I have a 20 volt, 90 watt laptop power supply I would like to use to power a 12 volt winch I use to open and close a heavy overhead shop door. The amps drawn are within that provided by power supply.

There is no documentation available for this winch.

A cars electrical system charges at anywhere from 12 volts to 14.8 volts and the 14.8 volts does not seem to adversely affect the winch.

20 volts is only another 5.2 volts above the 14.8 volts provided but there must be some limiting factor. A point where the overvoltage damages the motor after a short time of use, say the time it takes to open the door. I don't want to hit that point.

I realize that more info is needed for a definitive answer but I'm really just asking for a "rule of thumb" type answer and an explanation why this is so.


  • 2
    \$\begingroup\$ How many amps does the winch need \$\endgroup\$
    – Passerby
    Dec 17, 2017 at 8:27
  • 2
    \$\begingroup\$ How do you know that the supply will provide enough current particularly when the motor is starting? It sound pretty unlikely to me. \$\endgroup\$
    – RoyC
    Dec 17, 2017 at 8:31
  • 1
    \$\begingroup\$ measure the DC resistance of the winch, that will help you calculate how much current is needed to start the winch. \$\endgroup\$ Dec 17, 2017 at 8:40
  • \$\begingroup\$ Put a charge controller between the power supply and a car battery, and run the winch off that. \$\endgroup\$
    – user16324
    Dec 17, 2017 at 11:38

1 Answer 1


Even though you failed to specify the load rating on a say 1~2 ton winch, you overlooked the basics of Ohm’s Law for motors with equivalent load resistance and power for no load + load. I doubt you measured the DCR of the coil to see if that did not exceed the OCP limit of the supply.

Consider ; Is motor DCR < V²/Pd=20²/90=4.4 ohms? Then the supply will fault out and shutdown.

The rule of thumb is the voltage source should be capable of supplying Vbat/DCR without blowing a fuse and >10% of Vbat/DCR at rated load and max current about 1% V drop desired and 10% V drop tolerated.

These simple equations and winch milliohm DCR test measurement will tell you must have at least a motorcycle battery and probably a car battery at 12V+- 20% and never 20V @4.5A.

not even close

Edit —— Plan B

Get a 500W+. PC ATX supply and use heavy gauge wire to the 12V supply 20A to 30A+ capacity. 5V may need a 5% dummy load of its rating to prevent under voltage trip on 12V. Gnd jumper to PWR enable on ATX connector is mandatory.

  • \$\begingroup\$ Thanks for the answer. I don't know the formulas you speak of but I do get the gist of what you are saying. Please understand, I'm trying to eliminate having a battery and charger inside the building. I will read up on your recommendations and try to work out a plan of attack. \$\endgroup\$
    – Don6558
    Dec 18, 2017 at 5:14
  • \$\begingroup\$ 12V 20A supply is far better than 20V, 4A \$\endgroup\$ Dec 20, 2017 at 16:24

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