I don't have much experience with electronics, I'm simply trying to get (at least what I think) is a fairly simple circuit running.

I've got a blower fan that I need to get running. It uses 12VDC at 4.3 amps. I've got a DC converter that converts 120VAC to 19VDC, but that's too high. I want to step down the voltage from the converter to around 12 volts DC, and I was thinking simply using a resistor might work, though I'm none too sure how the calculations are worked out. I tried an Ohm's law calculation and arrived at that I would need a 2ohm resistor for this, but I'm not sure.

  • \$\begingroup\$ Suggest you buy a 24V to 12V converter module with an 8A rating approximately and an input range that includes 19V. A ~2\$\Omega\$ resistor would probably work but would get very hot - about 30W. \$\endgroup\$ Commented Aug 2, 2015 at 20:50
  • \$\begingroup\$ @Spehro: If he's going to buy a converter, he might as well just get a 12 V power supply in the first place. \$\endgroup\$ Commented Aug 2, 2015 at 21:16
  • \$\begingroup\$ @OlinLathrop Yes, this is a good point. A good & safe power supply. \$\endgroup\$ Commented Aug 2, 2015 at 21:21
  • \$\begingroup\$ Possible duplicate: electronics.stackexchange.com/questions/75448/… \$\endgroup\$
    – The Photon
    Commented Aug 2, 2015 at 23:51

1 Answer 1


You have a 19 V power supply, but what you want is a 12 V power supply. Instead of converting the 19 V to 12 V, just get a 12 V power supply in the first place. That's a common power supply voltage, so there will be lots of choices. Then you still have the 19 V supply to do other things with.

Using a resistor to drop the voltage can work if you know the current well, but it will waste a lot of power. The extra power usage may not be a concern, but getting rid of the heat may not be so simple.

You want the resistor to drop 19V - 12V = 7V at 4.3 A. From Ohm's law, that would required a (7 V)/(4.3 A) = 1.6 Ω resistor. The power dissipated will be (7 V)(4.3 A) = 30 W. Not only does the resistor have to be rated for that, but you have to allow 30 W of heat to be moved away from the resistor.

Look up how big such resistors are, what they cost, and how you have to mount them to allow the 30 W to be removed in steady state. The plain old 12 V power supply will look pretty good about now.


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