3V Hobby Motor in a 6V Circuit.

Hi. Hope I'm posting appropriately. If not, please forgive an older guy!
I'm working on a Useless Box project that I power from a 6V wall wart. I've already completed one and it works great. I use two standard servos with an Uno. But now, rather than being satisfied, I'd like to add a 3V hobby motor, which naturally would only run randomly for short periods of time. I have purchased an AMS1117 step down module which is rated at 800mA from Amazon to get the correct voltage for my little motor. My question is, do I need to add a capacitor across the leads or to the casing. If so, how do I determine what size? And while I'm asking, is there anything else I need to think about? I appreciate any help as this is a new hobby for me.

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randofo3 days ago

Running a motor rated for 3V at 6V (even with PWM), is generally not a good idea. That said, if it is only on for short bursts and using a PWM signal, you may be able to get away with it.

The issue is that if you run the motor with more voltage than it is rated for, the coil inside will start to heat up, things can start to melt, and it may eventually short circuit (or just stop working).

If possible, try to find a motor rated for the voltage you are working at.

Steve_Lane63 (author)  randofo2 days ago

Hi Randofo! I took a robot course that you taught, which included your Telepresence Robot. You were an amazing teacher, and there have been many times I wanted to ask you a question b/c you don't talk down to people. So thanks for responding. I wasn't planning on powering my 3V with 6V directly. My plan is at this point use two equal resistors to get to 3V. I think they call it a chopper circuit? Anyway, It's a voltage divider. And after I get the motor to 3V, I have to worry about the current, so was planning on using a MOSFET with flyback diode and resistors to switch on the motor. I use a stuffed animal (cat) in my useless box, and the small motor is to be mounted inside the cat's head as a vibration motor. This is only an add on to a box I've already completed, and works great. I still go back to my notes from you class! Do you have any ideas? Am I way wrong with my thoughts? And I was thinking about an 2N7000 or IRF540. Would they be suitable or should I use something else? Thanks for taking the time!!!

Oh yeah! You did complete the class. I knew your screen name seemed familiar. :)

The suggestion below about using a zener diode might be worth investigating, instead of using the resistors. The MOSFET and diode sound about right otherwise.

I have been using a IRLB8721PBF recently, but the ones you listed should work (I would think).

Steve_Lane63 (author)  randofo2 days ago

Awesome. I had just now ordered the MOSFET you suggested. The spec sheets are way above my pay grade - lol. At my age, I don't really want to learn the equations to figure out what transistor I need out of the 1000's out there. And wow, I hadn't seen the suggestion below. Again, thanks! But now I'm confused. At the risk of you hating me, I want to ask why my voltage divider isn't ideal. I have two equal value resistors in series from my PSU positive lead, and the voltage between the resistors is 3v which I tie to my motor and diode & cap, then why would I need to use the method below? I'm a pain, huh?

Hi Steve,

Your voltage divider would be very lossy and if you used two equal value resistors, the addition of the motor would unbalance the divider (i.e. one of the resistors would seem a lower value due to the motor).

If you'd go that way (you won't, trust me), it would be a bit better to find the current of the motor running on 3V (e.g. 2 fresh AA batteries) and then calculate the equivalent resistor - then match this with a single equal value resistor in series, to get 3V over the motor.

Since the current is likely in the range 50..100 mA (with perhaps a 2..3 times that surge at start). you could either use a 3V/1W zener diode or 4..5 serially connected 1N400n (1A) diodes to replace the single resistor, taking the calculation out of it, but the loss will be the same, as well as in a zener controlled transistor - hence the PWM suggestion, as it's virtually lossless.

Please don't put anything that could potentially get too hot into your cat, especially if it's made of poly-something :)

Regards

Steve_Lane63 (author)  Omniventyesterday

Hello again, I decided to bag the divider idea after digging a little more. At this point I'm leaning towards the zener. Do you have a suggestion for a transistor? As far as safety is concerned, I am planning on mounting only the motor in a medicine bottle with a bit of rubber and zip ties. All my other electronics are away from cat parts. Most of the cat has been cut away to allow for space so the box can close easily. Thanks again!

Steve_Lane63 (author)  Steve_Lane632 days ago

I did some more research, and now I see why a divider isn't the best choice at all. So I guess it's the zener. However, I watched a video that said linear regulator was a better option. Seriously though, what would you do?

For what you are doing, I think you can get away with a zener diode. I think getting a regulator is probably overkill.

Steve_Lane63 (author)  randofo2 days ago

Thanks. That's what I'll do. Can you, by chance, point me to a place that explains when a zener circuit, a voltage divider and a linear regulator are the appropriate choices. If not, that's fine. Just thought I'd ask.

Hi randofo,

I beg to differ.

6V at a 50% duty cycle equals 3V RMS into a DC load like a resistor.

With a DC motor (the model is a voltage source in series with the inner resistance and an inductance),the average will be even lower, determined by the parameters of the actual motor and the frequency of the PWM signal.

The motor will run on this without the same amount of heating as with 3VDC, as the inertia are way too large to not notice the much faster 0V-6V-0V-6V pulses, which will typically be around 25kHz (to keep the vibrations of the windings due to electromagnetic push-pull action inaudible) equating to 40µs a period, while a small vibe motor going 15kRPM (probably a bit optimistic) will take 4ms/rev, leaving 100 periods of PWM each revolution. Never caused any of my motors any grief either, when I have done so (even needing to go to 1/4 the input voltage at some instances) - Just look at how SMPSU's work, they're closely related.

But... If you have conflicting evidence, I'd be very interested in your source.

Regards

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