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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!

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Multimeters are not ideal for finding the peak current draws. Even if 1.7A is what is shown on the screen as the peak, it doesn't mean that it is actually the max that was pulled. Since 2 Amps is close to what you are reading as peak, there is a good chance that you are pulling too much current from the power supply.

Also, many power supplies drop their voltage even before you get to their current limit. You would have to find the datasheet for your supply to know what its current/voltage curve looks like. It is possible that your PSU just isn't providing the required power to get engage what ever is doing the work in the gun. The stall current on mechanical items can actually be much larger than the current that is pulled afterward.

Ideally you would want to get a scope setup to see what is going on, but if you don't have one it will be much easier to just get yourself a larger power supply.

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  • \$\begingroup\$ Gotcha, thank you for the response it is as i feared. Luckily my external hardrive power source seems to work decently well.. I should probably look into how to use a scopse, have my pops old one somewhere :) \$\endgroup\$ – AlexS Feb 29 '12 at 23:15
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As Kellenjb said, the initial current when starting the motor will be higher, possibly quite a bit higher, than what a multimeter will tell you. The right answer is to get a power supply rated for more current, like 5A. That will most likely do it.

Another possibility is to get a large capacitor and put it accross the power leads as close to the motor as possible. By "large", I mean on of those electrolytic power supply caps, sometimes called "computer" caps. For this to have any chance of working it will need to be the size of a good size soup can. Something like 22 mF at 15 V is a starting point.

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