Costume electronics help


#1

So I’m trying to run a motor and lights 9n a costume. I’ve got the lights working in 12v,but the small motor I have goes too fast at 12v. Luke and the internet suggested using a 7805 voltage regulator to drop the 12v down to 5v. But now it seems like the motor isn’t drawing enough current at startup to get going.

@gibbtall uploaded a file: IMG_20170618_190930.jpg and commented: This is the dumpy motor. It runs fine on 5 or 12v alone. But if I try to put 12v through the regulator it doesn’t run or goes suuuuper slow.

I’m willing to change the motor, I have another 12v motor sitting here, but it’s too fast. Gearing either motor would be too complicated mechanical and would be prone to breakage.

End result should be a roughly 60rpm motor with a reasonable amount of torque, in case of slosh, running off of a 12v power source.


#2

Uploading…

So Mostafa did this, but neither him nor Mimi can explain why it works? Something about stores voltage?


#3

since the picture isn’t loading, the solution was to put a 330nF cap between the input and ground. I assumed the cap in the application note is just for decoupling? why does adding it all of a sudden allow the regulator to drive the motor?


#4

Is it possible that the cap provides just enough current storage to help
the motor overcome static friction?


#5

Do you mean 600 rpm ? 60 rpm is 1 turn per second - pretty slow without a gearhead. But looking again at the photo, it seems to have a gearhead already.
Another idea is use batteries: 4 x 1.2 = 4.8v
Or just put a series resistor to the 12v instead of the regulator…


#6

Yes 1 turn per second, it’s going to drive a large foam wheel.

I was trying to avoid resistors or just batteries because I don’t want it
to slow down and possibly stop as the charge goes down. I’m ok with it not
lasting forever, but I’d like it to keep working with the lights until the
last second, within reason.


#7

It’s not really clear if your problem was fixed. Low speed DC motors don’t
exist, they have to be geared down - A common / easy way of doing this is
taking a hobby servomotor, and modifying it for continuous drive. There a
lot of guides on the internet to do this.


#8

I haven’t soldered it all together again to see if it’s the speed that I need but so far the solution Mostafa gave me seemed to work. The smaller yellow motor is already geared down, @bruce knows them far better than I do and has a few that are slower than this one that I’ll probably try in case this one is going too fast.


#9

Low speed DC motor’s don’t exist? Why is that?

Aren’t low speed dc motor’s what are used on electic bikes and electric cars
with direct drive to the wheel?


#10

Those are brushless DC motors - You’ll see them referred to as BLDC motors
more often. Despite the name, they’re actually three-phase AC.

These are also the same kind of motors used in quadcopters, and they’re
also taking over most modern applications because they’re very efficient,
and can be controlled very precisely. They’re closer in topology to stepper
motors than conventional DC motors, but with three windings and they’re
totally awesome.

So, they’re better for a lot of reasons, but you need purpose-built
controllers for them, instead of just hooking them up to a battery.


#11

The regulator might be taking a lot of abuse from the motor (back emf,brush noise etc) that might end up killing it. I’d test it out for longevity.


#12

You may want to get some of these el-cheapo PWM controllers:

https://www.aliexpress.com/item/Top-Sale-10pcs-lot-DC-6V-28V-3A-PWM-Motor-Speed-Varible-Regulator-Controller-Switch-1203B/32797820334.html

Doesn’t help you right now but order a handful and you’ll have some for the next costume you need one in.


#13

These motors have a gearbox and differential in them to drive small model cars. They’re geared down a lot.

Is this absolutely fixed? You’d be better off with a 6V supply direct to the motor.

If you’re stuck at 12V, with a 7805 you’re burning off more than 50% of the supplies energy as heat. It might be best to just use a big resistor.


#14

Have you measured the load current that the motor draws? It’s hard to judge the size of the motor from the photos, but small DC hobby motors typically draw 500-1000mA in stall. Without some decent amount of cooling, this is way too much power dissipation for a 7805 to handle (500mA * 7V = 3.5W). Even if the load current is only say 200mA, it’s still going to get very hot and may go into thermal shutdown on you. It’s also wasting more power as heat than it is turning your things. A 5W resistor of appropriate size is a better choice if you don’t have other options (different battery, switching converter, PWM control etc.).

Do you have any other electronics on the costume that might be able to generate a PWM signal for the motor? Simple PWM works well with DC motors and would be much more power efficient. All you’d need is a suitable MOSFET.

Also you should put a reverse diode across the motor to protect the regulator from back EMF and other garbage from the motor windings.


#15

If you are looking to design your own BLDC motor driver.

You might find this useful.

http://www.ti.com/tool/tida-01417?DCMP=mytinwsltr_06_18_
2017&userInfo=Mx4rxlB/JPCyXF77TjqV5g==&article_name=
sys-ind-app-bldcretrofit-nsl-rd-tida01417-wwe&newsletter=
11-JUN-17&eloquaCampaignId=522&utm_campaign=myTI%
20newsletter%202017-06-18&utm_medium=email&utm_source=Eloqua


#16

the cap is on the input side, where the supply current should be theoretically unlimited.
would make more sense to me if it worked with a cap on the output if that was the case?
black magic!


#17

On second thought though, you can push a regulator by giving it more heat sink, but resistors not so much… assuming the problem is current draw and not something else like just the part not being installed as per the data sheet requirements (black magic)!


#18

That’s amazing. And free shipping. I’m seeing a lot of stuff on ebay going for what must surely be less than even the cost of shipping it from China. Like what’s going on I wonder.


#19

There are a few good articles around on this. China is still treated as a 3rd world country wrt mail pricing so it is subsidised. The mailing cost of stuff like this into Canada is covered by the cost of mailing stuff out of Canada.


#20

I use small buck converters for projects like this. At around 80% efficiency, They are far better than the battery killing linear regulators (like the 7805) which waste nearly 2/3 of 12V battery power. Output voltage is adjustable too. Ok, you may not want to use them to build pro audio devices ( bucks are noisier), but at about one US dollar each, what’s not to like?