12v to 5v DC resistor - need confirmation

Discussion in 'General Electronics Chat' started by Carsten Svendsen, Oct 10, 2015.

  1. Carsten Svendsen

    Thread Starter New Member

    Oct 2, 2015
    I've got a Harddrive which I want to power from the wall outlet and plug into the PC via e-SATA.
    On the harddisk there are these ratings:


    Now since the load is 0,7A we know that there will be the same amount over R1 which means the resistor would have to be 10 ohms right?
    I know it's better to do with a voltage regulator, but I don't have one and I just happen to have an 11 ohm resistor lying around.
    Could the HDD still work with 4,3 volts?
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    Unlikely. As the HDD drive draws from its 5V (or 4.3V) supply, the current draw will likely vary. Ohms Law then states the voltage dropped by your resistor will change. You will end up with more or less than the expected voltage and the drive may fail.
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    7 V drop at 0.7 amp = 10 ohms 5 Watt

    it may work ok with a 11ohms, i personally would use a regulator...or transistor and zener..

    Better still use the 5v rail in the PC,...
    Last edited: Oct 10, 2015
  4. crutschow


    Mar 14, 2008
    I think there's a good chance a resistor will blow the HD circuit.
    Better to use a regulator such as the common LM317 or 7805 (note that, since it's dissipating about 5W it will need to be on a small heatsink).
  5. Carsten Svendsen

    Thread Starter New Member

    Oct 2, 2015
    Yeah you guys are probably right, it's just that there's only one retailer in Denmark that sells loose electronic parts with everything you could imagine. The shipping is just so expensive though. Maybe they have one down in my local radio shack.
  6. umphrey


    Dec 1, 2012
    How do you have a PC without a 5V rail somewhere? USB 3.0 provides up to 900mA.
  7. garyy

    New Member

    Jun 4, 2014
    it is better to get a dedicated 5V rail.
  8. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    If you measured the 5 V current and got a value of 0.7 A, then maybe. If it says 0.7 A on the device label, then no. That is a max value, not average, and a lower average current will mean a lower voltage drop across the resistor, and a higher voltage at the device.

  9. Carsten Svendsen

    Thread Starter New Member

    Oct 2, 2015
    Actually, I have now hacked a 12v DC converter and a usb wire onto the sata power cable but the HDD won't even start.
    I tried supplying it directly from my PC PSU and then it worked fine. Not sure why it won't work on the mains power then.

    Here's what I did

    I have not measured if there's any power in the sata cable as I don't have any connectors or pins or anything small enough to measure it inside the plug.
  10. Carsten Svendsen

    Thread Starter New Member

    Oct 2, 2015
    Omg I'm so stupid. I accidentally used the wrong power supply that was rated at 9v.
    That's what I get for having too many projects going at the same time I guess.
    I exchanged it with the correct 12v powers supply and now the drive works exactly like I expected it to.
  11. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Thanks for posting a conclusion. I'm happy to hear it is working ...for now.