Guys,

I have a simple project that I have to do for work. Basically I have a
power supply that is putting out 24v of AC current. I need to convert that into 12v DC current, the DC voltage is going to a PC Cooling Fan. Does anyone have a quick schematic that will help me do this. I tried using a Full Bridge Rectifier from Radio Shack and a 1000uf cap, but no dice. You see it get some current but it just doesn't want to spin as if the fan is confused.

Thanks for any help
ScarEye.


P.S. Love this site. It rocks for newbs like me. =)

 

Here is a link to www.discoverycircuits.com for a power supply that converts 20V AC that is produced using a step down transformer and converts it to 18 volts dc.

To change the output to 12 volts just replace the 3.3K resistor with 2K. You won't need the transformer since you have 24 Volts AC to start with. If your fan draws more than a 50 milliamps then you will need to consider a small heat sink for the LM350T.

hgmjr

 

hgmjr,

Thanks for your reply. I saw that circuit but I wasn't sure if this would work. I hope I can get the parts from Radio Shack. I know I can get it from digikey and mouser and jameco but I would like to pick this up and try it asap.

I will try this out and report back.


Thanks for your help
ScarEye

 

Would you guys know if there is a chart where I can see if what is comparable to a part that I have access to ?

For ex:


5001 -----> 1N4742A (Radio Shack)
LM350T ----> LM317T (Radio Shack)


Again any comments or suggestions are appreciated.



Thanks again
ScarEye

 

Would you guys know if there is a chart where I can see if what is comparable to a part that I have access to ?

For ex:


5001 -----> 1N4742A (Radio Shack)
LM350T ----> LM317T (Radio Shack)


Again any comments or suggestions are appreciated.



Thanks again
ScarEye

You can use the LM317T in place of the LM350T but a better choice for the 1n5001 would be the 1n4001. I think the 1n4742 is a 12V zener diode part number.

hgmjr

 

hgmjr,

Thanks for your quick reply. I am going to Radio Shack to pick up these parts and I will report back my results.

Thanks for your help. I really do appreciate it.

ScarEye

 

Draw up your circuit diagram and post it here. That way, someone can assist you if you encounter any problem with the design.

hgmjr

 

Yes I will do that. I will draw one up tonight and post it. Constructive criticism is a good thing. =)


Thanks again

 

Also, I had another question. I might need to add 2 more fans to this project. So that's a total of 3 fans that require 12v @ 130ma. This circuit should be able to give the power required to power all three fans ?

Also, 130ma x 3 = 390ma I should throw a heat sink on the LM317T voltage regulator correct ?


Thanks
ScarEye

 

The successful powering of three fans rather than a single fan is going to hinge on how much power you have available from your 20 VAC source.

As for the amount of heatsink material you are going to require, you will need to consider what happens in a linear power supply.

An LM317T adjustable voltage regulator requires a minimum of 2 volts above the regulated output to operate properly. Your application appears to be able to meet this criteria.

In your application, the input voltage is stated to be 20 VAC. You haven't stated whether this is the peak voltage, peak-to-peak voltage, or the average voltage of your source. If it is the peak-to-peak voltage then the fullwave rectified voltage is not going to be great enough to achieve the regulated voltage target of 12 Volts.

Assuming that the 20 VAC figure is the average voltage, then the voltage being dropped across the regulator will be 20-12 or 8 volts. Taking the 8 volts and multiplying it by the load current for a single fan of 0.130 Amps, you will come up with around 1 Watt. This is the power that will dictate the size of the heat sink that will be needed to transfer the heat generated by LM317T to its surroundings.

If you decide to power all three fans, then this power will become approximately 3 Watts.

If the 20 VAC is the peak voltage rather than the average voltage then you will be dealing with an average voltage of 0.707*20 or 14.14 VAC. With this lowering of the average voltage, there is a substantial decrease in the power dropped across the LM317T which would then become approximately 2*0.130 or 260 milliwatts for a single fan and around 780 milliwatts for all three fans. This would mean that a much smaller heat sink would be required.

hgmjr

 

Thanks for your response hgmjr. Here is the circuit I drew out. Please criticise.

The resisitor under the LM317T is 240 Ohms, The resistor that is going to ground is 2K OHM, The Cap to the right is 47uf 35v, And the cap next to the diodes (bridge) is 4700uf 35v.




Thanks

 

Seems ok, you want protection diodes on it in cause of shorts. The datasheet for the LM317 should show these.

 

mrmeval's suggestion to use protection diodes is worth strong consideration. Take a look at the datasheet and post any questions if you need clarification on the hookup of the protection diode.

hgmjr

 

Guys,

The LM317T only comes with a few instructions, I cannot find anything on the datasheet about using a diode for protection. I looked at data sheet on the internet but I still can't find any references to this. Please advise.


Thanks In Advance
ScarEye

 

Check out this application note AN-181 on National's website. Figure 4 shows the diodes and describes the effect of including them. The 1N4002 can be replaced with your 1N4001 diodes.

BTW, you have a minor component connection error in your circuit. You show the 2K resistor connected from the output of the regulator to ground. It should be connected from the voltage adjustment terminal to ground.

hgmjr

 

hgmjr,

So I should use the 1N4002 instead of the 1N4001 diodes ? And looking at the datasheet. I see data for the LM117 I am using the LM317.

Also, I will look over my circuit carefully.



Thanks

 

hgmjr,

So I should use the 1N4002 instead of the 1N4001 diodes ? And looking at the datasheet. I see data for the LM117 I am using the LM317.

Also, I will look over my circuit carefully.

Thanks

For your application, you can use either the 1N4002 or the 1N4001 for the diodes. The LM317 and the LM117 are both the same basic part with the LM117 having slightly tighter operating specs so the application note for the protection diode implementation applies to both.

hgmjr

 

Okay, So I put this circuit together it only took about 20 minutes. After it was completed I threw 1 fan on and it was spinning up with no problem. I am getting 13.75VDC output.

I could not find a 240 Ohm Resistor for the LM317T so I used a 220 Ohm resistor instead. And I could not find a 2K Ohm resistor so I used a 2.2k Ohm. Now if I wanted more juice I just have to lower the value on the 2.2K Ohm resistor correct ?

BTW, With three fans running the LM317T cooks. With 1 fan it's not that bad but with three fans running in 3-4 seconds that thing is hot. I am going to use
this heat sink Radio Shack (276-1178) & With this I am going to use this Silicone Based Heat Sink Compound. Radio Shack (276-1372)


Agan Thank For Your Help Guys..


ScarEye

 

Guys,

Since we are on this topic. I also tired this circuit

http://www.allaboutcircuits.com/vol_6/chpt_5/6.html

It's a Bridge Rectifier and Cap. Without the cap I get about 25VDC which is more than what I need. If I wanted to drop the voltage down to 12-14 VDC
what would I need to do ?

I am thinking just add a few resistors and thats it. But then again I am a newb so that is why I am here.

Thanks again.

ScarEye

 

Guys,

Here is an update. I threw a heat sink on and I powered up the 3 fans. But after about 15 seconds one of the fans stop spinning and then another onee goes. Only 1 fan runs and it's not spinning as fast as it should be. Any ideas ? I think the LM317T is overloading and suggestions ?

Thanks
ScarEye