Trying to drive a motor through a transistor

Hi,

I am trying to drive a coreless DC motor (used in RC planes), for this I use a NPN transistor, with base connected through a 4.7K resistance, the collector is connected to one end of the motor and the other end is connected to +4.7V (motor supply). Emitter is connected to ground

When the resistance is made high, for some reason the motor does not run at full speed, the current drawn is only 110 mA.

How can i make the motor run at full speed ? When the motor is directly connected to the +4.7V supply, the current drawn is 600 mA.

The basic idea is to use a micro-controller, with PWM output to control the speed of the motor.

thanks
a

It appears your 4.7k base resistor applies 1 mA. Therefore your gain would need to be 600x, if you want it to conduct 600 mA.

You can try reducing the base resistance. Then it's a good idea to hold the transistor between your fingers, because there is a chance it will overheat. Then you'll need either a larger package, or heatsinking, etc.

Hi,

there are different transistors. It´s a good idea to give us the partname.
Then we can calculate.
***

Consider to use a n-ch MOSFET. Use one with logic level threshold. U and I is known. r_ds_on should be 200mOhms or less.

In either case use a diode across the motor.

Klaus

When I first saw the name of the thread I thought about this circuit:


Now I am asking myself, which one is better ? What is the difference between placing the motor on the Emisor or in the Collector?

Consider to use a n-ch MOSFET

Click to expand...

Why use a MOSFET instead of a BJT transistor? Just because the gate current is zero ?

Both will work but if you put the motor in the emitter, the voltage to drive the transistor is higher by the motor voltage itself, it follows you can never drive it to full power unless the PWM is at least 0.7V higher than the motor voltage. Remember it's the voltage between the base and emitter that lets base current flow.

If you put the motor in the collector, the voltage required at the base will only be about 0.7V so it's much easier to drive and the base current will be far more predictable.

A MOSFET when turned on, behaves more like a low value resistor (maybe only a few milli-Ohms) so it will allow more current to flow than a bipolar transistor and also dissipate less heat. The drawback is that being voltage rather than current driven, you must ensure the gate voltage is high enough to make it fully conduct.

Brian.

Whatever the choice of the transistor type, it is highly advisable to place a capacitor in parallel with the terminals of the motor to prevent the generation of voltage spikes. Honestly, I would prefer add even few more components to implement a switched power supply - as the Buck topology, for instance - in order to ensure a better working, due this way would hold an average current through the motor with a ripple amplitude not too big.

it would be highly advisable to insert a capacitor in parallel with the terminals of the motor, to prevent the generation of voltage spikes

Click to expand...

Those voltage spikes weren't supposed to disappear by connecting the diode ?

In fact, the diode should suffice, but to be honest I'm unsure if there could happen some kind of damped oscillation, so that at the opposite cycle would be not caught by the diode; in the lack of certain, I allways put a capacitor there.

Hi,

Now I am asking myself, which one is better ?

Click to expand...

Motor control.. I prefer a low power solution. Therefore i'd prefer to connect the motor to the collector.
With a usual bjt you should apply at least 1/20 of the collector current to the base.
With 600mA motor current this means 30mA base current.
Some controllers have problems to drive 30mA.

Therefore i'd use a Mosfet. It needs about no drve current. On the other hand the mosfet are low ohmic. This causes low voltage drop and low power dissipation. Surely a SOT23 can do.

I don't like the motor to emitter solution. I can't find a benefit in this. It needs low impedance base drive, and because of the high voltage drop there is a lot of power dissipation.

Klaus

Thanks everyone its working OK now, the problem was in the 4.7 K resistor.

Now that is working, could you please give me this information about your final circuit ?

How much current can your microcontroller source?

Partname of your transistor, base resistance and characteristics of your motor.

Thanks

- - - Updated - - -

And if your Vcc is 4.7 V, what is the amplitude of your PWM then ?

I am using the cy8ckit-042-4200 board from cypress. The transistor is BC149C, base resistance is 330E, the motor is a tiny one used in RC helicopters, I thinks its called size 0.

Operating voltage of motor is 3.7 V, but for some mine works better at 5V.

I have managed a simple PWM, but I have abandoned the approach, as it is getting needlessly complicated. I am thinking of using wired control connected to pots instead of wireless.

The RC helicopter I have has a small chip, which is presumably the infrared receiver, and motor controller, but there is no name on the chip


thanks
a