Run a 12V fan from 50V rail

hannobisschoff@gmail.com · Jun 12, 2011

Hi all, I built a 50V 2A regulated power supply with variable current, very nice n' all.

But now the 2n3055 transistors on heatsinks get very very hot, and I want to install a 12V computer fan in the case. Please note, I do have a uA7812c regulator, but the input max is 40V.
What shall I do...
Thank you.

carbonzit · Jun 12, 2011

How 'bout 4 12-volt fans (identical) in series? You can get them pretty small, and cool multiple transistors.

hannobisschoff@gmail.com · Jun 12, 2011

HaHa, great idea, and I do have about 40 old cpu(the small ones) fans. Thanks, if I don't get anything "regulator related", I'll try your Idea.
Bit of an overkill, but hey...

hannobisschoff@gmail.com · Jun 12, 2011

carbonzit said:
How 'bout 4 12-volt fans (identical) in series? You can get them pretty small, and cool multiple transistors.

forgot to mention, there are only 2 transistors.

ronv · Jun 12, 2011

I think you could use a LM317 regulator. It allows 40 volts from the input to the output. Might be ok if your supply only goes to 50 volts maximum.

Still a lot of power to waste.

hannobisschoff@gmail.com · Jun 12, 2011

Or a 680 ohm 3Watt resistor should work, but where do I find one now?

Diver300 · Jun 12, 2011

You want a buck converter. One based on a LM2576HVT-12 would do.

hannobisschoff@gmail.com · Jun 12, 2011

by the way, is there a way to make your own high wattage transistors

cr0sh · Jun 12, 2011

hannobisschoff@gmail.com said:
by the way, is there a way to make your own high wattage transistors

Care to post your schematic for the power supply...? I wouldn't mind checking it out...

As far as home brew transistors, I would say this is basically a "no" - I remember from tech school that a transistor was approximately something like a couple of diodes connected together (its one way you can test transistors with a meter), but that a working transistor can't be built this way (I don't remember the specifics, but it has to do with the PN junctions being directly connected - as in manufactured that way). I also seem to recall (somewhere) a homebrew transistor (or transistor-like) device being made with salt-water and glass jars or similar; it was more meant as a "science-project" demonstration device, and not something for actual use (not very efficient, and so therefore likely wouldn't meet your desires anyhow).

Reloadron · Jun 12, 2011

12 volt fans are pretty forgiving. I would just (as suggested) place a resistor in series with the fan. Note the current at 12 volts and calculate the resistor.

Another option is simply order a 48 VDC fan as they are pretty common. That or use a mains powered fan which is what I likely would have done.

Short of getting involved with more circuits and electronics those are likely the simplest options.

Ron

hannobisschoff@gmail.com · Jun 13, 2011

cr0sh said:
Care to post your schematic for the power supply...? I wouldn't mind checking it out...

As far as home brew transistors, I would say this is basically a "no" - I remember from tech school that a transistor was approximately something like a couple of diodes connected together (its one way you can test transistors with a meter), but that a working transistor can't be built this way (I don't remember the specifics, but it has to do with the PN junctions being directly connected - as in manufactured that way). I also seem to recall (somewhere) a homebrew transistor (or transistor-like) device being made with salt-water and glass jars or similar; it was more meant as a "science-project" demonstration device, and not something for actual use (not very efficient, and so therefore likely wouldn't meet your desires anyhow).

HAHA, sorry cr0sh, I actually meant REsistors. But now I leart something about transistors as well.
Thanks

hannobisschoff@gmail.com · Jun 13, 2011

I got the resistance (at 50mA) needed as 680ohms 3 Watts.

hannobisschoff@gmail.com · Jun 13, 2011

I also have a uA7824, which gives its max input voltage as 40V. This looks pretty attractive, should I risk it...?

stk2008 · Jun 13, 2011

hannobisschoff@gmail.com said:
I also have a uA7824, which gives its max input voltage as 40V. This looks pretty attractive, should I risk it...?

Hi all.

I am not questioning I am a noob and I am inquisitive .

Why would you pros suggest using a uA7824 (what ever that is) at 50v when it says max V is 40v?.

Like I said I am asking to learn.

Thanks

Reloadron · Jun 13, 2011

With about 50 volts I would not think about using any 78 series regulator. However, your call on that note.

Not sure how you got this:

hannobisschoff@gmail.com

I got the resistance (at 50mA) needed as 680ohms 3 Watts.

Most of the 12 volt computer fans I have worked with generally draw about 150 to 160 mA with 12 volts applied. Therefore I see:

50 - 12 / .160 = 237.5 Ohms so around a 240 Ohm resistor rated at about 6 watts so use a 10 watt resistor.

Even if you were to use a 7824 24 volt regulator then what? Now you have 24 VDC and a 12 volt fan. Are 220 VAC fans that hard to find in SA? I assume you have 220 VAC mains?

Ron

hannobisschoff@gmail.com · Jun 14, 2011

I was thinking I could then just hook two fans in series. It would also help for airflow. And my fan drew 50mA when I measured it.
Yes, we have 220V in SA. Well, the reason I don't want to use a 220V fan, is because that would be a bit of an overkill, I have a LOT of 12v computer fans, and the noise levels in my psu would be quite high with a 220V fan.

Thanks for the responses.

hannobisschoff@gmail.com · Jun 14, 2011

stk2008 said:
Hi all.

I am not questioning I am a noob and I am inquisitive .

Why would you pros suggest using a uA7824 (what ever that is) at 50v when it says max V is 40v?.

Like I said I am asking to learn.

Thanks

The uA 7824 is a positive voltage regulator. Generally , if you see "78" in a part number, it's a voltage regulator.

crutschow · Jun 14, 2011

ronv said:
I think you could use a LM317 regulator. It allows 40 volts from the input to the output. Might be ok if your supply only goes to 50 volts maximum.

The problem is that, at startup, there may be 50V across the regulator momentarily until the fan gets up to speed. That could be enough to blow the regulator.

ronsimpson · Jun 14, 2011

Fairchild LM317AHVT Vin up to 60 volts (note AHVT)
TI TL783CKCSE3 Vin 21 to 150 volts
TL783CKC
ST Micro VB409 12 to 500 volts (low current)
Will need heat sink!

Birdman Adam · Jun 14, 2011

I'd just get higher voltage rated fans, 24V fans are rather common. Hook two in series, and your good to go!

Run a 12V fan from 50V rail

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