Battery Vs. Power Supply In Toy Motor Circuit - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: April 16, 2020, 02:15:01 pm
content: Hi,I have a toy motor hooked up to two AAA batteries. That's working. When I try to hook up a similar motor to my bench power supply with a small resistor, it doesn't though? Am I not getting enough current from the bench power supply?I know I'm misunderstanding something very basic here.Thanks,Carl

author: Manul
author_rank: Super Contributor
timestamp: April 16, 2020, 02:37:34 pm
content: 2 AAA batteries is ~3V. So adjust your power supply to 3V and connect motor, it will be same. Why use resistor?

author: eev_carl
author_rank: Regular Contributor
timestamp: April 16, 2020, 02:43:43 pm
content: Without the resistor, the bench power supply current protection kicked on. (It's set to 2A.)

author: Manul
author_rank: Super Contributor
timestamp: April 16, 2020, 03:05:42 pm
content: So power supply goes into constant current of 2A and the motor is not spinning? But it spins from 2 AAA batteries? I can hardly believe that.

author: rstofer
author_rank: Super Contributor
timestamp: April 16, 2020, 03:09:01 pm
content: "Small motor" without a photo doesn't mean much. The very small motors used in robotics toys will easily draw more than 1A at startup. Batteries will ride through a certain amount of inrush current. Current limiting power supplies won't."Small resistor" doesn't mean much either. What value? 6V at 2A implies the entire circuit has a resistance of 3 Ohms or less. Part of that is the motor, part is just a heat producing resistor. In any event, the resistor probably shouldn't be much larger than 2 Ohms probably not more than 1 Ohm. Measure the DC resistance of the motor and work backwards to 2A.Or, set the current limit a lot higher...

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2020, 03:11:59 pm
content: Quote from: Manul on April 16, 2020, 03:05:42 pmWhy can't you believe that? I can. It makes perfect sense. AA batteries can provide much more than 2A, especially NiCd and NiMH.Quote from: eev_carl on April 16, 2020, 02:15:01 pmYou need a power supply which can provide more current to start to start the motor, which will be considerable more than the running current. Try adding a large capacitor, say 10 000µF after the resistor. Turn on the power supply. Wait for a minute for the capacitor to charge up to 3V. Bypass the resistor with a wire. Now connect the motor. Hopefully the capacitor will be able to supply the current surge required to start the motor.

author: eev_carl
author_rank: Regular Contributor
timestamp: April 16, 2020, 03:13:50 pm
content: Here's the motor listed on Amazonhttps://www.amazon.com/dp/B07FDRRCQ1/ref=cm_sw_em_r_mt_dp_U_CxhMEbVBFZNXC

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2020, 03:21:03 pm
content: Quote from: eev_carl on April 16, 2020, 03:13:50 pmIt appears to be the same motor as sold by the websites linked below, which specify a stall current of 2.5A, which would also be required to start the motor. Another thing which might work is to slowly increase the supply voltage.https://www.amazon.co.uk/Hobby-Components-Ltd-1-5v-3v-Motor/dp/B071YGQYRBhttps://hobbycomponents.com/motors-and-servos/345-130-motor-eco-friendly-6v-12v-micro-dc-motor

author: eev_carl
author_rank: Regular Contributor
timestamp: April 16, 2020, 03:24:11 pm
content: Quote from: rstofer on April 16, 2020, 03:09:01 pmI tried 3A which is the max setting...all my resistors are <250mW so i can't go down to 10 ohms.Should I go a transistor route? I built a motor circuit with an Arduino and a 9V battery once.

author: Manul
author_rank: Super Contributor
timestamp: April 16, 2020, 03:41:35 pm
content: Zero999, I can not believe that, because I'm not thinking about max peak current it will take on startup if you apply constant voltage. I'm thinking about how much current is enogth to generate enogth starting torque to start spining from a stand still. And 2A will probably spin a car starter motor from stand still (without load). If it is CC of course. If 2A is some kind of cut-off protection, then yes, it will just cut-off immediately and there will be no more current.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2020, 03:44:10 pm
content: Quote from: Manul on April 16, 2020, 03:41:35 pmThe trouble is we don't know whether his power supply goes into constant current mode or simply shuts down.Quote from: eev_carl on April 16, 2020, 03:24:11 pmResistors can be connected in series/parallel to increase the power handling.

author: eev_carl
author_rank: Regular Contributor
timestamp: April 16, 2020, 03:55:02 pm
content: Quote from: Manul on April 16, 2020, 03:41:35 pmI don't think you're right about your assessment of currents. Don't starter motors require hundreds of amps?

author: Manul
author_rank: Super Contributor
timestamp: April 16, 2020, 04:06:18 pm
content: They require hundrets of amps, because they need to rotate an engine and they need to generate something like 10-20Nm to do that, which is a lot.

author: eev_carl
author_rank: Regular Contributor
timestamp: April 16, 2020, 04:08:36 pm
content: Thanks everyone...like the other posters mentioned I had CC on so this was operator error. I usually just play around with ICs so this was set to <1A. I was raising a different current parameter that wasn't the OCP.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 16, 2020, 06:02:35 pm
content: 2 amps would barely activate the solenoid on a car starter motor.Even if you bypassed that, I think you'll find no-load current is still of the order of 10 amps or so, not to mention the initial current while it spins up to speed, probably a spike of 50 amps or so.And that's on a small petrol engine, on a big diesel engine you can probably double that.