Pot value for motor speed control

[Father Haskell]

In the following diagram, what value do I need for the pot (R1)
so that it can run 24-7 constant duty without overheating?

Simple speed control for 12vdc 200 mA muffin fan, 0 - full RPM
as listed on the motor hub:

+ 12v ------------------------
(1 amp max) |
R1 ---------------
| |
| M1
| |
gnd ---------------------------------------------

 

[Tom Bruhns]

R1 must have a wiper rated to handle 200mA, at least (or whatever the
fan draws at maximum). Not all potentiometers like such a heavy wiper
current. I suppose if you don't need the control to be particularly
linear, a standing current around 100mA in the potentiometer would be
reasonable. That would be 120 ohms. Some value close to that should
be OK. At 100 ohms, you'd draw 120mA in the potentiometer plus
whatever the fan load is. At 100 ohms, the potentiometer dissipates
1.44 watts with 12 volts applied, but you should use one rated for
320mA current, because that's what will be in the upper section if the
fan is drawing 200mA, when the wiper is near the top. That would be a
10 watt potentiometer.

(It would be a lot easier to use a simple emitter follower, base driven
from a potentiometer, though you wouldn't get to quite the input
voltage at the output. But it would cut down on the requirements for
the potentiometer a lot.)

Cheers,
Tom

 

[Father Haskell]

R1 must have a wiper rated to handle 200mA, at least (or whatever the
fan draws at maximum). Not all potentiometers like such a heavy wiper
current. I suppose if you don't need the control to be particularly
linear, a standing current around 100mA in the potentiometer would be
reasonable. That would be 120 ohms. Some value close to that should
be OK. At 100 ohms, you'd draw 120mA in the potentiometer plus
whatever the fan load is. At 100 ohms, the potentiometer dissipates
1.44 watts with 12 volts applied, but you should use one rated for
320mA current, because that's what will be in the upper section if the
fan is drawing 200mA, when the wiper is near the top. That would be a
10 watt potentiometer.

(It would be a lot easier to use a simple emitter follower, base driven
from a potentiometer, though you wouldn't get to quite the input
voltage at the output. But it would cut down on the requirements for
the potentiometer a lot.)

Cheers,
Tom

I could also use an LM317 variable voltage regulator, but a single pot
seemed a bit simpler.

 

[ehsjr]

In the following diagram, what value do I need for the pot (R1)
so that it can run 24-7 constant duty without overheating?

Simple speed control for 12vdc 200 mA muffin fan, 0 - full RPM
as listed on the motor hub:

+ 12v ------------------------
(1 amp max) |
R1 ---------------
| |
| M1
| |
gnd ---------------------------------------------

You really don't want a pot to control the speed directly.
That will overheat the pot and may cause motor stalling at
very low RPM. You want a PWM (pulse width modulation) circuit.

Here's a very simple circuit to control the fan:
View in Courier font

+12 ---+---+----------+--------------+-----+
| | | | |
[1K] | ---------- [D2] [FAN]
| | | 8 | a| |
| +-----|4 | | |
/ | | +-----+
25K \<---+----|7 555 | |
/ | | | /
| a| | 3|---[150R]---| TIP31
[1K] [D1] | | \e
| | | | |
+----+----|6 | |
| | | | |
| +----|2 5|---+ | Diodes: 1N4001
| | | | |
[C1] ---------- [C2] |
| .1uF | .01uF |
Gnd ---+------------------------+----------+

This will avoid both overheating the pot, and motor
stalling at very low RPM.

Ed

 

[Rich Grise]

The fan load, which would have been right here if you had quoted
context, is 200 mA at 12V - that's 12 * .2 watts (2.4?) and 12 / .2
ohms = 60?

At 100 ohms, the potentiometer dissipates

I could also use an LM317 variable voltage regulator, but a single pot
seemed a bit simpler.

If all you want is a variable resistance, then you should use a rheostat.
Don't wire the pot like a voltage divider - as can be seen, it will
dissipate more than the fan motor. Find a 50-100 ohm rheostat that's
good for 200 mA - lessee, I^2 * R = .2 * .2 * 100 = 4 watts.

Of course, this is assuming that the fan has an ordinary DC motor that's
amenable to running with a resistor in series with it - if it's one of
those newfangled, fancy-schmancy brushless DC things, (essentially an
induction motor, or maybe stepper) I wouldn't want to try this.

Good Luck!
Rich