the processor pins and the rest could be implemented with a few lines of code. If you want to explore this route, I can supply book titles and links. - FG

Rory Buszka · Post by **Rory Buszka** » Mon Dec 12, 2005 2:42 pm

Right now I'm looking at an analog solution - the whole PWM noise thing is kind of scary. This is my first time doing something application-specific and I want to do it right. Your option might be more "right" but I don't have any experience in using microprocessors.I do have some experience with analog components, so I think I could do this more easily. Not like any mistakes would be expensive. Right now I'm looking at a solution using an opamp and three channels of a quad comparator. One looks at the 10k thermistor and checks it against a voltage set using a potentiometer. The output from the first comparator should equal the rail voltage, which for the control circuitry will be 5 volts. The output from that comparator is sent to an RC circuit which charges a capacitor over the course of one minute. Then, the output from that gets compared to a voltage of 0.1V, and when it drops below that 0.1V, it will automatically kick over from the starting voltage level to the operating voltage level. One comparator is looking for that voltage to be above 0.1v and the other is looking for that voltage to be below 0.1V, and each comparator activates the base of a small NPN transistor. The collector of the "start" transistor (which supplies starting voltage) is connected through a 10k pot to the 5-volt rail. The collector of the "run" transistor is fed from both the 10k thermistor and its 10k potentiometer, so a minimum fan voltage can be set, which can be less than the starting voltage. Finally, the output from whichever transistor is operating at the time is fed into an LM741 opamp whose voltage gain is set such that it will drive the output transistor over its range of operation. To be sure, there are still some things I need to check out and make sure they'll be correct, but I hope to build this thing sometime over my Christmas break.

Bob_the_lost · Post by **Bob_the_lost** » Mon Dec 12, 2005 3:13 pm

Micro processors are almost always the easier option. I built a 40 Chip behemoth for a project, only realising after i'd started to fabricate it i could have done it in half the time with a PIC.

It all worked of course, but it was a monstrosity. As i suspect would your analog system.

spolitta · Post by **spolitta** » Mon Dec 12, 2005 4:26 pm

Isnt there a software that could do this?

Mccoyness · Post by **Mccoyness** » Mon Dec 12, 2005 5:39 pm

frankgehry wrote:If you want to explore this route, I can supply book titles and links.

I have little or no experience with electronics at this level. I would been keen to learn though. In the past I have done plenty of low level programming if this is useful. Where do you think I should start?

I'm obviously interested in applying this knowledge as above but honestly how difficult is it to make useful devices like these?

frankgehry · Post by **frankgehry** » Mon Dec 12, 2005 7:02 pm

M,

Check your messages. - FG

Rory Buszka · Post by **Rory Buszka** » Wed Dec 14, 2005 7:47 am

I have one more question, regarding to the design of this circuit. In my circuits class, I worked with a Hewlett-Packard "Triple Power Supply" which had five binding posts on the front, one for +18V, one for COM (basically 0V), one for +20V, and one for -20V, plus an earth ground terminal. Working with the computer's power supply is different. It's always been my understanding that the molex connector has a +12, +5, -12, and -5 volt connection. Does that mean that there is a net EMF of 10V between the +5V and -5V cpins on the Molex? Or are the -12V and -5V pins like the "COM" terminal on the triple power supply? If there's no COM or 0V connection, then I need know this in order to calculate the correct values of the resistors in my opamp and comparator circuits.

SUSPIRIA · Post by **SUSPIRIA** » Wed Dec 14, 2005 12:01 pm

You can try the MCUBED T-BALANCER

http://www.mcubed-tech.com/eng/produkte.htm

Rory Buszka · Post by **Rory Buszka** » Wed Dec 14, 2005 3:49 pm

Actually, the new MCubed FanAmp looks interesting to me - I might check that one out if I give up on building this thing.

streety · Post by **streety** » Wed Dec 21, 2005 5:02 pm

Rory Buszka wrote:I have one more question, regarding to the design of this circuit. In my circuits class, I worked with a Hewlett-Packard "Triple Power Supply" which had five binding posts on the front, one for +18V, one for COM (basically 0V), one for +20V, and one for -20V, plus an earth ground terminal. Working with the computer's power supply is different. It's always been my understanding that the molex connector has a +12, +5, -12, and -5 volt connection. Does that mean that there is a net EMF of 10V between the +5V and -5V cpins on the Molex? Or are the -12V and -5V pins like the "COM" terminal on the triple power supply? If there's no COM or 0V connection, then I need know this in order to calculate the correct values of the resistors in my opamp and comparator circuits.

A molex has 2 ground wires, one 12V line and one 5V line.