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Kendall

Powering Nerf Gun With Wall Adapter

I have the Nerf Stampede gun that i am planning on making an automated turret from and need some help running it from a wall adapter, so here is where i stand:

The gun normally requires 9V (6 batteries) but i have been running it off of 8 for 12V and that is fine so far. After measuring with a multimeter and external battery pack it seems like the peak draw for the gun is ~1.7A which seems right and easy enough to cope with. My problem begins here:

I've tracked down a few power supplies, the current one i'm trying to use is is an audiovox 12v and 2A adapter, seems regulated and likely by switching as opposed to linear regulation as it's very light and i get a good ~12V even with no load.

But for some reason when i hook it up in the place of the battery i hear the motor whine a little a few times a second and the AC adapter seems to switch on and off since the power light blinks on and off.. Seems like it doesn't have enough juice but it's easily rated for enough...

Is this common? If so is there any solution? Or tough luck find a different supply...

Thanks!
Sparky2504

Is the power supply 12v ac or dc? If it's ac that will be your problem as the motor is dc
OldNoob

Your current draw calculation is off. God knows why you are using a wall wart, but the Stampede draws far more than 1.5A. The PSU you have is tripping it's overload protection.
If you want a proper, reliable way to run it you should just put a Nimh pack in. A 3000mah one will run a Stampede for hours and hours, along with any accessories. You can run 3s Lifepo as well if you need 12v.
Otherwise you will need a more powerful PSU. A 100w laptop PSU is about the minimum. I would also swap the output plug to XT60.
I have seen 6kg Stampedes pull in excess of 15A
SSGT

EDIT

Moved to Q&A.

SSGT



As above, it sounds like the PSU is tripping it's overload protection. There are a couple of problems trying to measure current with a multimeter. Firstly the act of measuring current effectively changes the amount of current drawn. It's very hard to measure current directly so you have to measure a byproduct, in this case it measures current by instead measuring the voltage drop across a known resistance in series with the device (a "shunt" resistance). This can be a problem with any ammeter if you don't know what you're doing since selecting an inappropriate shunt resistance will either result in too low a voltage drop for the equipment to gain an accurate measurement or cause a significant voltage drop which will, in turn, result in reduced current draw - you need to vary the shunt resistance based on the magnitude of the current you're trying to measure.

That said, this probably isn't the main issue in this case - the main issue here is likely that the multimeter is simply incapable of reacting quickly enough to measure the peak current (and even if it could you probably wouldn't be able to actually read the peak current from the display in time to write it down). What you would really need to measure it is an oscilloscope (along with a couple of different valued shunt resistors to allow you to accurately measure both high/low areas of the current trace), although you can estimate based on datasheet specs.

I don't have specs for the exact motor used in the Stampede (RS360PH-3560) but I do have data for a very similar motor (RS360SH-3560 - should be the same wind but with slightly different magnets - if I'm right about "S" spec magnets having a stronger field than "P" spec magnets then the no-load speed should be slightly lower, and stall torque slightly greater, but stall current specs should be practically identical). At the instant you close the switch the motor isn't spinning and so will demand it's stall current at that voltage - it'll never quite draw the listed stall current (due to inductance of the wire causing current to ramp up rather than instantly heat peak and due to some voltage sag due to the internal resistance of the power supply) but, as long as the supply voltage doesn't sag too much, it'll be close. For the case of a 3560 wind 360 motor that's 22.31A @ 12VDC. It won't draw that for long but if the PSU can't supply it, or has a protection circuit, it will either take longer than it should to spin up the motor or it will simply cut power to the motor.

That is only at startup although even if you could "soft start" the motor to curb the initial current draw you'd still likely trip the circuit once per cycle. Since the gearbox has to compress a spring, the load on the motor (and therefore current draw) cycles between next to no load and the load required to compress the spring fully - it isn't a constant draw. The gearbox has a ratio of 102:1 with a final drive pitch circle diameter of approximately 40mm so the stock spring (supposedly rated to 2.5kgf) will put a max load of at least 48.5gf.cm on the motor at full compression (potentially more depending on gearbox efficiency). At that load the motor will be demanding at least 2.27A peak per dart fired (closer to 2.6A if we assume the gearbox is only 80% efficient). That is still more than enough to trip your wall supply once per shot. If you replace the spring with something rated to 6kgf that goes up to at least 4A once per cycle (5A if gearbox is only 80% efficient). The motor will never draw less than the no-load current of 0.94A @ 12VDC (probably more like 1-1.5A depending on how inefficient the gearbox is/how much friction it adds to the drivetrain - the motor will never be completely unloaded) and will only draw that when it's freewheeling immediately after firing.

tl;dr: You really need to spec the supply to meet the inrush current of the motor. In this case, for a 3560 wind 360 motor on 12VDC, you really need something rated to at least 25A (i.e. a 300W 12VDC output PSU).

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