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### Topic: 3V motor to motor shield? (Read 1 time)previous topic - next topic

#### wolly981

##### Apr 12, 2014, 11:04 pm
Hi there,

I am quite new to this, and I have done a lot of research, but I was just wondering if I can connect a 3V motor to the motor shield using a 9V battery as power supply and simply slap a load of resistors between the shield and the motor until the voltage is correct for the motor? I'm guessing this is pretty inefficient and I know your not recommended to use 3V motors but I was just wondering is it possible?

Thanks.

#### Grumpy_Mike

#1
##### Apr 12, 2014, 11:10 pm
Quote
and simply slap a load of resistors between the shield and the motor until the voltage is correct for the motor?

Yes
Quote
I'm guessing this is pretty inefficient

Yes and the voltage dropped will not be constant it will depend on the current draw on the motor. This increases as the load increases and so the more load a motor has the lower the voltage it will have across it. Rendering the motor much less effective.

#### raschemmel

#2
##### Apr 12, 2014, 11:13 pmLast Edit: Apr 12, 2014, 11:15 pm by raschemmel Reason: 1
Technically you can do that but you have to calculate the power dissipation of the dropping resistor:
the formula is as follows:
Can you post a link to the website that sells it ? (can you ask anyone who knows ? (like maybe your professor ?)

Power
example:
Let I motor = 300mA
P= I x E = 0.3A  x 3V = 0.9 Watts ( 1W or greater dropping resistor ok)
Let I motor = 800mA
P = I * E = 0.8A * 3V = 2.4 Watts (need 3W resistor)

I suppose you don't want to hear that your 9V battery will either not work or only last a minute or two.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

#### wolly981

#3
##### Apr 12, 2014, 11:21 pm
Ok thanks everyone for your replies.

I have not much info on the motor other than it's a motor used in the gearbox of a k'nex kids construction kit which takes 2 1.5V batteries in series. The weight of the whole project should weigh no more than 800 grams but I've got no idea about the load on the motor. Unfortunately I have no professor or anything I'm still in high school so I'm all alone on this

Once again thanks for the help

#### raschemmel

#4
##### Apr 12, 2014, 11:22 pm
TWO 1.5 volt batteries..
D, C , AA or AAA ?
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

#5
AA sorry

#### raschemmel

#6
##### Apr 12, 2014, 11:32 pm
http://en.wikipedia.org/wiki/AA_battery
Alkaline AA = 1800 -2600 mAh depending on manufacturer.
So can you tell us how long the motor can run on two AA batteries at full speed ? (try it)
OR , you could get a meter and move the positive lead to the current jack and measure the current by putting it in series ..
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

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