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Simply relay circuit - help please

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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90% of the worlds desktop PC's have 12v fans running at much lower voltages, usually using variable resistance controllers. Even the 5v supply we are talking about could be a PWN running at a startup of 3v.

I can't say if this is true or not but I do know that all pertinent information is important to include in our threads. I'm sure you'd agree. ;)

If your port is 5V PWM it can be used with a 12V wallwart and a transistor switch. It will essentially produce a 12V PWM signal to the motor.

Chris
 

KrisBlueNZ

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If the fan connector is 5V PWM then a transistor driver with the fan running from a 12V supply will just translate the percentage of 5V into a percentage of 12V. IOW if the PWM duty cycle is 60%, so you would measure 3V on the connector, then the fan will see 60% of 12V which is 7.2V.

I'm not sure why you haven't built up Chris's little test device to see whether your fan header is 5V PWM or a steady 3V DC voltage. You seem to have just ignored the suggestion, despite the importance of knowing what type of voltage is on that connector.

One more thing. If you use a transistor to drive the fan, it's a good idea to put a reverse-connected diode across the fan to suppress any back EMF from the fan. (The fan is considered an inductive load.) A 1N4001 should be fine for this. Connect cathode to positive, anode to negative, directly across the fan wires.
 

nemo1966

Jan 31, 2013
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I'm not sure why you haven't built up Chris's little test device to see whether your fan header is 5V PWM or a steady 3V DC voltage. You seem to have just ignored the suggestion, despite the importance of knowing what type of voltage is on that connector.

Well, with the fan being 12v and running it at 5v it's started to worry me (the boxes aren't cheap and I don't want anything un-toward happening). So I have been thinking about using an external 12v supply, using some sort of voltage controller to keep the fan at the speed I need and then using a thermistor to switch the fan on at a certain temperature level.

Something like this http://www.bit-tech.net/modding/2001/12/03/pwm_fan_controller/1 the very bottom diagram. However this increases fan speed with temp, I want to keep the fan speed constant.

Simply put, I like this circuit idea, but just need to set the fan speed and then have the fan come on at above say 30c and go off at below 30c.

Sorry if it seems like I am ignoring stuff, I'm really not, I'm reading like mad and changing ideas every few minutes. Your help really is appreciated.

regards
Nemo

PS just noted that the IC put the fan on at full for 2 seconds when it stats,

Edit*** Just seen this http://www.circuitstoday.com/fan-speed-controller-using-lm2941 This seems a simpler format that I'd just need to add a variable thermistor into????
 
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KrisBlueNZ

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It's nice to try to find the method you think will be best, but at some point you need to build something and try it out. You can spend forever going over ideas in your head, but without experimentation and testing, you have no real world knowledge to base your choices on, and you can run off on tangents forever. It's not just YOUR time that this wastes.

I'll tell you what I think you should do.

If the unit has a connector for a fan, it makes sense to use it, if possible.
Build up Chris's circuit and find out whether the connector has a steady DC voltage or a 5V PWM signal.
If it's a PWM signal and you have a 12V fan, use a transistor to drive the fan as I explained.
If it's a DC signal... well, let's cross that bridge if we come to it.
 

nemo1966

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Right built circuit and get nothing from the LED.

The fan control header doesn't seem very reliable.

regards
 

KrisBlueNZ

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The fan control header doesn't seem very reliable.
Well, check it out. Can you get access to the bottom of the board? Check that the fan header pins are properly soldered and haven't become loosened. If you can't get to the bottom of the board, wiggle the connector pins gently to see if they're loose.

Right built circuit and get nothing from the LED.
OK that indicates that the fan connector voltage is 3V DC not a 5V PWM signal.

1. Are you sure this connector is meant to be connected to a fan?
2. Can you post a photo of the circuit board in the area of the fan connector?
3. Does the fan voltage appear only when the unit gets hot?

If the voltage is present on the connector all the time, you might as well just run the fan continuously from 12V or some slightly lower voltage. You can use the LM2941-based circuit you linked to in an earlier post.
 
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