I just wanted to check that this setup is safe as I am not an electrical engineer, nor am I experienced...

I am using an AC adapter which outputs 12 volts DC. It has a max current rating of 6 amps. I have wired it directly to a DC motor (rated for 12 volts) using wire rated for 21 amps. The AC adapter has a pin output like on a laptop charger, so I used an adapter which allows me to screw in the positive and negative wires. (Looks like this: https://uge-one.com/image/cache/catalog/catalog/0%20UGE%20JACK%20ADAPTOR%201-550x550.jpg) I intend on using some insulation tape around this to make it safer.

The motor drives a system of pulleys to lower/raise a shelf in a linear motion, so it has some load. I tested the current drawn with this load with some batteries. With 5.9v across the motor, it drew 1.8 amps (with a load).

The eBay listing for the motor (I couldn't find a decent datasheet) states that its locked-rotor current is 3.25 amperes. I assume that this is the maximum current it can draw, although could the starting current exceed this or exceed the max current rating of the AC adapter (6 amps max)? I ask this because the load could increase as it is driving a shelf with items on. I am not sure if this is something to worry about as the item description for the AC adapter says that it does have over current protection, so what's the worse that could happen?

So, my question is, does this circuit sound safe with my current, tested load, and would it be safe if the load were to increase? Is the safety gap between the amps the motor is drawing and the AC adapter rating large enough?

Many thanks in advance!

The AC adapter that I am using: https://www.amazon.co.uk/gp/product/B074HX4GJ3/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

The motor that I am using: https://www.ebay.co.uk/itm/775-DC12V-4500RPM-Motor-D-Style-Axle-Mini-Generator-DIY-High-Torque-Bearing/312542643980

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    \$\begingroup\$ Not an expert, but AFAIK, start current should be the same as locked rotor current, \$\endgroup\$ – immibis Aug 1 at 3:13

If the motor 3.25 A locked rotor current and the 6 A power supply output current rating are reliable, that should be ok. A lot of power supplies have over-current protection that is quite sensitive and shut down when a motor is switched on.

I would be concerned about the motor current at full speed. Your 5.9 V test would have only driven the motor at half speed. Even the 1.8 amps that you measured may overheat the motor depending on duty cycle.

  • \$\begingroup\$ Thanks for your answer. Is there a safe way to test this at 12 volts? Perhaps by powering the motor for short periods of time with the power supply to see if the motor gets hot? \$\endgroup\$ – Callum Aug 1 at 2:09
  • \$\begingroup\$ Yes, testing under load, starting with short duty time would be good. You can also search the motor numbers to find another seller of the same motor with more complete specs. \$\endgroup\$ – Charles Cowie Aug 1 at 2:49
  • \$\begingroup\$ I found a motor that has the same specifications with one difference. Instead of the 3.25 A figure described as the "locked rotor current," it is described as "current with load." I suspect that 3.5 amps is the maximum continuous current. The locked rotor current mat be more like 30 amps. If your power supply shuts itself down or emits a puff of smoke the second you connect the motor, you will know that the motor seller's "specification" is garbage, pretty much the norm for eBay. \$\endgroup\$ – Charles Cowie Aug 1 at 3:32
  • \$\begingroup\$ Thank you for your help. I'll give it a test. \$\endgroup\$ – Callum Aug 1 at 13:14

Stall current is(theoretically) the maximum that the motor will draw, so your power supply should be more than sufficient for your use.

Some power supplies don't use very large output filter capacitors, though, so it never hurts to add an additional somewhat-largish capacitor across the output of the power supply and/or input of the motor, at least a 16 Volt electrolytic at 1000uF or greater, maybe a lower 330uF/470uF or so on the motor if it's current is toggled between the power supply and the motor. Keep in mind that the additional capacitors will require the power supply to attempt to provide slightly more current than normal upon startup than it otherwise would, so low ESR capacitors should not be used as they could be exposed to a still-existing switching-frequency component if the inbuilt capacitors are not ample.

  • \$\begingroup\$ What do you hope to gain by adding capacitance? Note that this challenges the turn-on behavior of the supply. \$\endgroup\$ – Chris Stratton Aug 1 at 1:56
  • \$\begingroup\$ @ChrisStratton - Strictly to add better regulation between motor startup and shutdown situations, in case the power supply has lower value output capacitors. You are correct about the added current draw on startup, which is why I did not recommend low ESR capacitors. \$\endgroup\$ – Hitek Aug 1 at 2:19
  • \$\begingroup\$ @ChrisStratton - I had already edited my answer before I noticed your comment, so I did lower the recommended capacitances in a switched motor scenario. After considering your last comment, I do think I should stipulate that low ESR capacitors should not be used. Edited, and thank you! \$\endgroup\$ – Hitek Aug 1 at 2:27
  • \$\begingroup\$ Whomever is down-voting, please feel free to let me know of any errors in my answer so I can correct them, or even tell me I'm outright wrong, in which case I can delete the answer... \$\endgroup\$ – Hitek Aug 1 at 2:53

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