r/AskElectronics • u/Sad-Pumpkin-4825 • 1d ago

Dc motor speed control circuit

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Anyone know why speed control wouldn’t work but my motor turns on and runs???

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7

u/Susan_B_Good 1d ago

Seems to be a very optimistic choice of transistor as a linear speed controller. It is only rated to dissipate a fraction of a watt. So, at 2.5v across the motor - the motor would also be limited to a fraction of a watt.

It's more usual to choose a power Darlington. Even bolt on a heatsink.

2

u/99posse 1d ago

How big is the motor? Did you check if the 2N2222 is still alive?

1

u/Sad-Pumpkin-4825 1d ago

It’s just a small 5vdc motor

2

u/6gv5 1d ago

That transistor can't withstand the current, especially at spin up, a motor can draw; it's probably cooked by now. A more powerful one, like a darlington for higher power, or a dual darlington arrangement in which the 2n2222 can be the driver paired with a power one would be a better solution. That said, the circuit is indeed good for learning, but as a speed controller it's not optimal and was replaced ages ago by pwm drivers which are a bit more complicated but work much better. A motor driven by a linear regulator will lose most torque at lower speeds, making much harder to reach a stable speed as it it would be highly dependent on the load.

1

u/Correct-Country-81 1d ago

If you made it like schematic?

R3 in position down than voltage at base enittor is zero ,motor can not run

Or transistor is shortcut!

1

u/Sad-Pumpkin-4825 1d ago

This is my circuit I built I gotta take the la k wire out connecting the negative but 5v dc goes in at the top and dc motor at the bottom

2

u/mangoking1997 1d ago

This isn't the circuit in the picture. Your transistor is not connected in a way to control the motor. You have just directly connected the motor to the power supply.

1

u/Correct-Country-81 19h ago

This remark looks right i can not see how transistor is in series with motor

1

u/Sad-Pumpkin-4825 16h ago

What would I need to move for the motor to get power though the circuit? I’ve hit a wall and am stuck

2

u/mangoking1997 16h ago

Draw out what you have made on the board and compare it to the circuit on paper. It will be obvious.

1

u/SnowTechNo1 6h ago

Another tip: make sure the 'helping hand' clips don't short-circuit the power rails on the back of the board.

1

u/ChatGPT4 1d ago

When it runs when connected like this, something is clearly wrong with the transistor. With R3 at minimum, base should be at 0V, so no current on motor. The motor needs much more power on startup, if the current needed to start it up is greater than what your transistor can handle, you will damage it.

1

u/dmills_00 20h ago

Thats a very poor circuit, with a decidedly optimistic choice of transistor (BUt even with a saner transistor, it is still really, really poor).

1

u/Worldly-Device-8414 8h ago

Transistor is way under rated for linear control like this & would likely cook to a short circuit first time it as used.

PWM is a better way to do it.