# Drop 48VDC to 12VDC ?

#### SteveDouglas

Joined Apr 30, 2009
42
I have a 48 volt, 7 amp PS and need to drop down to 12 volts to run two 12 volt 60mm cooling fans each drawing 20ma. I know a LM7812 wont work and think maybe a simple 2 resistor voltage divider will do it.

Can someone advise what resistor values and wattages are required ?

The 48v PS is in my project case with very little room left and this PS is required for the main supply of my project, I just want to add the cooling fans to keep the case cool.

Thanks,

Steve

#### MrChips

Joined Oct 2, 2009
22,100
Put the two 12V fans in series. Now your load is 24V @ 20mA.
The effective resistance is 24V/0.02A = 1200Ω
Now you need to burn off 48V - 24V @ 0.02A, i.e. the same amount as the load.
Hence you need a 1200Ω resistor in series.

Calculate the wattage = 24V x 0.02A = 0.48W

Use a 1200Ω resistor, 1W or higher.

#### bountyhunter

Joined Sep 7, 2009
2,512
DC motors may not like a resistor in series with their power source. I like stacking the fans in series, but you could feed them from a really cheap voltage source with a 24V Zener and a 100V NPN transistor in a TO-220 case, no heatsink needed if you use TO-220 and load is 20 mA.

I think you could also use an LM317HV to create 24V from 48V.

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#### Bernard

Joined Aug 7, 2008
5,686
Or how about a 5W, 20V zener [ NTE 5135A ] in series with 240 Ω, providing about 60 mA starting current? ; or just a 24 V zener, 5 W [NTE5137A ], about 2.5W at start up.

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#### Wendy

Joined Mar 24, 2008
22,176
I may try to draw a crude buck boost converter for this, if you are interested.

#### bountyhunter

Joined Sep 7, 2009
2,512
I may be slow in my old age but enlighten me:

Why would anybody go through all that trouble when you can just use an LM317HV which will not need a heatsink? It will also provide short circuit and thermal protection if the fan gets blocked and draws excess current.

Sometimes life can be simple.

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#### nigelwright7557

Joined May 10, 2008
532
I would put the two fans in series with a high power 1K8 resistor.
I did the same for a disco amplifier I had and it worked a treat.
Although I think the fans were 200mA not 20mA !

#### Wendy

Joined Mar 24, 2008
22,176
I may be slow in my old age but enlighten me:

Why would anybody go through all that trouble when you can just use an LM317HV which will not need a heatsink? It will also provide short circuit and thermal protection if the fan gets blocked and draws excess current.

Sometimes life can be simple.

Assuming a LM317T could regulate 48V to 12V (which it can't, out of spec)...

36V X 0.02 = 0.64W

Hot enough to burn fingertips.

#### bountyhunter

Joined Sep 7, 2009
2,512
Assuming a LM317T could regulate 48V to 12V (which it can't, out of spec)...

36V X 0.02 = 0.64W

Hot enough to burn fingertips.
You need to read my post:

I said to use an LM317HV, not an LM317 standard. The HV version handles input working voltage up to 60V.

And FYI: the thermal resistance of the TO-220 pkg with no heatsink is 65C/W. At 0.64W, that raises the junction temp to maybe 65C. Absolutely no problem. Solder it down to a small copper patch on the PCB and it drops to about 30C/W thermal resistance.

Like I said: Use the LM317HV in the TO-220 pkg.

http://pdf1.alldatasheet.com/datasheet-pdf/view/8608/NSC/LM317HV.html

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