Current limiter for fan

Thread Starter

yverde

Joined Dec 18, 2020
6
Hey,
I'm am trying to use computer fans for other purpose, I tried hooking one up to a 12v power source and it blew up.
The fan was rated 12v, but 0.37A and I used a 12v, 2A power supply, I thought the fan would use only the current it needs, but I was obviously wrong.

So my question is can I limit the current to the fan with a resistor and how do I calculate the value of the resistor for that?

Sorry for the beginner's question!
 

Thread Starter

yverde

Joined Dec 18, 2020
6
Welcome to AAC!

What makes you think you're wrong?
The fan blew up, I don't think this is the expected behavior
:p

I tired with another fan (same model) and a 12v 0.34A and it worked as expected. So if the fan really only draws the current that it needs, I don't understand what went wrong the first time :\
 

Thread Starter

yverde

Joined Dec 18, 2020
6
Are you certain the fan that burned up was rated for 12V?
Yes, it's written on it and it's the same model that the other fan that worked on 12v 0.34A.

I am actually using an adjustable power supply purchased on ebay, I am starting to think that this thing is the issue as I saw it cause short circuits in the past.
 

dl324

Joined Mar 30, 2015
12,871
I am actually using an adjustable power supply purchased on ebay , I am starting to think that this thing is the issue as I saw it cause short circuits in the past.
Those Chinese supplies on eBay are a crap shoot. Some have had supplies that had hot and neutral reversed.

I have a Velleman that looks much like the cheap things from China. I haven't had it long, but it seems to be working correctly. The output banana jacks are retarded. I have some dual banana plug leads that I made in the 70's and I can't use them on that power supply because they put earth ground between the + and - outputs. Unbelievably retarded, but I'll likely buy more because they're so inexpensive.
 

Tonyr1084

Joined Sep 24, 2015
5,966
If two exactly the same fans were hooked up to one power supply and one of the fans blew up then it had problems internal to the fan that blew up. If the second fan worked normally, this proves the hypothesis. The first fan was defective.

OR it was hooked up wrong. Computer fans these days are brushless fans. That means they will only run in one direction when hooked up correctly. If hooked up incorrectly they may simply block the reverse current and protect themselves - OR they may self destruct.

I've run plenty of old computer fans, but I've never had one blow up. And given that WE ALL sometimes make the mistake of polarity, it's totally conceivable that you got something reversed, albeit unintentionally.

Fan speed control can be done by limiting the current. But it would be better if you limited the voltage. A bit more circuitry involved, but you avoid the wasteful heat of a large resistor. Assume your 12V fan draws 370mA (0.37A); that's 4.44 watts. If you wanted to reduce the fan speed by 50% then you'd have to waste 2.22 watts somewhere. Enter the resistor. You would need a resistor capable of handling that much wattage. I personally know of 1 watt, 2W, 5W resistor ratings. I don't know if they make a 3W resistor. Since your fan is drawing 370mA its calculated resistance is 32.4Ω; so you would need a 32Ω resistor in order to reduce fan speed by 50%. At 32Ω, 12V - you have 375mA flowing through that resistor. And we're back to 4.5W. You would have to use a 5W resistor.

Now, when it comes to motors and inductances, I'm highly inexperienced. Yes, IN-experienced. That is to say "NOT" experienced. So I'm sure I got something wrong here, but the idea still remains the same; using a resistor to change the speed of a DC fan isn't the best way to go. You have to do something with all that heat. Granted, 4 watts isn't a lot of heat, but just try picking up that resistor when it's operating. Tell me if you don't get a good burn. Or if other close by items don't suffer heat damage.

You're better off finding a different power source. One that suits the speed of the fan you wish. ME? I have a variable voltage power supply. I can vary the DC voltage from zero to 170VDC. Not that I have much use for that high a DC voltage. The point is I can vary the voltage for a DC fan and get different speeds as may be desired. So my suggestion is not to limit the current but rather control the voltage. You MIGHT look into an LM317 regulator. But remember, you are still dissipating a lot of heat energy. Truth is - it must go somewhere. The 317 gives you better control over the voltage.
 

Thread Starter

yverde

Joined Dec 18, 2020
6
If two exactly the same fans were hooked up to one power supply and one of the fans blew up then it had problems internal to the fan that blew up. If the second fan worked normally, this proves the hypothesis. The first fan was defective.

OR it was hooked up wrong. Computer fans these days are brushless fans. That means they will only run in one direction when hooked up correctly. If hooked up incorrectly they may simply block the reverse current and protect themselves - OR they may self destruct.

I've run plenty of old computer fans, but I've never had one blow up. And given that WE ALL sometimes make the mistake of polarity, it's totally conceivable that you got something reversed, albeit unintentionally.

Fan speed control can be done by limiting the current. But it would be better if you limited the voltage. A bit more circuitry involved, but you avoid the wasteful heat of a large resistor. Assume your 12V fan draws 370mA (0.37A); that's 4.44 watts. If you wanted to reduce the fan speed by 50% then you'd have to waste 2.22 watts somewhere. Enter the resistor. You would need a resistor capable of handling that much wattage. I personally know of 1 watt, 2W, 5W resistor ratings. I don't know if they make a 3W resistor. Since your fan is drawing 370mA its calculated resistance is 32.4Ω; so you would need a 32Ω resistor in order to reduce fan speed by 50%. At 32Ω, 12V - you have 375mA flowing through that resistor. And we're back to 4.5W. You would have to use a 5W resistor.

Now, when it comes to motors and inductances, I'm highly inexperienced. Yes, IN-experienced. That is to say "NOT" experienced. So I'm sure I got something wrong here, but the idea still remains the same; using a resistor to change the speed of a DC fan isn't the best way to go. You have to do something with all that heat. Granted, 4 watts isn't a lot of heat, but just try picking up that resistor when it's operating. Tell me if you don't get a good burn. Or if other close by items don't suffer heat damage.

You're better off finding a different power source. One that suits the speed of the fan you wish. ME? I have a variable voltage power supply. I can vary the DC voltage from zero to 170VDC. Not that I have much use for that high a DC voltage. The point is I can vary the voltage for a DC fan and get different speeds as may be desired. So my suggestion is not to limit the current but rather control the voltage. You MIGHT look into an LM317 regulator. But remember, you are still dissipating a lot of heat energy. Truth is - it must go somewhere. The 317 gives you better control over the voltage.
It was hooked up to the same power supply only with different current and it was not possible to hook it wrong since the connector connects only one way so like you said, maybe the fan was defective to begin with.

I was planning to have some kind of speed control for it, I was going to try with a transistor, I have a TP-120 on hand, do you think it would suit this purpose? Like turning it on/off at variable speed...
I will look into LM 317 otherwise, thanks for the advice!
 

Tonyr1084

Joined Sep 24, 2015
5,966
I'm not familiar with that transistor. For simple speed control of a motor rotating a mirror I'm using an LM317 in this configuration. It works fine. The motor is small and its current draw is low.
1608394506817.png
A better way to control motor speed is with PWM (Pulse Width Modulation), but that requires a 555 timer chip and some more components. By the way - my motor is a brushed DC motor. Reversing polarity results in reversing the rotation. This motor is NOT like a fan motor.
 
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