1
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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\$ – Spehro Pefhany Aug 2 '15 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\$ – Olin Lathrop Aug 2 '15 at 21:16
  • \$\begingroup\$ @OlinLathrop Yes, this is a good point. A good & safe power supply. \$\endgroup\$ – Spehro Pefhany Aug 2 '15 at 21:21
  • \$\begingroup\$ Possible duplicate: electronics.stackexchange.com/questions/75448/… \$\endgroup\$ – The Photon Aug 2 '15 at 23:51
2
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.